Important Notes of Cell Structure and Function

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Subject – NEET
Unit – III
Cell structure and Function
Organization of Cell
Introduction
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Cell is regarded as the unit of structure and function of all living organisms from simplest microorganisms to complex
multi-cellular organisms.
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“Cellula” in Latin means small compartments.
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Definition of cell – The simplest integrated organization in living beings, capable of independent survival.
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Study of structure, function, organization, growth and reproduction of cell is called as Cell Biology or cytology.
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Many scientists contributed to the study of cell, which are as follows1.
The word cell was introduced in 1665 by Robert Hooke.
2.
Anton van Leeuwenhoek discovered other types of cells such as human sperms, bacteria, RBCs etc. under
microscope and called them as animacules.
3.
Robert Brown discovered the existence of nucleus in the cell in 1831.
4.
M.J. schleiden and T.S. Schwann in 1839 proposed the cell theory.
5.
J.E Purkinje (1839) and Von Mohl (1846) named the protoplasm as formative substance found in the cells of
animal and plant embryo respectively.
6.
Waldeyer (1888) stated that the nucleus contains chromosomes on which the genes are linearly arranged.
Contributions of all these scientists and the development of electron microscopy lead to the modern version of cell theory which
states thata.
All living organisms are made up of cells.
b.
Cell is the basic structural unit of life.
c.
Cells contain genetic information which is passed on from cell to cell during cell division.
d.
All cells arise from pre existing cells.
e.
Cells are self duplicating, self contained units that are sometimes totipotent.
Definition of Totipotency – (totus = entire, potential = power)
The capacity of living nucleated cell to differentiate into any other type of cell and thus can form a complete new organism is
called as Totipotency.
Exceptions for cell theory
1.
Viruses do not follow the cell theory as they do not have cellular organization.
2.
Bacteria and blue – green algae are prokaryotes and they do not possess true cellular structures.
3.
Few species of fungi and algae are multinucleated.
Characteristics of a living cell
1.
Presence of plasma membrane
2.
Presence of protoplasm consisting of organic and inorganic substance in water.
3.
Presence of genetic material.
4.
Performs protein synthesis.
5.
Reproduce and pass the genetic information from one generation to next.
All living organisms are grouped into two main categories, the Prokaryotes and Eukaryotes.
A.
Prokaryotes
(Gr, pro = primitive, Karyon = nucleus)

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These are the cells in which, the nuclear material is not separated from cytoplasm by nuclear membrane.
eg. Bacteria and blue – green algae.
The average diameter of these cells is 1 um.
There are 4 forms of bacteria –
Cocci (spherical) Bacilli (rod - like)
Vibrio (comma shaped) spirilla (long and twisted)
Cyanobacteria or blue – green algae are photosynthetic prokaryotes. These may be filamentous or non – filamentous and
many times nitrogen – fixers.
2.
3.
4.
Prokaryotic cell shows following structures
1.
Cell envelope – is made up of 3 layers –
i.
Glycocalyx – Outer most layer composed of proteins and polysaccharides which helps in adhesion. It may be in the form
of loose sheath called slime layer or thick tightly bound structure called capsule.
ii.
Cell wall – Present below glycocalyx is a layer made – up of peptidoglycan pseudopeptidoglycan called cell wall. It gives
definite shape and support to the cell.
iii.
Plasma membrane – Inner most layer composed of lipids and proteins.
It functions as a permeability barrier, helps in inter cellular communication, is a site for respiration, photosynthesis and
act as a receptor for external environmental signals.
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Depending upon the composition of cell wall, bacteria are classified as Gram positive or Gram negative.
2.
i.
ii.
iii.
iv.
v.
Cytoplasm – Semi fluid ground substance or matrix present inner to the plasma membrane.
It has many organic and inorganic compounds useful for cell but lack cellular organelles.
It has mesosomes, chromatophores and inclusion bodies in it.
Mesomosmes are invaginations of plasma –
Membrane in the cell in the form of vesicles, tubules and lamellae. These are believed to participate in DNA replication
and respiration.
Chromatophors – These are usually found in photo synthetic bacteria and blue – green algae as these contain pigments
such as bacteriochlorophylls, bacteriophaeophytin and carotenoids.
Inclusion bodies – These are kind of storage granules found in cytoplasm.

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Diagram of Prokaryotic cell
vi.
These contained stored Organic Compounds (starch, glycogen) or inorganic compounds (phosphates and sulphur
granules)
3.
Ribosomes – These are dense particles made from RNA and proteins that participate in protein synthesis.
Prokaryotic ribosomes are of 70s type having 50s and 30s sub units.
4.
Genetic material (Nucleoid) – It is a large double – stranded circular DNA molecule present in the cytoplasm bound to
inner side of plasma membrane through mesosome.
5.
Plasmids – These are the additional, extra chromosomal, small, circular and self – replicating DNA molecules present in
many prokaryotes.
These plasmids carry certain genes for example antibiotic resistance, tumor formation in plants etc.
B.
Eukaryotic cell and its ultrastructure –
(Gr, Eu = advanced, karyon = nucleus)
Eukaryotes are the cells that contain a well developed nucleius i.e. the DNA of these cells is separated from
cytoplasm as it is enclosed inside a nucleus surrounded by nuclear membrane.
i.
Eukaryotic cells are usually 10 times larger than prokaryotes.
ii.
These cells also contain several intracellular organelles that carry various functions.
iii.
All the cells except bacteria and blue – green algae are eukaryotic.
Eukaryotic cell shows following structures
1.
Cell wall – It is the protective, semi – transparent, outermost covering and is a characteristic of plant cell.
i.
Its size varies depending upon the type of the cell and metabolic stage of the cell.
ii.
Chemically, it is composed of cellulose, pectin, lignin, hemicellulose, and cutin and suberin depending upon
type of the cell.

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Cell wall gives a definite shape to the plant cell and performs the functions such as protection from injury,
transport of material and storage of metabolites etc.
Plasma membrane – It is the outer most covering in animal cells and is present below cell wall in plant cells.
i.
It is also called as cell membrane or plasma lemma.
ii.
The plasma membrane of a cell is a wall organized, three dimentional structure, which is described by Singer
and Nicholson (1972). They named the structure as Fluid Mosaic Model.
Ultra structure of plasma membrane –
i.
According to the Fluid mosaic model, the plasma membrane is present as a bilayer made up of phospholipids.
ii.
Each phospholipid molecule has two parts – head and two tails.
iii.
Heads are hydrophilic in nature and tails are hydrophobic.
iv.
In a bilayer, the tails of the phospholipids face towards each other, whereas, the heads form external and internal
surfaces.
v.
Along with the phospholipids, several proteins are present which are globular in nature.
vi.
Depending upon the location of these proteins, they are categorized into 3 groups.
1.
Extrinsic proteins / peripheral proteins – are present on either surfaces of the phospholipid bilayer and
are loosely held.
2.
Intrinsic proteins / Integral proteins – are present embedded inside the bilayer and are tightly bound.
3.
Tunnel proteins – These are the large proteins that run across the phospholipid bilayer and thus are
visible on both the surfaces.
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The entire thickness of the plasma membrane is approximately 45Aº.
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The hydrophobic surfaces of the plasma membrane prevents the entry of hydrophilic solutes into the
cell.
Functions of plasma membrane –
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Gives the shape to the cell and protects the cell from external environment.
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It is selectively permeable and hence regulates the transport of molecules.
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Participates in absorption, excretion and secretion.
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It is responsible for intercellular communication.
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It helps in maintaining the turgidity of the cell.
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It serves as a receptor for various chemical stimuli such as amino acids, hormones, and sugars.
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In certain unicellular organisms like amoeba, plasma membrane performs the function of ingestion of food
(endocytosis) and locomotion too.
3.
Cytoplasm – or cytosol is a semi – solid, jelly like ground substance present inside the plasma membrane.
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The cytosol is composed of mainly water along with many organic and inorganic substances such as minerals,
sugars, amino acids, t-RNA, nucleotides, vitamines, proteins and enzymes.
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In this matrix various organelles such as mitochondria, chloroplast, ribosomes, Endoplasmic reticulum etc. are
suspended.
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In some plant cells such as spirogyra, the cytoplasm shows streaming movement called cyclosis.
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The main function of cytoplasm is that it acts as the site for many vital metabolic activities of the cell eg. protein
synthesis.

