2. Learning Objectives:
(a) state the importance of mitosis in growth, repair and asexual
reproduction.
(b) explain the need for the production of genetically identical cells
and fine control of replication.
(c) identify, with the aid of diagrams, the main stages of mitosis.
(d) state what is meant by homologous pairs of chromosomes.
(e) identify, with the aid of diagrams, the main stages of meiosis.
(f) define the terms haploid and diploid, and explain the need for a
reduction division process prior to fertilisation in sexual
reproduction.
(g) state how meiosis and fertilisation can lead to variation.
3. Human ChromosomesHuman Chromosomes
• Each cell in the human body contains two sets of 23
chromosomes.
• Cell divides by mitosis to produce the same number of
chromosomes as the parent nucleus. Each cell therefore has
the same genetic material.
• Reproductive cells (sex gametes) only have
one set of chromosomes. These combine to make a new
person with different genetic material to
both parents.
6. Mitosis
• A cell divides to produce two daughter cells, each
having the same number of chromosomes as the
parent cell. (diploid number, 2n)
• Daughter cells have the same hereditary materials or
genes as the parent cell.
• Young or offspring produced asexually will therefore
be genetically identical to the parents.
• Mitosis is mainly responsible for
(a)cell growth in both plants and animals.
(b)repair for worn-out cells
(c)asexual reproduction in plants (vegetative
10. INTERPHASE (INTERPHASE ( ‘Resting’ Stage‘Resting’ Stage))
• The cell is replicating its DNA content before the cell divides.
• DNA replication is a process that faithfully copies all the
information stored within the chromosomes to ensures that
daughter cells produced by mitosis contain all the DNA
needed for subsequent cell division and differentiation.
• The resulting daughter cells can be said to be genetically
stable.
• In adult human, mistakes made in DNA replication or mitosis
may cause the uncontrolled division of cells, which lead to
cancer. Cancer can be fatal in most cases because the cells
lose the ability to control the way they divide.
11. INTERPHASE (INTERPHASE ( ‘Resting’ Stage‘Resting’ Stage))
Chromatin
thread
(contains one
DNA molecule)
sister chromatids
(contain two identical
DNA molecule
Chromatin
threads coil
and shorten
One chromosome
DNA replication
12. • Each chromosome consist of
two identical DNA molecules
known as sister chromatids.
• The sister chromatids are joined
at a point known as the
centromere.
DNA replication
sister chromatids
centromere
13. PROPHASEPROPHASE
• The chromatin (unravelled DNA) in the nucleus, condenses to
form pairs of chromosomes.
• The centrioles move to opposite ends of the
nucleus.
• As this is happening the nucleolus begins to break
down
• Nuclear membrane begins to break down
14. METAPHASEMETAPHASE
• The spindle becomes
fully developed.
• The nuclear membrane has completely gone
• The chromatid pairs place
themselves onto individual fibres
and are aligned along the
centre of the spindle.
15. ANAPHASEANAPHASE
• The chromatid pairs are split into
two (This is done by movement of
the spindle fibres)
• The pairs then travel to opposite
ends of the spindle.
• The halved chromatids are now
called chromosomes
16. TELOPHASETELOPHASE
Two new nuclei are formed when the chromosomes
reach the opposite poles of the cell
The nuclear membrane is formed- the nucleolus
reappears
The chromosomes disperse in the nucleus
18. Common Misconception
• X During interphase, a chromosome
replicates to form two new chromosomes.
• √ During interphase, DNA replicates and two
chromatids are formed, but both are still
considered as one chromosome.
19. A B C
D E
Quick Review
What’s the order of stages in mitosis?
21. Quick review:
What’s the importance of mitosis?
• Mitosis is mainly responsible for
(a)cell growth in both plants and animals.
(b)repair for worn-out cells
(c)asexual reproduction in plants
(vegetative propagation to produce new
plants)
23. Meiosis
• A type of cell division that results in the nucleus
of each daughter cell containing half the number
of chromosomes as the nucleus of the parent
cell.
•This choromosome number in the sex gamete is
the haploid number.
•Twice the haploid number of chromosomes
make up the diploid number.
• In man, normal cell has diploid number of 46.
