1. Sec 3 Biology
Chapter 20

2. Learning outcomes 1 - 4
Outline the relationship between DNA, genes and
chromosomes
State that DNA is made up of nucleotides
State that nucleotides consist of bases, sugars and
phosphate groups
State the rule of complimentary base pairing

3. Who discovered the structure of DNA?Watson and Crick
were awarded the
Nobel Prize in 1962
for discovering the
double helix
structure of DNA,
but work was
started long before
by others like
Rosalind Franklin.

4. 20.1 What is DNA?
A molecule carrying genetic
information
A small segment of DNA makes a gene
consists of two parallel strands twisted
together to form a double helix
A DNA molecule is wrapped around
proteins to form a chromatin thread.
During cell division, chromatin threads
coil tightly into structures called
chromosomes inside the cell nucleus.
proteins called
histones

5. 5
Basic Unit of DNA: nucleotide
P O
O
O
CH2
H
OH
H H
H
H
OH
HO
phosphate
base
sugar
C
N CH
NC
C
N
HC
N
NH2

6. one of these 4 bases + 1 sugar + 1 phosphate =
1 nucleotide
Basic units of DNA

7. Nucleotides can join together to form long chains called polynucleotides
Each gene is made up of a sequence of nucleotides and the varying
sequence results in different genes. For a gene made up of n
nucleotides, there are 4n
different combinations of nucleotides (why?)

8. 8
The Watson-Crick Model of DNA Structure
Double Helix

9. DNA is a double helix, so two strands of polynucleotides must come together. The
rule of base pairing states that adenine will bind to thymine (A-T) while guanine
will bind to cytosine (G-C).
A and T are known as complementary bases, and so are G and C.

10. 10
Nitrogenous Base Pairing in DNA
N H O CH3
N
N
O
N
N
N
N H
Sugar
Sugar
Adenine (A) Thymine (T)
N
N
N
N
Sugar
O H N
H
NH
N OH
H
N
Sugar
Guanine (G) Cytosine (C)
H

11. 11
Purine:
-Adenine
-Guanine
Pyrimidine:
-Thymine
-Cytosine
Nitrogenous Base Pairing in DNA

12. 12
Nitrogenous Base Pairing in DNA

13. Learning outcomes 5 - 7
State that DNA is used to carry the genetic code,
which is used to synthesize polypeptides
State that each gene is a sequence of nucleotides on a
DNA molecule
State that each gene controls the production of one
specific polypeptide

14. 20.2 GenesA gene is a small segment of DNA which controls the formation of a
single protein.
Each gene stores a message that determines how a protein should be
made in the cell.
Each protein then determines a certain characteristic in your body.

15. Structure of a geneOnly one of the polynucleotide chains determines the protein to be
made. This is known as the template.
Every three nucleotides in the template code for one amino
acid. This is known as the triplet code or codon.
Stop and recall: Many amino acids make up a polypeptide, and one
or more polypeptides form a protein.
A gene carries the message for making a polypeptide. If a protein
consists of many polypeptides, many genes will contribute to the
making of this protein.
Synthesis of proteins can be broken down into two steps –
transcription and translation

16. The Central Dogma of Molecular Genetics
 The information in
genes flows from DNA
to RNA to polypeptides
 DNA RNA→ →
polypeptide
16
The DNA on your 23 pairs of chromosomes is the
blueprint for life, while the over 10,000 different
proteins in your cells determine traits and
functions.

17. Gene expression
 Transcription
 The synthesis of RNA (mRNA) under the direction of
DNA (template strand)
 Translation
 The process through which nucleotide sequence on
mRNA is translated into amino acid sequence of a
polypeptide.
17

18. Transcription and Translation
1. The message in the
DNA template has
to be copied into a
mRNA molecule
first.
2. The mRNA carries
the message to the
cytoplasm, where a
ribosome
translates the
message into a
protein.

19. DNA vs RNA
DNA
Deoxyribonucleic acid
RNA
Ribonucleic acid
Bases: A T G C Bases: A U(uracil) G C
Large and insoluble Small and soluble
Permanent molecule in
the nucleus
Temporary molecule
made only when needed
Sugar unit is deoxyribose Sugar unit is ribose

20. First, the gene unzips.
1 part of a gene
Transcription and Translation

21. template
mRNA molecule is
made
One of the strands in the gene is
used as the template to make
mRNA. This is transcription. The
mRNA molecule copies the genetic
code in the DNA template,
following the rule of base pairing.
1
Note that mRNA does not
contain T (thymine). A
(adenine) in DNA pairs with U
(uracil) in mRNA.
Transcription and Translation

22. mRNA molecule is
made
ribosome
mRNA
nuclear
envelope
The mRNA leaves the
nucleus and attaches to a
ribosome in the cytoplasm.
2
nuclear pore
Transcription and Translation

23. The Genetic code
 43
= 64 codons
 20 amino acids
 Redundancy, but no
ambiguity
23

25. Fill in the blanks!
During protein synthesis, one strand of DNA is used
as a _________ for the synthesis of ______. This
process is called __________. The mRNA moves from
the ________ to the _________, where it attaches itself
to a _________ for the process of __________. During
this process, the ________ moves along the mRNA. In
this way, __________ are linked to form a polypeptide.