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Mitochondria – are rod – like organelles that have a surrounding double membrane and are present in the cytoplasm of
all eukaryotic cells.
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The size of the mitochondria varies from 0.2 to 2.0 µ in diameter.
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These are present in huge numbers about 50 to 5000 per cell.
Ultra structure of Mitochondria –
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Mitochondria is enclosed by two membrane called as outer membrane and inner membrane.
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The outer membrane is smooth, thick (about 60Aº) and continuous.
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The inner membrane is of the same thickness but is folded many times in the inward direction and hence is a highly
convulated structure.
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Each infolding of the inner membrane is called as Cristae. Due to the infoldings, the surface area of the inner membrane
increases many times.
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The outer membrane is permeable whereas, the inner membrane is selectively permeable.
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The inner membrane carries all the enzymes required for the cellular respiration enclosed in the Oxysomes or F, particles
that are arranged linearly on the inner membrane.
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Each F, particle has 3 parts – head, stalk and base.
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Inner to the inner mitochondrial membrane, there is a dense fluid called matrix.
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The matrix is granular and contains proteins, lipids, few ribosomes (70s type) and a small DNA and RNA.
Functions of Mitochondria
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The inner mitochondrial membrane carries all the enzymes required for electron transport chain.
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The coenzymes of electron transport chain namely cytochromes, dehydrogenases and flavoproteins present on inner
mitochondria membrane are responsible for cellular respiration.
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Due to the transfer of electrons, ATP molecules are generated and stored in the matrix. Hence these are called as the
power houses of the cells.
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Mitochondria converts pyruvic acid into CO2 and water thus liberating energy
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Endoplasmic Reticulum – is a membranous tube – like structure embedded in the cytoplasm.
It is present in all cells except Ova and RBCs.
These are thought to be originating from nuclear envelope.
Morphology and Ultra structure –

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ER forms a kind of network in the cytoplasm extending between the plasma membrane and the nuclear
membrane,
It is composed of 3 types of structure –
i.
Cisternae – flattened parallel sacs
ii.
Tubules – irregular branching
iii.
Vesicles – oval sacs
All these structures forms a network of continuous system.
The inner space within the ER is called as lumen which is 400 to 700 Aº in diameter.
This lumen is filled with a matrix which mainly consists of Glycogen and lipids.
Many ER have ribosomes arranged linearly on its outer surface, which gives it a rough texture. Such ER are
called as Rough Endoplasmic Reticulum (RER).
Whereas, if the ER is devoid of ribosomes, it is called as Smooth Endoplasmic Reticulum (SER)
Functions of Endoplasmic Reticulum –
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It provides mechanical support to the intracellular matrix.
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The membranes of the ER provide increased surface area for the metabolic activities.
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It is involved in many metabolic activities such as synthesis of lipids and glycogen, detoxification of certain
drugs and antimetabolites
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It provides precursors for different secretary substances to golgi apparatus.
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RER is involved in proteins synthesis.
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Processing and transport of many proteins takes place inside the tubules of the ER.
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The cavities of ER act as temporary storage compartments of metabolites which are later secreted out.
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Golgi apparatus These bodies were first demonstrated in the cells by Camillo Golgi and hence they are called as Golgi bodies / apparatus.
It is a unit membrane bound organelle present near the nucleus.
Morphology and Ultrastructure
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Golgi apparatus is present inside the cytoplasm as numerous aggregates of tubules and vehicles.
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There are about 3 to 30 flattened tubules or sacs which look like SER.
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These sacs, also called as cisternae are curved or cup shaped that lie in a parallel array.
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Several spherical vesicles of about 60-80 nm in diameter are also seen associated with cisternae.

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The lumen of Golgi complex consists of an amorphus matrix.
The matrix is made – up of several enzymes that are actively involved in metabolic activities.
Functions of Golgi apparatus
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The golgi complex is involved in the synthesis of lysosomes.
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These are actively involved in secretion of glycoproteins and glycolipids.
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Golgi complex stores proteins, carbohydrates and lipids in it
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These take part in enzymatic processing of proteins
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Plastids – are the semi – autonomous, double membrane bound organelles that enclose certain kinds of pigments in
them.
Plastids have their own DNA that is smaller than the genomic DNA.
These are found only in algae, plant cells and certain protists.
Depending upon the type of pigment enclosed, these are classified into 3 groups –
i.
Leucoplasts – colourless, stores starch
ii.
Chromoplasts - coloured (orange, red, brown, and yellow), contain pigments other than chlorophyll.
iii.
Chloroplasts – green coloured, stores chlorophyll in them.
Ultrastructure of chloroplasts
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Chloroplasts are double membrane bound organelles enclosing colourless matrix.
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Inside the matrix (stroma), there are many grana.
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The grana are made up of flattened sacs called thylakoids.
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The thylakoids are packed together in such a way that the grana look like a pile of 25 – paisa coins.
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There may be 40 to 60 grana per chloroplasts and each granum is made up of 2 to 100 or more thylakoids.
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The chlorophyll is enclosed in these thylakoids.

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Functions of chloroplasts
Due to the presence of photosynthetic pigment chlorophyll, chloroplasts are the sites for photosynthesis.
9. Lysosomes
Lysosomes are simple tiny spherical sac-like structures evenly distributed in the cytoplasm. Each lysosome is a small vesicle
surrounded by a single membrane and contains powerful enzymes. These enzymes are capable of digesting or breaking down all
organic materials.
Structure of Lysosomes
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Irregular, sac like structure bounded by single membranes.
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Under Electron microscope they appear dense and finely granular, with about 50 types of hydrolytic enzymes.
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In animal cells they are usually spherical
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They are polymorphic in which suggests that lysosomes are highly dynamic in nature and lights up on what is the
lysosome's function.

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Significance of Lysosome
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In WBC or leucocytes: Cells of leucocytes digest foreign proteins, bacteria and virus
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In autophagy: During starvation, the lysosomes digest stored food contents such as proteins, fats and glycogen of the
cytoplasm and supply the necessary amount of energy to the cell.
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In fertilization: The lysosomal enzymes present in the acrosome of the sperm cells digest the limiting membrane of the
ovum. Thus, the sperm is able to enter the ovum and start fertilization.
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Lysosomes also remove the worn out and poorly working cellular organelles by digesting them to make way for their
new replacements. Since they remove cell debris, they are also known as scavengers, cellular housekeepers or
demolition squads
10.Microbodies:
Microbodies are roughly spherical in shape, single membrane bound organelles.
These are of two types: Peroxysomes and Glyoxysomes
PEROXYSOMES:
An intracellular organelle found in all eukaryotes which is the source of the enzymes that catalyze the production and breakdown
of hydrogen peroxide, and are responsible for the oxidation of long-chain fatty acids.
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The peroxisome is contained by a single membrane and is involved in oxidation.
Peroxysomes contain at least 50 different enzymes, which are involved in a variety of biochemical pathways in different
types of cells.
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Peroxysomes have diverse functions ranging from cellular respiration to alcohol detoxification and result in the
production of hydrogen peroxide.
Hydrogen peroxide production and the enzymes that break down this toxic byproduct are sequestered to
prevent cell damage.
Peroxysomes incorporate proteins and lipids made in the cytosol and ER into the peroxisome itself in order to grow.
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Glyoxysome
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Peroxysomes are the specialized organelles found in plants (particularly in the fat-storage tissues of germinating seeds)
and also in filamentous fungi.
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They serve to break down fatty acids and provide enzymes to produce intermediates for the synthesis of sugars by
gluconeogenesis.