• Sperm/egg has only have half the number of
chromosomes, which is 23 (haploid number).
24. 16.3
1 diploid parent
cell with 2 pairs
of chromosomes
Meiosis I
Each daughter
cell contains 2
chromosomes,
each consisting
of 2 chromatids
4 haploid
gametes, each
containing 2
chromosomes
Meiosis II
Overview of Meiosis
Replication of
chromosomes
25. During Meiosis gamete (sex) cells undergo a
“double division”, maintaining the DNA, but
reducing the chromosomal count to 23
+ =
Sperm (23) + Egg (23) = Fertilized Cell (46)
26. Importance of Sexual Reproduction
• Sexual reproduction by meiosis shuffles the genes to
produce genetically unique offspring.
• Variation in offspring provided by sexual reproduction
confers a large evolutionary advantage
27. Common Misconception
• X Meiosis II is the same as mitosis.
• √ The stages in meiosis II are the same as
mitosis, but while mitosis begins with a
diploid number of chromosomes, meiosis
begins with a haploid number of
choromosomes.
28. Stages of Meiosis
• Meiosis consists of meiosis I and II.
• Meiosis I and II consists of the prophase,
metaphase, anaphase and telophase stages.
• During prophase I, homologous pair up and crossing
over occurs.
• Crossing over is the process during which
chromatids of homologous chromosomes cross
each other and exchange parts. The points where
homologous chromosomes cross over are called
chiasmata.
• Anaphase I separates homologous choromosomes.
• Telophase I produces 2 daughter cells that have
haploid number of chromosomes.
29. Stages of Meiosis
• Meiosis consists of meiosis I and II.
• Meiosis I and II consists of the prophase,
metaphase, anaphase and telophase
stages.
30.
31. What is ‘Homologous Chromosomes’?
• In diploid organism (2n),
one chromosome in the
pair comes from the male
parent and the other
from the female parent
during Meiosis I.
33. Importance of Meiosis
• Meiosis results in variations in the gametes
produced.
• Variations occurs during
(a) Crossing over at Prophase I;
(b) Independent assortment of
chromosomes at Metaphase I and II
(c) Random fertilisation of gametes
34. (a) Crossing over:
• During prophase I, homologous
chromosomes pair up and
crossing over occurs.
• Crossing over is the process
during which chromatids of
homologous chromosomes cross
each other and exchange parts.
35. (a) Independent Assortment
• During metaphase I (pairs of
homologous chromosomes)
and metaphase II
(chromosomes)
arrange themselves in
the
equator of the cell.
36. Comparison of Mitosis and Meiosis
Mitosis MeiosisParent cell
(before chromosome replication)
Chromosome
replication
Chromosome
replication
2n = 4
Prophase
Duplicated
Chromosome
(2 sister chromatids)
Metaphase
Anaphase
Telophase
2n 2n
Daughter cells of mitosis
Chromosomes align
at the metaphase plate
Sister chromatids
separate during
anaphase
Prophase I
Metaphase I
Anaphase I
Telophase I
Meiosis I
Meiosis II
Tetrads align
at the metaphase plate
Daughter cells of meiosis II
n n n n
Sister
chromatids
Remain together
Homologous
chromosomes
separate
Sister
chromatids
separate
during
anaphase II
Tetrads formed by
synapsis of homologous
chromosomes
5112109
37. Cell division Mitosis Meiosis
No. of chromosomes
produced in daughter
cells
Diploid (2n) number
(same as parent cells)
Haploid (n) number (half the
number of choromosomes
as parent cells)
Function Growth and repair Formation of gametes
Variation No genetic variation Recombination of
chromosomes gives rise
to variation.
Location All plant and animal
cells
In sexual reproductive cells
Editor's Notes
Introduction. Will automatically lead into the next slide.
A bit of background knowledge on chromosomes in general.
Genes are instructions for all activities of the cells and determine what they will be.
Each chromosome contains approximately 4000 genes.
The total of DNA in a human cell can consist of 60 000 genes. This is known as the genome.
Notes:
This diagram shows a parent cell containing only 4 chromosomes. In human cells, there are 46 chromosomes.
