IGCSE Biology
Section 5: Use of Biological Resources
c. Genetic Modification (Genetic engineering)
1. Structure of DNA
Describe a DNA molecule as two strands coiled to form a double helix, the strands being
linked by a series of paired bases: adenine (A) with thymine (T) and cytosine (C) with
guanine (G).
The Structure of DNA:
 The DNA molecule is in the shape of a double helix (liken to a twisted ladder).
 It is made up of two backbones (the sides of the ladder) and joined together by base
pairs (the rungs of the ladder).
 The backbone consists of alternating sugar and phosphates.
 The rungs consist of complementary base pairs.
 The bases always pair adenine with thymine and guanine with cytosine.
 A single nucleotide consists of a phosphate, sugar and one base.
2. Restriction Enzymes and Ligase
Describe the use of restriction enzymes to cut DNA at specific sites and ligase enzymes to
join pieces of DNA together.
Restriction Enzymes
These are enzymes that cut DNA at specific sites. Some restriction enzymes make a
staggered cut in the two strands, forming ‘sticky ends’.
The cut ends are “sticky” because they have short stretches of single-stranded DNA with
complementary sequences. These sticky ends will stick to another piece of DNA by
complementary base pairing, but only if they have both been cut with the same restriction
enzyme.
DNA Ligase
This enzyme repairs broken DNA by joining two nucleotides in a
DNA strand. It is commonly used in genetic engineering to do the
reverse of a restriction enzyme, i.e. to join together
complementary restriction fragments.
The sticky ends allow two complementary restriction fragments
to stick, but only by weak hydrogen bonds, which can quite easily
be broken. The backbone is still incomplete.
DNA ligase completes the DNA backbone by forming strong bonds
between the sugar and phosphates. Restriction enzymes and DNA
ligase can therefore be used together to join lengths of DNA
from different sources.
3. Vectors
Describe how plasmids and viruses can acts as vectors, which take up pieces of DNA, then
insert this recombinant DNA into other cells.
In biology a vector is something that carries things between species. For example the
mosquito is a disease vector because it carries the malaria parasite into humans. In genetic
engineering a vector is a length of DNA that carries the gene we want into a host cell. A
vector is needed because a length of DNA containing a gene on its own won’t actually do
anything inside a host cell. Since it is not part of the cell’s normal genome it won’t be
replicated when the cell divides, it won’t be expressed, and in fact it will probably be broken
down pretty quickly.
Plasmids
Plasmids are by far the most common kind of vector, so we shall look at how they are used in
some detail. Plasmids are short circular bits of DNA found naturally in bacterial cells.
Foreign genes can quite easily be incorporated into them using restriction enzymes and DNA
ligase.
Viruses
Viruses always infect cells. They do this by injecting their DNA into the host cell. If we
want to insert a piece of DNA into a host cell we can use a virus to do this. DNA introduced
into a virus will be injected into a host cell when the virus infects a cell.
Plasmids and viruses are both used to carry the gene we want into the host cell. E.g. we can
insert the gene for insulin into a plasmid or a viruses and it will carry it to the host cell – a
bacteria.
4. Insulin Production
Understand that large amounts of human insulin can be manufactured from genetically
modified bacteria that are grown in a fermenter.
Genetically modified bacteria has already been used to produce large quantities of human
insulin using bacteria. It has been a great help to sufferers of diabetes.
It has been a great help to sufferers of diabetes. This process avoids the older method of
extracting insulin from dead bodies or pigs. That wasn't a very attractive idea if you had to
inject yourself with insulin every day!
In genetic engineering the gene that you want is cut out of a human chromosome using
special enzymes (restriction enzymes).
The gene is then fitted ('spliced') into a length of DNA (usually a plasmid) using ligase
enzymes. The plasmid is returned to the bacterial cell.
The bacteria is tricked into carrying out the instructions on the human gene and producing
the protein, insulin.
Once the bacteria has been cultivated so that it multiplies many times, enough insulin is
produced so that it can be filtered off and collected.
This whole process is carried out in an industrial fermenter (covered in “Food ProductionMicro-organisms) so that masses of insulin is produced in a continuous process.
5. GMOs and Food Production
Evaluate the potential for using genetically modified plants to improve food production
(illustrated by plants with improved resistance to disease).