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11.Ribosomes
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Ribosomes are tiny spherical non membrane bound organelles that make proteins by joining amino acids together.
Many ribosomes are found free in the cytosol, while others are attached to the rough endoplasmic reticulum.
The purpose of the ribosome is to translate messenger RNA (mRNA) to proteins with the aid of tRNA.
In eukaryotes, ribosomes can commonly be found in the cytosol of a cell, the endoplasmic reticulum or mRNA, as well
as the matrix of the mitochondria.
Ribosomes are composed of two subunits, one large and one small, that they only bind together during protein synthesis.
All prokaryotes have 70S (where S=Svedberg units) ribosomes while eukaryotes contain larger 80S ribosomes in their
cytosol.
The 70S ribosome is made up of 50S and 30S subunits while the 80S ribosome is made up of 60S and 40S subunits.
The ribosomes play a very important role in protein synthesis, which is the process by which proteins are made from
individual amino acids.
12.Centrioles
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The Centrosome is an organelle that serves as the main microtubule organizing center of the animal cell as well as a
regulator of cell-cycle progression.
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In animal cells, centrosomes contain two barrel-shaped structures called centrioles.
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The walls of each centriole are usually composed of nine triplets of microtubules.
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Mature centrioles vary in diameter from 1500A° - 2500 A° and in length from 1600 A° - 1800 A°.
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An associated pair of centrioles, arranged perpendicularly and surrounded by an amorphous mass of dense material,
constitutes the compound structure of the Centrosome.
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Centrioles are involved in the organization of the mitotic spindle and in the completion of cytokinesis.

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13. Cytoskeleton
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The cytoskeleton is a cellular scaffolding or skeleton protein structure and is contained within the cytoplasm. The
cytoskeleton is present in all eukaryotic cells.
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The cytoskeleton is made up of three kinds of protein filaments: actin filaments (also called microfilaments), intermediate
filaments and microtubules.
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Microfilaments (actin filaments) are the thinnest filaments of the cytoskeleton. They are composed of linear polymers of
actin subunits and generate force by elongation at one end of the filament coupled with shrinkage at the other, causing net
movement. They also act as tracks for the movements like gliding, contraction and cytokinensis.
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Intermediate filaments are more stable (strongly bound) than actin filaments, and heterogeneous constituents of the
cytoskeleton. Like actin filaments, they function in the maintenance of cell-shape by bearing tension. Intermediate
filaments organize the internal three-dimensional structure of the cell and anchoring organelles.
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Microtubules are hollow cylinders, most commonly comprising alpha and beta tubulin. They play key roles in:
intracellular transport, the axoneme of cilia and flagella, the mitotic spindle and the synthesis of cell wall in plants.
14. Vacuoles
A vacuole is a membrane-enclosed compartment which performs a variety of functions within the cell, including storage and
transport.
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Vacuoles arise in the Golgi apparatus and the endoplasmic reticulum and are an important component of the
endomembrane system.

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The vacuole is bound by single unit membrane called tonoplast.
Vacuoles are vesicles with a selectively permeable membrane and perform specific functions based on the cell type they
are found in.
Food vacuoles arise as a result of the engulfment of food by a cell or phagocytosis.
Contractile vacuoles are common in protists and are used to maintain appropriate ion and molecule levels within the cell.
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In plants, vacuoles have diverse roles including, enzymatic hydrolysis, storage of organic compounds, protection against
herbivores through poison storage and pigment storage to facilitate pollination.
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Smaller plant vacuoles merge to form a larger central vacuole in mature plants that acts as a storage center of inorganic
ions called cell sap.
The central vacuole is critical in cell growth; as water is absorbed by the plant cell, the central vacuole increases in size
allowing the cell to expand without using valuable cell resources to make more cytoplasm.
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15. Cilia and Flagella
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Cilia and flagella are slender projections which are specialized to serve a variety of functions.
Flagella are tail-like projections which protrude from the cell bodies of certain prokaryotic and eukaryotic cells and
function in locomotion.
Eukaryotic flagella consist of bundles of nine fused pairs of microtubule doublets ("9+2”) surrounding two central single
microtubules.
Flagella and cilia are important in many kinds of cellular motility including, propulsion of unicellular eukaryotes through
water, and the movement of the sperm of animals, some plants, and algae.
Flagella beat in an undulating pattern to generate force that is the same as its axis, similar to the tail movement of a fish.
Cilia generate force perpendicular to their axis and move more like the oar on a boat.
Though cilia and flagella differ in functions, they are similar in composition, specifically the "9+2" pattern (a ring of nine
doublets of microtubules surrounding two single microtubules).
Cilia that are non motile lack the central pair of microtubules and therefore have a "9+0" pattern.
NUCLEAR ORGANIZATION
The nucleus is the most prominent organelle as compared to other cell organelles, which accounts for about 10 percent of the
cell's volume.
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In general, an eukaryotic cell has only one nucleus. However, some eukaryotic cells are enucleate cells (without nucleus),
for example, red blood cells (RBCs); whereas, some are multinucleate (consists of two or more nuclei), for example,
slime molds.

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Nucleus usually remains located in the centre but its position may change from time to time according to the metabolic
state of the cell.
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Shape of the nucleus may be spherical, ovoid, disc shaped, bilobed or multilobed.
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The size of the nucleus depends on the volume of the cell, amount of the DNA and proteins and metabolic phase of the
cell.
Structure of Cell Nucleus
The cell nucleus consists of a nuclear membrane (nuclear envelope), nucleoplasm, nucleolus and chromosomes.
Nuclear Membrane (Karyotheca)
The nuclear membrane is a double-layered structure, each being 7-8 nm thick, which encloses the contents of the nucleus.
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The outer layer of the membrane is connected to the endoplasmic reticulum.
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A fluid-filled space or perinuclear space is present between the two layers of a nuclear membrane and is usually 20nm
thick.
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The nucleus communicates with the remaining of the cell or the cytoplasm through several openings called nuclear pores.
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These pores are enclosed by electron dense rings called annuli which functions as a kind of diaphragm for selective
permeability.
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Such nuclear pores are the sites for exchange of large molecules (proteins and RNA) between the nucleus and cytoplasm.
Nucleoplasm
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Similar to the cytoplasm of a cell, the nucleus contains 'nucleoplasm' (nucleus sap) or karyoplasm.
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The nucleoplasm is one of the types of protoplasm, and it is enveloped by the nuclear membrane or nuclear envelope.