Before the genetic material can be distributed equally to both daughter nuclei, DNA needs to be replicated (i.e. another exact copy has to be made). Thus the replicated chromosomes appear as X-shaped structures where the sister chromatids are genetically identical.
Mitosis ensures that daughter cells have the same number and type of chromosomes as the parent cell.
A mnemonic to help remember the stages of mitosis.
A brief description of Interphase on the whole. The following pages go into greater depth about Interphase.
The three stages follow on from each other and are in order-if these would like to be viewed then continue with the forward button. Alternatively an animated diagram can be viewed of the cell cycle, by clicking on the “cell cycle” button.
Interphase is the time lapse stage between the end of telophase and the beginning of prophase. The cell is recovering its body mass and preparing for division once again, at the same time. A cell at interphase can be seen in the photograph.
This could be mistaken for a cell at early prophase as there is only slight differences between the two, however the large nucleus is an indication of its stage.
A brief description of Interphase on the whole. The following pages go into greater depth about Interphase.
The three stages follow on from each other and are in order-if these would like to be viewed then continue with the forward button. Alternatively an animated diagram can be viewed of the cell cycle, by clicking on the “cell cycle” button.
Interphase is the time lapse stage between the end of telophase and the beginning of prophase. The cell is recovering its body mass and preparing for division once again, at the same time. A cell at interphase can be seen in the photograph.
This could be mistaken for a cell at early prophase as there is only slight differences between the two, however the large nucleus is an indication of its stage.
Note:
During replication, a new identical DNA strand is formed using the existing DNA on the chromosome as a template. The two identical DNA strands are attached to each other by a centromere, each strand known as a sister chromatid.
The photographs are of a cells at their resting states. The diagram is indicating the chromosomes condensing during prophase.
Prophase is the first stage of mitosis, it prepares the cell for the next stage- Metaphase, by condensing the unravelled DNA into a helix which can be split into its two halves.
The chromatin begins to condense to form chromosomes. Each chromosome has already replicated itself, fprming two sister chromatids which contain exactly the same genetic information.
This could, however not be true if a mutation has occurred. Other events occuring in the cell are; the nucleolus begins to break down so the genetic information can be equally distributed between the two daughter cells.
The nucleolus completely disappears because the genetic information is dispersed as condensing chromosomes within the cytoplasm. The contrioles (‘Points’ of the spindle) move to opposite ends of the nucleus as the spindle begins to form.
The nuclear membrane begins to break down so there is no longer a nucleus as such in the parent cell.
The information, ‘I’, will take you to a slide about the spindle and what it is made of.
The dark area within the photographed cells are the developed spindles and is simplified with the diagram below it.
Metaphase literally means ‘middle stage’, it is the moment before the chromosomes reach opposite ends of the cell at anaphase. The spindle can be seen in the green picture, and the chromosomes arranged upon it.
The easy way to identify the start of metaphase is the disappearance of the nuclear membrane, this breaks up into individual vesicles, it then joins up onto the endoplasmic reticulum (so is not just lost within the cytoplasm).
The spindle is now fully developed and takes the place where the nucleus was.
The most obvious stage of metaphase;- the chromatid pairs align themselves along the centre of the spindle after being attached to the spindle fibres themselves.
The photograph shows the cell at the proceeding stage of mitosis-anaphase and is highlighted to show the direction in which the spindle is moving.
The aligned chromatid pairs are pulled apart from the centre of the spindle due to movement of the microtubule spindle fibres. They are pulled to opposite poles of the cell.
This is the most recognisable event during mitosis, it can be seen clearly in the films featured in the ‘movies section’ which can be accessed through the contents page.
The photographed cell now shows telophase. Two cells “stuck” next to each other. The forward button will now take you to a slide about Cytokinesis, or the home button will take you back to the contents page.
The chromosomes have reached opposite ends of the spindle, they unravel and disperse into the cytoplasm so cannot be seen.
The nuclear membrane reforms.
The nucleolus reappears- much of the events that happen during telophase are the opposite of the events that happen during prophase.
Cytokinesis begins around anaphase and is the splitting of the cytoplasm, mitosis is mainly the replication of the DNA.
Click the return button to return to the mitosis contents page, or the house button to return to the main menu.