26. Fill in the blanks!
During protein synthesis, one strand of DNA is used
as a _________ for the synthesis of ______. This
process is called __________. The mRNA moves from
the ________ to the _________, where it attaches itself
to a _________ for the process of __________. During
this process, the ________ moves along the mRNA. In
this way, __________ are linked to form a polypeptide.

27. Control of genes
Each cell in the body contains a complete set of
genes, but many of them are switched off / not
expressed, so they do not produce the corresponding
protein.
E.g., the genes to make insulin are found in both the
liver and islets of Langerhans cells in the pancreas,
but liver cells do not produce insulin, so the insulin
making genes in the liver cells are not expressed. The
islets of Langerhans cells however can control when
to produce insulin, so they can control when to switch
on or off the insulin making gene.
In short, different cells express different genes.

28. When things go wrong - mutations

29. Genetic Basis of Sickle-Cell Disease
Glu Val

30. Learning outcomes 8 – 10
Explain that genes may be transferred between cells
Briefly explain how a gene that controls human
insulin production can be inserted into bacterial DNA
to produce human insulin
Outline process of large scale production of insulin
using fermenters

31. 20.3 Transferring genes between organismsGenetic engineering – a
technique used to transfer genes
from one organism to another.
A vector is a DNA molecule that
is used to carry the genes of one
organism into the other.
Plasmids (circular DNA from
bacteria) can be used to transfer
genes  a plasmid is an example of
a vector

32. 20.3 a) Inserting the human insulin gene
into a bacteria
Insulin injections are needed to treat diabetics
who cannot control their blood glucose level.
Insulin used to be harvested from the pancreas of
animals but prolonged treatment caused the
patients to develop antibodies against the animal
insulin and there was also the fear of transmitting
disease from animal to human.
Inserting the human insulin gene into a
bacteria will result in the bacteria expressing
the human gene and insulin can be mass
produced and harvested.

33. 33
insulin gene
• Obtain the human chromosome
containing the insulin gene.
• Cut the gene using a restriction
enzyme. This enzyme cuts the two
ends of the gene to produce
‘sticky ends’.
• Each ‘sticky end’ is a single
strand sequence of DNA bases.
These bases can pair with
complementary bases to form a
double strand.
1
cut by restriction
enzyme
fragment of DNA containing
the insulin gene
sticky end
How the human insulin
gene is inserted into
bacterial DNA
Genetic Engineering

34. 34
insulin gene
• Obtain a plasmid from a
bacterium.
• Cut the plasmid with the
same restriction enzyme.
This produces
complementary sticky
ends.
2
cut by restriction
enzyme
fragment of DNA containing
the insulin gene
sticky end
plasmid
cut by same
restriction enzyme
sticky ends
How the human insulin
gene is inserted into
bacterial DNA
Genetic Engineering

35. 35
insulin gene
• Mix the plasmid with the
DNA fragment containing
the insulin gene.
• Add the enzyme DNA
ligase to join the insulin
gene to the plasmid.
3
cut by restriction
enzyme
fragment of DNA containing
the insulin gene
sticky end
plasmid
cut by same
restriction enzyme
sticky ends
insulin gene
inserted into
plasmid
How the human insulin
gene is inserted into
bacterial DNA
DNA
ligase
Genetic Engineering

36. 36
Genetic Engineering
insulin gene
• Mix the plasmid with E.
coli bacteria.
• Apply temporary heat or
electric shock. This opens
up pores in the cell surface
membrane of each
bacterium for the plasmid
to enter.
4
cut by restriction
enzyme
fragment of DNA containing
the insulin gene
sticky end
plasmid
cut by same
restriction enzyme
sticky ends
insulin gene
inserted into
plasmid
plasmid
bacterial
DNA
plasmid enters
the bacterium
trangenic bacterium
E. coli
bacterial DNA
How the human insulin
gene is inserted into
bacterial DNA
DNA
ligase
The organism that receives a new gene is known as a transgenic organism.

37. Large-scale fermenters
As the transgenic
bacteria multiples, it
will use the new gene
to produce insulin.
These bacteria can be
isolated and grown in
fermenters for mass
production of
insulin.
*Insulin production by
transgenic bacteria is
not a fermentation
process!

38. A fermenter is designed to keep the internal
environment favourable (optimum pH, acidity,
oxygen, temperature, nutrition) for the biological
process occurring inside.
Do you think the production of insulin using
transgenic bacteria requires an aerobic or anaerobic
fermenter?