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The nucleoplasm is a highly viscous liquid that surrounds the chromosomes and nucleoli.
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Many substances such as nucleotides (necessary for purposes such as the replication of DNA) and enzymes (which direct
activities that take place in the nucleus) are dissolved in the nucleoplasm.
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Chromosomes
Chromosomes are present in the form of strings of DNA and histones (protein molecules) called chromatin.
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The chromatin is further classified into heterochromatin and euchromatin based on the functions during interphase.
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The former type is a highly condensed, transcriptionally inactive form, mostly present adjacent to the nuclear membrane.
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On the other hand, euchromatin is a delicate, less condensed organization of chromatin, which is found abundantly in a
transcribing cell.
Nucleolus
The nucleolus (plural nucleoli) is a dense, spherical-shaped structure present inside the nucleus.
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Some of the eukaryotic organisms have nucleus that contains up to four nucleoli. The nucleolus plays an indirect role in
protein synthesis by producing ribosomes.
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Ribosomes are the protein-producing organelles of a cell.
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Nucleolus disappears when a cell undergoes division and is reformed after the completion of cell division.
Functions of Cell Nucleus
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It controls the hereditary characteristics of an organism.
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This organelle is also responsible for the protein synthesis, cell division, growth and differentiation.
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Storage of hereditary material, the genes in the form of long and thin DNA (deoxyribonucleic acid) strands, referred to as
chromatin.
Storage of proteins and RNA (ribonucleic acid) in the nucleolus.
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Nucleus is a site for transcription in which messenger RNA (mRNA) are produced for protein synthesis.
Exchange of hereditary molecules (DNA and RNA) between the nucleus and the rest of the cell.
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During the cell division, chromatins are arranged into chromosomes in the nucleus.
Production of ribosomes (protein factories) in the nucleolus.
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Selective transportation of regulatory factors and energy molecules through nuclear pores.
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As the nucleus regulates the integrity of genes and gene expression, it is also referred to as the control center of a cell.
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The nucleus contains all the genetic material of an organism like chromosomes, DNA, genes, etc
Cells Contain Organic Molecules
A. Most Common Elements
1. Most common elements in living things are carbon, hydrogen, nitrogen, and oxygen.
2. These four elements constitute about 95% of your body weight.

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3. Chemistry of carbon allows the formation of an enormous variety of organic molecules.
4. Organic molecules have carbon and hydrogen; determine structure and function of living things.
5. Inorganic molecules do not contain carbon and hydrogen together; inorganic molecules (e.g., NaCl) can play
important roles in living things.
1. Carbon has four electrons in outer shell; bonds with up to four other atoms (usually H, O, N, or another C).
2. Ability of carbon to bond to itself makes possible carbon chains and rings; these structures serve as the
backbones of organic molecules.
3. Functional groups are clusters of atoms with characteristic structure and functions.
a. Polar molecules (with +/- charges) are attracted to water molecules and are hydrophilic.

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b. Nonpolar molecules are repelled by water and do not dissolve in water; are hydrophobic.
c. Hydrocarbon is hydrophobic except when it has an attached ionized functional group such as
carboxyl (acid) (
COOH); then molecule is hydrophilic.
d. Cells are 70-90% water; degree organic molecules interact with water affects their function.
4. Isomers are molecules with identical molecular formulas but differ in arrangement of their atoms
C. Large Organic Molecules Have Monomers
1. Each small organic molecule can be a unit of a large organic molecule called a macromolecule.
2. Small organic molecules (e.g., monosaccharides, glycerol and fatty acid, amino acids, and nucleotides) that
can serve as monomers, the subunits of polymers.
3. Polymers are the large macromolecules composed of three to millions of monomer subunits.

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4. Four classes of macromolecules (polysaccharides or carbohydrates, triglycerides or lipids, polypeptides
or proteins, & nucleic acids such as DNA & RNA) provide great diversity.
D. Condensation Is the Reverse of Hydration
1. Macromolecules build by different bonding of different monomers; mechanism of joining and breaking
these bonds is condensation and hydrolysis.
2. Cellular enzymes carry out condensation and hydrolysis of polymers.
3. Condensation involves a dehydration synthesis because a water is removed (dehydration) and a bond is
made (synthesis).
a. When two monomers join, a hydroxyl (
hydrogen is removed from the other.
OH) group is removed from one monomer and a
b. This produces the water given off during a condensation reaction.
4. Hydrolysis (hydration) reactions break down polymers in reverse of condensation; a hydroxyl
(
OH) group from water attaches to one monomer and hydrogen (
H) attaches to the other.
Carbohydrates
Carbohydrates are among the most abundant compounds on earth. They are normally broken down into five major classifications
of carbohydrates:
1.
2.
3.
4.
5.
Monosaccharides
Disaccharides
Oligosaccharides
Polysaccharides
Nucleotides
Monosaccharides
The word monosaccharide is derived from mono, meaning "one", and saccharide, meaning "sugar". The common
monosaccharides are glucose, fructose, and galactose. Each simple sugar has a cyclic structure and is composed of carbon,
hydrogen and oxygen in ratios of 1:2:1 respectively. Although each sugar mainly exists as a cyclic compound, it is important to
note that they are all in equilibrium to a small extent with their linear forms.

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Monosaccharides
While galactose and glucose are composed of six-membered rings, fructose has only five carbon atoms bonded to each other in
ring form.
Glucose
Glucose is the main sugar metabolized by the body for energy. The D-isomer of glucose predominates in nature and it is for this
reason that the enzymes in our body have adapted to binding this form only. Since it is an important energy source, the
concentration of glucose in the bloodstream usually falls within a narrow range of 70 to 115mg/100 ml of blood. Sources of
glucose include starch, the major storage form of carbohydrate in plants.
Galactose
Galactose is nearly identical to glucose in structure except for one hydroxyl group on carbon atom number four of the six-sided
sugar
Galactose is not normally found in nature in large quantities, however it combines with glucose to form lactose in milk. After
being absorbed by the body, galactose is converted into glucose by the liver so that it can be used to provide energy for the body.
Fructose
Fructose is a structural isomer of glucose, meaning it has the same chemical formula but a completely different three-dimensional
structure. The main difference is that fructose is a ketone in its linear form while glucose is an aldehyde. Upon consumption,
fructose is absorbed and converted into glucose by the liver in the same manner as lactose. Sources of fructose include fruit, honey
and high-fructose corn syrup.
Disacharides
Disaccharides, meaning "two sugars", are commonly found in nature as sucrose, lactose and maltose. They are formed by a
condensation reaction where one molecule of water condenses or is released during the joining of two monosaccharides. The type
of bond that is formed between the two sugars is called a glycosidic bond.
Condensation Reaction resulting in Glycosidic Bonds in Maltose
Lactose
Lactose is a disaccharide formed through the condensation of glucose and galactose. The bond formed between the two
monosaccharides is called a beta glycosidic bond . The alpha glycosidic bond, found in sucrose and maltose, differs from the beta
glycosidic bond only in the angle of formation between the two sugars. Unfortunately, unlike alpha glycosidic bonds, betaglycosidic bonds are unable to be digested by some people. Therefore, many people are lactose intolerant and suffer from
intestinal cramping and bloating due to the incomplete digestion of the substance.
Sucrose
Sucrose is found in common table sugar and is composed of glucose and fructose linked via a 1-2 alpha glycosidic bond.

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Figure %: Sucrose
Sucrose is an excellent preservative because it has no "reducing end" or reactive group like the other sugars. Other natural sources
of sucrose are found in plants such as sugar cane, sugar beets, and maple syrup.
Maltose
Maltose is the final disaccharide and consists of two glucose molecules joined by an alpha glycosidic bond. Maltose is an
interesting compound because of its use in alcohol production. Through a process called fermentation, glucose, maltose and other
sugars are converted to ethanol by yeast cells in the absence of oxygen. Through an analogous process, muscle cells convert
glucose into lactic acid to obtain energy while the body operates under anaerobic conditions. Although maltose is uncommon in
nature, it can be formed through the breakdown of starch by the enzymes of the mouth.
Oligosaccharides and Polysaccharides
Carbohydrates that contain more than two simple sugars are called oligosaccharides or polysaccharides, depending upon the
length of the structure. Oligosaccharides usually have between three and ten sugar units while polysaccharides can have more than
three thousand units. These large structures are responsible for the storage of glucose and other sugars in plants and animals.
Oligosaccharides
Important oligosaccharides are raffinose and stachyose. Composed of repeating units of galactose, glucose and fructose, these
oligosaccharides are of nutritional importance because they are found in beans and legumes. Because of their unique glycosidic
bonds, raffinose and stachyose cannot be broken down into their simple sugars. Therefore, they cannot be absorbed by the small
intestine and are often metabolized by bacteria in the large intestine to form unwanted gaseous byproducts.
Polysaccharides

Polysaccharides or complex carbohydrates are usually monomers and consist of thousands of repeating glucose units.

Naturally, they allow for the storage of large quantities of glucose. Starch is the major storage form of carbohydrate in
plants and has two different types: amylose and amylopectin.

Like glycogen is a highly branched polymer of glucose that is the main storage form of carbohydrate in humans. The
main chain of the structure is composed of alpha 1, 4 glycosidic bonds, while alpha 1,6 glycosidic bonds give rise to the
branch points of the polymer. Glycogen is stored in the liver and muscle where it is synthesized and degraded depending
upon the energy requirements of the body.

Indigestible forms of polysaccharides are known as dietary fiber and come in many different forms including cellulose,
hemicellulose, pectin, gum and mucilage.

Cellulose is by far the most abundant biochemical compound on the earth because it forms part of the structure of many
plants. It is unique among polysaccharides in that it forms intramolecular hydrogen bonds between adjacent glucose units
as well as beta 1,4 glycosidic bonds present in other carbohydrates. These special bonding characteristics allow cellulose
to form long, straight chains of glucose and give it strength and rigidity that many plants require for proper growth.
Cellulose and most forms of hemicellulose are insoluble fibers while pectin, gum and mucilage are all soluble fibers and
readily dissolve or swell when mixed with water.
Nucleotides
Other sugars of importance are found in nucleotides such as deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Both
RNA and DNA are five sided cyclic sugars; however, RNA has one more hydroxyl group than DNA. Glucose-6-phosphate, an
intermediate in the breakdown of glucose for energy, can be used for the synthesis of these compounds.
Lipids
A. Lipids
1. Lipids are varied in structure.

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2. Many are insoluble in water because they lack polar groups.
B. Fats and Oils Are Similar
1. Each fatty acid is a long hydrocarbon chain with a carboxyl (acid) group at one end.
a. Because the carboxyl group is a polar group, fatty acids are soluble in water.
b. Most fatty acids in cells contain 16 to 18 carbon atoms per molecule.
c. Saturated fatty acids have no double bonds between their carbon atoms. (C-C-C-)
d. Unsaturated fatty acids have double bonds in the carbon chain.(C-C-C-C=C-C-)
e. Saturated animal fats are associated with circulatory disorders; plant oils can be substituted for
animal fats in the diet.
2. Glycerol is a water-soluble compound with three hydroxyl groups.
3. Triglycerides are glycerol joined to three fatty acids by condensation

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4. Fats are triglycerides containing saturated fatty acids (e.g., butter is solid at room temperature).
5. Oils are triglycerides with unsaturated fatty acids (e.g., corn oil is liquid at room temperature).
6. Fats function in long-term energy storage in organisms; store six times the energy as glycogen.
C. Waxes Are Nonpolar
1. Waxes are a long-chain fatty acid bonded to a long-chain alcohol.
a. Solid at room temperature; have a high melting point; are waterproof and resist degradation.
b. Form protective covering that retards water loss in plants; maintain animal skin and fur.
D. Phospholipids Have a Polar Group
1. Phospholipids are like neutral fats except one fatty acid is replaced by phosphate group or a group with both
phosphate and nitrogen
2.Phosphate group is the polar head: hydrocarbon chain becomes nonpolar tails
3. Phospholipids arrange themselves in a double layer in water, so the polar heads face outward toward water
molecules and nonpolar tails face toward each other away from water molecules.
4. This property enables them to form an interface or separation between two solutions (e.g., the interior and
exterior of a cell); the plasma membrane is a phospholipid bilayer.

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E. Steroids Have Carbon Rings
1. Steroids differ from neutral fats; steroids have a backbone of four fused carbon rings; vary according to
attached functional groups.
2. Cholesterol is a precursor of other steroids, including aldosterone and sex hormones.
3. Testosterone is the male sex hormone.
4. Functions vary due primarily to different attached functional groups.
Proteins
A. Amino Acids

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1. Amino acids are the monomers that condense to form proteins, which are very large molecules with structural
and metabolic functions.
2. Structural proteins include keratin, which makes up hair and nails, and collagen fibers, which support many
organs.
3. Myosin and actin proteins make up the bulk of muscle.
4. Enzymes are proteins that act as organic catalysts to speed chemical reactions within cells.
5. Insulin protein is a hormone that regulates glucose content of blood.
6. Hemoglobin transports oxygen in blood.
7. Proteins embedded in the plasma membrane have varied enzymatic and transport functions.
B. Peptide Bonds Join Amino Acids
1. All amino acids contain a carboxyl (acid) group (
2. Both ionize at normal body pH to produce
COOH) and an amino group (
COO- and
NH2).
NH+; thus, amino acids are hydrophilic.
3. Peptide bond is a covalent bond between amino acids in a peptide; results from condensation reaction.
a. Atoms of a peptide bond share electrons unevenly (oxygen is more electronegative than nitrogen).
b. Polarity of the peptide bond permits hydrogen bonding between parts of a polypeptide.

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4. Amino acids differ in nature of R group, ranging from single hydrogen to complicated ring compounds.
a. R group of amino acid cysteine ends with a sulfhydryl (
SH) that serves to connect one chain
of amino acids to another by a disulfide bond (
S
S).
b. There are 20 different amino acids commonly found in cells.
5. A peptide is two or more amino acids joined together.
a. Polypeptides are chains of many amino acids joined by peptide bonds.
b. Protein may contain more than one polypeptide chain; it can have large numbers of amino acids.
C. Proteins Can Be Denatured
1. Both temperature and pH can change polypeptide shape.
a. Examples: heating egg white causes albumin to congeal; adding acid to milk causes curdling. When
such proteins lose their normal configuration, the protein is denatured.
b. Once a protein loses its normal shape, it cannot perform its usual function.
2. The sequence of amino acids, therefore, forecasts the protein's final shape.
D. Proteins Have Levels of Structure
1. Final 3-D shape of a protein determines function of the protein in the organism.
a. Primary structure is sequence of amino acids joined by peptide bonds.
1) Frederick Sanger determined first protein sequence, with hormone insulin, in 1953.
a) First broke insulin into fragments and determined amino acid sequence of
fragments.
b) Then determined sequence of the fragments themselves.
c) Required ten years research; modern automated sequencers analyze sequences in
hours.
2) Since amino acids differ by R group, proteins differ by a particular sequence of
the R groups.
b. Secondary structure results when a polypeptide takes a particular shape.
1) The
(alpha) helix was the first pattern discovered by Linus Pauling and Robert Corey.
a) In peptide bonds, oxygen is partially negative, hydrogen is partially positive.
b) Allows hydrogen bonding between the C
of another.
O of one amino acid and the N
H
c) Hydrogen bonding between every fourth amino acid holds spiral shape of a helix.
d) helices covalently bonded by disulfide (S
amino acids.
S) linkages between two cysteine

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2) The
sheet was the second pattern discovered.
a) Pleated sheet polypeptides turn back upon themselves; hydrogen bonding occurs
between extended lengths.
b) keratin includes keratin of feathers, hooves, claws, beaks, scales, and horns; silk
also is protein with sheet secondary structure.
3. Tertiary structure results when proteins of secondary structure are folded, due to various
interactions between the R groups of their constituent amino acids
4. Quaternary structure results when two or more polypeptides combine.
1) Hemoglobin is globular protein with a quaternary structure of four polypeptides.
2) Most enzymes have a quaternary structure.