39. Characteristics of a fermenter
1. Cooling system – Heat from bacteria growth is removed by
pumping water in through the base of a cooling jacket (recall the
Liebig condenser)
2. Aeration system – Adequate aeration promotes growth and two
devices can help. Sterile air is forced through the tiny holes of a
sparger and the numerous bubbles dissolve in the nutrient broth.
An impeller spreads the oxygen and nutrients out evenly and also
ensures that bacteria does not clump together.
3. pH controller – A pH probe measures the pH of the broth and
makes the adjustments accordingly.
4. Nutrients – The nutrient broth should contain a carbon and
energy source e.g. glucose, a nitrogen source e.g. amino acids or
nitrates and essential mineral salts.
The bacteria harvested from the broth are then burst open to
release the insulin, which has to undergo purification before it can
be administered,

40. 20.3 b) Transferring foreign genes into
plantsGenetic modification of plants generally aims to provide
transgenic plants with
increased resistance to pests and pathogen
increased heat and drought tolerance
increased salt tolerance
a better balance of proteins, carbohydrates, lipids,
vitamins and minerals, resulting in more nutritious
crops
e.g. A weak solution of cyanamide kills weeds but damages
tobacco plants too. A soil fungus, Myrothecium verrucaria,
has a gene which produces cyanamide hydratase, an
enzyme which converts cyanamide to urea, which is
harmless to tobacco plants. When this gene is inserted
into the tobacco plant, the plant becomes resistant to the
herbicide and the urea is also a nitrogen source for the
plant.

41. Just for your info

43. 20.3 c) Transferring genes within the same
species
Incorporating resistant genes from wild species
into crop plants e.g. incorporating genes from
wild species wheat into common wheat confers
resistance to the Hessian fly, a major wheat pest.
Genes can also be transferred between people.
Cystic fibrosis (bronchial tubes produce mucus)
may be treated by replacing defective genes in the
damaged airway cells with healthy genes – gene
therapy

44. Selective breeding vs Genetic engineering
Factors to
consider
Selective breeding Genetic engineering
Species of
organisms
Only between closely related
species
Genes can be transferred
across non related / different
species
Which
genes are
transferred?
Defective genes may be
inherited along with healthy
genes
Only the desired (beneficial)
genes are transferred, so there
is less chance of genetic defects
Speed?
Efficiency?
Slow process involving
breeding over generations,
and may require large
amounts of land.
Less efficient – organisms
grow slowly and may require
more food
Targets individual cells which
reproduce quickly in lab
conditions
More efficient – e.g. transgenic
salmon grows to harvesting size
much faster than ordinary
salmon

45. Learning outcome 10
Discuss the social and ethical implications of genetic
engineering using a named example

46. 20.4 Genetic Engineering and Medical Biotechnology
While genetic
engineering may seem
highly beneficial,
there are many social
and ethical issues
involved. Let’s look at
the environmental,
economic, health,
social and ethical
hazards…

47. 20.4 Genetic Engineering and Medical Biotechnology

48. The ‘golden rice’ has had three genes
added to its normal DNA content.
Two come from daffodils and one
from a bacterium. Together, these
genes allow the rice to make beta-
carotene, the chemical that makes
carrots orange. More importantly, the
beta-carotene is converted to vitamin
A, which is essential for good eyesight,
and could save children in very poor
countries’ from going blind.
Bt (Bacillus thurigensis), is a bacterium that
produces a toxin that can kill larvae. The Bt
toxin has been sprayed onto plants so that
larvae on the leaves get killed when the toxin
digests their gut. In the 1980s. The gene for
Bt toxin that killed the European corn borer
was isolated and introduced into corn. The
corn expressed the toxin and was able to kill
the corn borer. Moreover, the toxin is
harmless to humans, fish, wildlife and most
insects. => Genetically engineered plants
can be environmentally friendly by reducing
pesticide use.

49. While genetic engineering can improve the quality of
our lives, it can also potentially
Disrupt the environment
Affect the economics of society
Harm human health
Affect the way an individual is looked upon in society

50. Issues of genetic engineering –
1. Environmental hazards
Crop plants have been genetically engineered to
produce insect toxins or be resistant to herbicides,
resulting in
Loss of biodiversity from insect deaths (long term)
Insects that feed on GM crops may adapt and develop
resistance to the toxins.
Herbicide resistant plants and weeds could cross-breed
to create superweeds. Although sterile male plants
could be created to prevent this, more problems are
inadvertly created.

51. 2. Economic hazards
The company that first engineered the GM seed can
patent their seeds to prevent others from planting such
seeds without their permission. Other biotechnology
companies also cannot produce such seeds – competition
from farmers and biotech companies is eliminated.
Some companies produce plants that produce non-
germinating seeds. This terminator technology means that
farmers have to spend money each year to buy new plants.

52. 3. Health Hazards
Genetic engineering could introduce allergens in food.
( Allergens cause a reaction from your immune system.)
E.g. Lectin found in beans is an effective pest control
against aphids, but lectin can be transferred to potatoes,
and people allergic to lectin may unknowingly eat those
GM potatoes.
Modifying a single gene in plants could alter metabolic
processes within the plant and result in the production of
toxins.
Genes that code for antibiotic resistance may be
accidentally incorporated into bacteria, making antibiotics
ineffective in treating these diseases.
People may deliberately create new combinations of genes
to use in chemical or biological warfare.

53. 4. Social and ethical hazards
In gene therapy, a gene inserted in the body cells may find
its way into the gametes. Should there be a mutation, the
offspring may be affected.
Genetic engineering is expensive. Only the rich can afford
it.
Some religions are against genetic engineering as
scientists are altering the natural genetic make-up of the
organism.