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Nucleic acids
Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are made up of nucleic acids found in the nuclei of
living cells. They are the vehicles of genetic inheritance.
Nucleic acids are condensation polymers of nucleotides. To understand their functions you will find it helpful to
look at how their molecules are built up and the structures of these molecules.
The building blocks
Three types of chemicals make up the building blocks for nucleic acids.
Phosphates
These are based on the inorganic acid H3PO4 (phosphoric acid).
Two other acids may be formed from phosphoric acid by condensation reactions
Sugars
The sugars in DNA and RNA are pentoses.


in DNA the sugar is deoxyribose
in RNA the sugar is ribose
Both these sugars have hydroxyl groups. Ribose has four and deoxyribose has three (hence the prefix 'deoxy').
These groups can react with carboxylic acids and phosphoric acid to form esters.
Organic bases
There are four organic bases involved in the formation of DNA molecules:


adenine
adenine and guanine (both purines containing two rings in their structures)
thymine and cytosine (both pyrimidines containing only one ring in their structures)
(A)
A and G are double-ring purine
bases.
guanine
(G)

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(T)
T and C are single-ring pyrimidine
bases.
cytosine
(C)
In RNA the four bases are the same except for thymine which is replaced by uracil (U), a pyrimidine base.
Adenine, guanine, thymine, cytosine and uracil are bases because of the presence of one or more of the
following groups: - NH2, - NH - and = N - groups. Importantly, - NH2 and - NH - groups can react with
carboxylic acids and phosphoric acid to form amides.
Formation of polymers:
The basic structure of a nucleotide is shown below.
In all nucleotide molecules the bonds holding the phosphate group to the sugar and the base to the sugar are
both products of condensation reactions. Water is eliminated when they form. In both cases the oxygen to form
the water has come from the sugar's -OH groups.
Nucleic acids
Nucleotides can link together by the formation of phosphate ester bonds. The hydroxyl group of a phosphate on
one nucleotide undergoes a condensation reaction with the hydroxyl group on the carbohydrate ring of another
nucleotide. The process may continue, building up nucleic acid molecules. These are polymers
called polynucleotides.
Nucleic acids are the 'building blocks' of DNA and RNA.
The structure of DNA

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DNA is formed from two polynucleotide chains. Each chain has a helical structure (a helix), in other words the
molecule is coiled like a spring.
The two helices are then intertwined to give a double helix. The bases are on the inside of the helix and the
phosphate groups are on the outside.
The two helices are held together by pairing of the nucleotides' bases through hydrogen bonding. Because the
double ring purines are bigger than the single ring pyrimidines the structure can only form with purine bases
opposite pyrimidine bases. A big one complements a little one to take up about the same space.
The structure is sometimes described as a ladder where the sugar-phosphate chains are the sides of the ladder
and the base-base bonds are the rungs. Intermolecular forces twist the ladder into a double helix shape.
The structure of RNA
The structure of RNA differs fundamentally from that of DNA in three ways:



It forms a single strand sugar-phosphate chain
The sugar in its nucleotides is ribose
The base thymine is replaced by the base uracil
Although RNA is single stranded it can form helical loops by folding back on itself. Hydrogen bonding between
base pairs holds the strand in shape. This allows RNA to carry out its important function in protein synthesis
ENZYMES
Enzymes are biological catalysts. There are about 40,000 different enzymes in human cells, each
controlling a different chemical reaction. They increase the rate of reactions by a factor of between
106 to 1012 times, allowing the chemical reactions that make life possible to take place at normal temperatures. They were
discovered in fermenting yeast in 1900 by Buchner, and the name enzyme means "in yeast". As well as catalysing all the
metabolic reactions of cells (such as respiration, photosynthesis and digestion), they may also act as motors, membrane pumps and
receptors. The active site of RUBISCO, the key enzyme in photosynthesis, contains just 6 amino-acids.
Enzymes are proteins, and their function is determined by their complex structure. The reaction takes place in a small part of the
enzyme called the active site, while the rest of the protein acts as "scaffolding".
The amino acids around the active site attach to the substrate molecule and hold it in position while the reaction takes place. This
makes the enzyme specific for one reaction only, as other molecules won't fit into the active site – their shape is wrong.
Many enzymes need cofactors (or coenzymes) to work properly. These can be metal ions (such as Fe2+, Mg2+, Cu2+) or organic
molecules (such as haem, biotin, FAD, NAD or coenzyme A). Many of these are derived from dietary vitamins, which is why they
are so important. The complete active enzyme with its cofactor is called a holoenzyme, while just the protein part without its
cofactor is called the apoenzyme.
1. Reaction Mechanism
In any chemical reaction, a substrate (S) is converted into a product (P):
In an enzyme-catalysed reaction, the substrate first binds to the active site of the enzyme to form an enzyme-substrate (ES)
complex, then the substrate is converted into product whilst attached to the enzyme, and finally the product is released, thus
allowing the enzyme to start all over again.
2. Molecular Geometry
The substrate molecule is complementary in shape to that of the active site. It was thought that
the substrate exactly fitted into the active site of the enzyme molecule like a key fitting into a
lock (the now discredited ‘lock and key’ theory). This explained why an enzyme would only
work on one substrate (specificity), but failed to explain why the reaction happened. It is now known that the substrate and the
active site both change shape when the enzyme-substrate complex is formed, bending (and thus weakening) the target bonds.

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Although enzymes can change the speed of a chemical reaction, they cannot change its direction, otherwise they could make
"impossible" reactions happen and break the laws of thermodynamics. So an
enzyme can just as easily turn a product into a substrate as turn a substrate into a product, depending on the local concentrations.
The transition state is the name given to the distorted shape of the active site and substrate. Before it can change into product, the
substrate must overcome an "energy barrier" called the activation energy. The larger the
activation energy is, the slower the reaction will be. This is because
only a few substrate molecules will have sufficient energy to
overcome the activation energy barrier. Imagine pushing boulders
over a hump before they can roll down hill, and you have the idea.
Most biological reactions
have large activation energies, so they without enzymes they happen far too slowly to be useful.
Enzymes reduce the activation energy of a reaction so that the kinetic energy of most
molecules exceeds the activation energy required and so they can react.
Factors that Affect the Rate of Enzyme Reactions
1. Temperature
Enzymes have an optimum temperature at which they work
fastest. For mammalian enzymes this is about 40°C, but there
are enzymes that work best at very different temperatures, e.g.
enzymes from the arctic snow flea work at -10°C, and
enzymes from thermophilic bacteria work at 90°C.
Up to the optimum temperature the rate increases
geometrically with temperature (i.e. it's a curve, not a straight
line). The rate increases because the enzyme and substrate molecules both have more kinetic
energy and so collide more often, and also because more molecules have sufficient energy to
overcome the activation energy.
Above the optimum temperature the rate decreases as more of the enzyme molecules denature.
The thermal energy breaks the hydrogen bonds holding the secondary and tertiary structure
of the enzyme together, so the enzyme loses its shape and becomes a random coil - and the
substrate can no longer fit into the active site. This is irreversible. Remember that only the
hydrogen bonds are broken at normal temperatures; to break the primary structure ( the peptide
bonds) you need to boil in strong acid for several hours – or use a protease enzyme!
2. pH
Enzymes have an optimum pH at which they work
fastest. For most enzymes this is about pH 7-8 (normal
body pH), but a few enzymes can work at extreme pH,
such as gastric protease (pepsin) in our stomach,
which has an optimum of pH 1.
The pH affects the charge of the amino acids at the
active site, so the properties of the active site change
and the substrate can no longer bind. For example a
carboxyl acid R groups will be uncharged a low pH
(COOH), but charged at high pH (COO-).
3. Enzyme concentration
As the enzyme concentration increases the rate of the reaction also
increases, because there are more enzyme molecules (and so more
active sites), available to catalyse the reaction therefore more
enzyme-substrate complexes form. In cells, the substrate is always
in excess, so the graph does not level out. In the lab, these
conditions need not apply and a plateau can be reached.
4. Substrate concentration

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The rate of an enzyme-catalysed reaction is
also affected by substrate concentration. As
the substrate concentration increases, the
rate increases because more substrate
molecules can collide with active sites, so
more enzyme-substrate complexes form.
Point of
saturation
At higher concentrations the enzyme
molecules become saturated with
substrate, and there are few free active sites,
so adding more substrate doesn't make
much difference (though it will increase the
rate of E-S collisions).
The maximum rate at infinite substrate concentration is called vmax, and the substrate
concentration that gives a rate of half vmax is called KM. These quantities are useful for
characterising an enzyme. A good enzyme has a high vmax and a low KM.
5. Covalent modification
The activity of some enzymes is controlled by other enzymes, which modify the protein chain
by cutting it, or adding a phosphate or methyl group. This modification can turn an inactive
enzyme into an active enzyme (or vice versa), and this is used to control many metabolic
enzymes and to switch on enzymes in the gut e.g. HCl in stomach → activates pepsin →
activates rennin.
6. Inhibitors
Inhibitors inhibit the activity of enzymes, reducing the rate of their reactions. They are found
naturally, but are also used artificially as drugs, pesticides and research tools. There are two
kinds of inhibitors.
(a) A competitive inhibitor molecule has
a similar structure to the substrate
molecule, and so it can fit into the
active site of the enzyme. It therefore
competes with the substrate for the
active site, so the reaction is slower.
Increasing the concentration of
substrate restores the reaction rate and
the inhibition is usually temporary and
reversible.
(b) A non-competitive inhibitor molecule is
quite different in structure from the
substrate and does not fit into the active
site. It binds to another part of the enzyme
molecule, changing the shape of the
whole enzyme, including the active site,
so that it can no longer bind substrate
molecules. Non-competitive inhibitors
therefore simply reduce the amount of
active enzyme.
This kind of inhibitor tends to bind tightly and irreversibly – such as the poisons cyanide and heavy metal ions. Many nerve
poisons (insecticides) work in this way too.
Naming Enzymes

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Enzyme names usually end in –ase.
The six classes of enzymes are defined on the basis of the types of reactions they catalyze.






Oxidoreductase – catalyzes redox reactions
Transferase – catalyzes transfer of functional groups
Hydrolase – splits chemical bonds by addition of water
Lyase – splits chemical bonds without using water (not a hydrolysis reaction)
Isomerase – rearranges atoms within a molecule
Ligase – forms a chemical bond between two atoms
General properties of Enzymes –
∙
React with both acidic and alkaline substrate hence amphoteric in nature.
∙
Enzymes are extremely sensitive to external factors.
∙
Enzymes are thermolabile.
∙
Enzymes are colloidal in nature (gel – like)
∙
Enzyme molecule is several times larger than the substrate molecule.
∙
Only a part of enzyme called ‘active site’ participates in enzymatic reaction.
∙
Small quantity of enzyme is required to change a large amount of substrate
∙
Enzymes are highly specific towards the substrate they act upon.
∙
Enzymes generally work best under certain narrowly defined conditions referred to as optima.
∙
Enzyme reactions are reversible i.e. same enzyme can regulate the reaction forward or backward.
Cell Cycle –
The cell cycle is the sequence of events in the life of a cell which starts immediately after previous cell division and ends
with the completion of the current division.
∙
Eukaryotic cell cycle can be divided into following stages –
1.
Interphase
2.
Karyokinesis
3.
Cytokinesis

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Interphase – It is the preparative phase during which cell is metabolically very active and prepares itself for cell division.
During interphase, following activities occur in the cell.
∙
replication of chromosomal DNA, synthesis of RNA and the nuclear proteins (eg. histones)
∙
Synthesis of energy rich compounds such as ATP, GTP, NADPH 2 etc. Through various metabolic pathways.
∙
Division of Centrioles in animal cells.
There are 3 sub stages of Interphase –
i.
G1 phase –
It is called as post division gap phase or first growth phase.
It is characterized by synthesis of RNA and large amount of proteins leading to the increase in size of the cell.
ii.
S-phase – or synthesis phase is the phase where the doubling of DNA by the means of DNA replication takes place.
∙
G2 Phase – It is the final stage of Interphase and occurs just before karyokinesis. It is also called as second growth phase.
In this phase protein synthesis for nuclear division occures.
2.
Karyokinesis – It is the phase nuclear division where the previously duplicated DNA gets distributed into two halves i.e.
daughter nuclei.
It is sub divided into prophase Metaphases Anaphase and Telophase.
3.
Cytokinensis – This is the division of cytoplasm it occurs after karyokinesis and divides the parent cell into daughter
cells.
Karyokinesis and cytokinesis together form The M-phase (i.e cell division)
The total duration of a cell cycle varies greatly in different organisms. It may be as short as 20-30 min in E. coli or may
take 12-24 hours in most of the higher plants and animals.
Go phase – Some cells are terminally differentiated. Neurons and red blood cells are examples of terminally differentiated cells.
When these cells reach their final mature state, they do not need to divide ever again. For this reason, the cells leave G1 and enter
an alternative state called G0 (read “G-zero”) where they stop dividing permanently. Not all cells can enter the G0 phase.
Cells in G0 may not undergo mitosis, but they are still very metabolically active. For example, half of a human brain consists of
neurons that are in G0. Yet the brain consumes about 25% of the body’s metabolic energy each day.
Diagram - cell cycle
Significance of cell cycle.

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∙
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In multi-cellular organisms, the cell cycle helps in reproduction, growth and replacement of dead cells, healing of rounds
etc.
∙
The interphase allows time for synthesis and growth of dividing cell.
∙
Property controlled and regulated cell cycle results in normal and proportionate growth of organisms.
∙
Loss of control over the cell cycle can lead to abnormalities in growth.
Mitosis phases:-
Mitosis is the characteristic division of the body cells, hence called as somatic division.
Definition – mitosis is an equational division, dividing the mother cell into two daughter cells which are identical to one
another and also to the original mother cell in every aspect. In mitosis, there is equal distribution of chromosomes of
mother cell in the two daughter cells.
∙
Mitotic cell cycle consists of interphase followed by karyokinesis and then Cytokinesis.
A.
Interphase – It is characterized by –
∙
Volume of nucleus increases
∙
Nuclear membrane and nucleolus are prominent.
∙
Chromosomes are present in thin thread like structures forming a network called chromatin network.
∙
In this stage the nucleus has double amount of DNA
B.
Karyokinesis –
It is characterised by series of changes in the nucleus which is a continuous process. Bat it is studied under 4 rule phases
as follows.
1.
Prophase –
∙
The chromatin fibre starts condensing slowly.
∙
Here, it separates as single separate threads of chromosomes called monads.
∙
The nuclear membrane and nucleolus are prominent and visible.
∙
As the prophase progresses, chromosomes become shorter and thicker.
∙
At the end of prophase, each chromosome appears to be consisting of 2 identical threads of monads (forming a dyad)
∙
At this stage the chromosomes are shorter thicker and consists of two identical sister chromatids held together by a
spherical body called centromere or kinetochore.
∙
Finally, the nuclear membrane and nucleolus starts disintegrating.
2.
Metaphase –
∙
Till the beginning of the metaphase, the nuclear envelope and nucleolus have completely disintegrated.
∙
Chromosomes are at highly condensed state and lie in the cytoplasm.

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∙
A bipolar spindle body appears in the centre of the cell. It consists of numerous spindle fibres.
∙
Spindle fibers are thin, threads of tubulin protein organized to form microtubules.
∙
The chromosomes move and get arranged in a plane along the equator of the spindle in such a way that the centromeres
are in the centre of the spindle body.
∙
This results in the formation of an equatorial plate (metaphasic plate).
Diagram – phases of Mitosis
3.
Anaphase –
∙
During early anaphase, the centromeres of each chromosome divides longitudinally into two

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∙
As a result, each chromosome is now completely divided into two identical halves (sister chromatids).
∙
The Centromere of each daughter chromosome remains connected to the pole through spindle fibre.
∙
During, late anaphase, the two groups of daughter chromosomes are pulled away from each other and start moving
towards the opposite poles.
∙
In both the groups, each chromosome has one chromatid and one centromere
∙
The chromosomes starts becoming longer and thinner as they move towards the poles.
4.
Telephone –
∙
This is the last phase of karyokinesis.
∙
The two sets of chromosomes reach the opposite poles.
∙
The chromosomes again become long and thin like prophase chromosomes.
∙
Two nucleoli appear near two poles, spindle body starts disappearing and nuclear membrane starts developing
surrounding the two groups of daughter chromosomes.
∙
Thus two daughter nuclei that are similar to each other and similar to their original mother cell are formed.
C.
Cytokinesis –
∙
It is the division of the Cytoplasm –
∙
It starts at the end of the telophase.
∙
In plant cells, a cell plate forms at the centre of equitorial plane and it goes on extending towards the peripheri thus
dividing the mother cell into two equal daughter cells.
∙
In animal cells, two groves form at the sides o the cell and go on deepening to finally meet each other thus, dividing the
mother cell into two equal daughter cells.
Significance of mitosis –
∙
Maintains equal distribution of chromosomes after each cell cycle.
∙
Produces genetically similar daughter cells.
∙
Maintains constant number of chromosomes in all the somatic cells.
∙
Dead cells are replaced by nearly formed cells by mitosis.
∙
It helps in asexual reproduction, growth and development of organisms.
MEIOSIS –
∙
Meiosis is the reduction division in which the diploid number of chromosomes is reduced to haploid during gamete or
spore formation.
∙
The gamets then fuse to form a diploid zygote through fertilization.
Definition : It is a type of division found in reproductive cells, in which the diploid (2n) number of chromosomes is reduced to
haploid in the daughter cells. In meiosis, the chromosomes divide once whereas, the nucleus divides twice.
∙
Four haploid daughter cells result from one diploid mother cell. These differ from each other as well as from the mother
cell.
∙
Meiotic cell cycle consists of the interphase, Karyokinesis and cytokinesis.
∙
The interphase consist of G1, S and G2 phases and involves changes same as mitotic interphase.
∙
The nucleus enlarges during interphase.
∙
The nuclear membrane and nucleolus are clearly visible while the chromosomes are not clearly visible and are thin and
long.

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Karyokinesis in Meiosis consists of two successive nuclear divisions Meiosis I and Meiosis II Separated by a phase
called interkinesis.
∙
Karyokinesis is followed by division of cytoplasm called cytokinesis.
A.
Meiosis I –
∙
involves division of diploid mother cell nucleus to form two haploid nuclei called as daughter nuclei
∙
Reduction in the number of chromosomes occur due to the separation of homologous chromosomes.
∙
Due to the reduction of chromosome number in daughter nuclei, this is also known as reduction division.
∙
The entire process can be studied under following phases –
1.
Prophase I -
i.
Leptonema - It is characterized by –
∙
Chromosomes become visible as long slender threads
∙
Nuclear envelope and the nucleolus are prominently visible.
∙
Thin chromosomes are scattered within the nucleus.
ii.
Zygonema (Zygotene) –
It is characterized by –
∙
Pairing of homologous chromosomes.
∙
Homologous Chromosomes from maternal and paternal set get paired to each other along the length and farm a bivalent.
∙
This stage is also called as a dyad stage.
∙
Chromosomes become shorter, thicker and more distinct.
iii.
Pachyanema (Pachytene) – It is characterized by –
∙
The chromosomes become shorter, thicker and more distinct.
∙
Each chromosomes has two sister chromatids joined by a centromere. Thus each pair of homologous chromosomes
(bivalents) consists of 4 chromatids forming a tetrad stage.
∙
At this stage, Crossing over between two non-sister chromatids of a homologous pair takes place.
∙
The non-sister chromatids taking part in crossing over break simultaneously at same point and exchange their segments
with each other followed by rejoining of the exchanged segments.
∙
Thus, crossing over results in recombination of genes, which play an important role in genetic basis of variation and
evolution.
iv.
Diplotene (Diplonema) – It is characterized by –
∙
In each pair, the homologous chromosomes start repelling each other; as a result, they start to separate and uncoil.
∙
The separation and uncoiling of the homologues begin at the centromeres and proceed towards the ends. This is called as
terminalization of chaisma (point of crossing over).

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v.
Diakinesis – It is characterized by –
∙
Chromosomes are still in pair and in contact with each other by terminal chaisma.
∙
The chromosomes become shorter, thicker and more prominent.
∙
By the end of prophase I, the nuclear envelope and nucleolus disappear completely and the pairs of chromosomes are
seen scattered in the nucleoplasm.
2.
Metaphase I – It is characterized by –
∙
Formation of bipolar spindle fibres.
∙
The homologous chromosomes, still in pairs, move towards the centre of the spindle.
∙
Each chromosome has one centromere and two sister chromatids.
∙
At the equatorial plane, the pairs lie randomly in such a way that some maternal and some paternal homologues are
facing each pole.
∙
These maternal and paternal chromosomes assort independently.
3.
Anaphase I – It is characterized by –
∙
The homologous chromosomes are pulled away from each other and finally separate completely (lie terminalisation is
completed).
∙
The two sets of homologous chromosomes separate and start moving towards opposite poles due to shortening of the
chromosomal fibers (spindle).
∙
Each separated set has haploid number of chromosomes that is a random mixture of chromosomes from the original
paternal and material sets.
4.
Telophase I – It is characterized by –
∙
The two sets reach the opposite poles.
∙
The chromosomes, each with two chromatids and one centromere, become thin and long.

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∙
Nucleolus and nuclear membrane arise for each set of chromosomes.
∙
Thus two daughter nuclei get organized having haploid set of randomly assorted chromosomes.
Interkinesis – The interval between Meiosis I and meiosis II is called as inetrkinesis.
B.
Meiosis II – Second meiotic division is similar to mitosis in which two haploid daughter nuclei formed at the end of
Meiosis I divide and produce 4 haploid nuclei. It can be studied under following phases –
1.
Prophase II –
∙
Chromosomes condense, become shorter and thicker distinct bodies.
∙
Each chromosome has two sister chromatids joined by a centromere.
∙
The nuclear membrane and nucleolus disappear by the end of prophase II.
2.
Metaphase II –
∙
Bipolar spindle body forms and chromosomes get arranged at the equatorial plane.
∙
Chromosomes are highly condensed
∙
Animal cells form asters.
3.
Anaphase II –
∙
Centromeres divide longitudinally into two, thus dividing each chromosome into two halves or daughter chromosomes.
∙
These two sets of daughter chromosomes are pulled away from each other and move towards opposite poles.
∙
In each set, every daughter chromosome has one centromere and one chromatid.
∙
Due to the result of crossing over, out of the total four sets of daughter chromosomes, no two sets are exactly identical to
each other.
4.
Telophase II –
∙
The sets of chromosomes reach the opposite poles and a Neal nucleus is organized at each pole.

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In all 4 daughter nuclei are formed that are haploid and dissimilar.
Cytokinesis – it is the division of cytoplasm which may be occurring due to cell plate formation or cleavage (groove)
formation.
Significance of Meiosis –
i.
Regulation of chromosome number in the life cycle of an organism.
ii.
Crossing over leads to recombination of genetic material.
∙
It is responsible for variations in offspring.
∙
Variation are responsible for evolution.