A comparison of the advantage and disadvantages of genetically modified plants.
Advantages:
 Higher yields of crop to feed a growing population.
 Engineered to grow in extreme conditions e.g. resistance to drought may help crop
production in many starving African countries.
 More predictable than selective breeding.
 Engineered to contain more vitamins or carry vaccines for developing countries.
 Resistance to pests – a gene is inserted to make a pesticide which will kill pests which
try to eat the plant, therefore increasing production.
 Resistance to herbicides – a field of crops can be sprayed to kill weeds but the
herbicide will not damage the crop plant.
 Resistance to wind damage – engineering plant to have stronger stems so crops are not
lost in adverse weather conditions.
 Tomatoes have been engineered with a gene which prolongs their shelf life; the gene
inhibits the enzymes which cause rotting.
Disadvantages:
 Pesticide and herbicide resistance may cross-pollinate with other plants and so the gene
is passed on possibly creating a new breed of ‘super weeds’.
 New breeds may be patented – this means the company who developed the technology
owns the rights to the product. This has serious implications when large biotechnology
companies by a seed from developing countries – alter it to help growth and then sell it
back at a higher price – the developing countries do not have the money to invest in
their own product.
6. Transgenic
Recall that the term transgenic means the transfer of genetic material from one species to
a different species
Simply learn the following definitions:
Transgenic = transfer of genetic material from one species to a different species.
Transgenic organism = contains genetic material from a different species.
Questions
Genetic engineering is a much more controlled and predictable process than that of
selective breeding.
1.
What is selective breeding?
(2)
The difference with genetic engineering is that we are actually picking the gene we want to
be past on and transferring it into an organism.
General Method
Genes that code for a characteristic valuable
to humans are identified.
The gene is removes from the organism that
normally shows the characteristic.
The gene is transferred to another organism,
usually one that grows very quickly.
The organism ‘reads’ the gene and will make
the specific protein the gene codes for.
Specific Example
The gene for human insulin.
A healthy gene for making insulin is removed
from a human cell.
The gene for insulin is placed inside a
bacterium.
The bacteria will now be able to make human
insulin
2. What is the process of moving a gene from one organism to another referred to?
(1)
3. Explain the importance of each of the following in genetic engineering.
a. Restriction enzymes.
(2)
b. Vectors.
(2)
c.
(2)
Culture Techniques
4. List three of the advantages of recombinant DNA technology.
(3)
5. Name two useful products of recombinant DNA technology.
(2)
6. Rearrange the following steps in the correct order.
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The restriction enzyme cuts the human DNA and plasmid with complementary sticky
ends.
The recombinant plasmid is introduced into a bacterial cell. Under the right
conditions some of the bacteria will take up the plasmid.
A gene of interest (e.g. human insulin) is isolated from a cell.
The human gene and the plasmid are mixed together and the complementary sticky
ends are attracted to each other by base pairing.
An appropriate plasmid vector is isolated from a bacterial cell.
The bacteria now has the gene to make human insulin and so begins to make it.
Both the human DNA and the plasmid are treated with the same restriction enzyme.
7. The following table lists the events from the identification of a human gene coding for a
growth hormone to the commercial production of that growth hormone. Genes can be
transferred into plasmids, tiny circles of DNA which are found in bacteria. Show the
correct sequence of events 1 to 8 by completing the table below.
1
8
Cutting of a bacterial plasmid using restriction endonuclease.
Cutting of human DNA with restriction endonuclease.
Identification of the human DNA, which codes for the growth hormone.
Many identical plasmids, complete with human gene, are produced inside the
bacterium.
Mixing together human gene and ‘cut’ plasmids to splice the human gene into the
plasmid.
Some of the cloned bacteria are put into an industrial fermenter where they breed
and secrete the growth hormone.
The bacterium is cloned.
Using the plasmid as a vector, inserting it, complete with human gene, into a
bacterium.
8. People produce small amounts of an enzyme that digests blood clots. People who have
suffered heart attacks can be treated with injections of large amounts of this enzyme.
The diagram shows stages in the production of large amounts of the enzyme using
hamster cells.
a.
What happens at stage A?
(1)
b. What happens at stage C?
(1)
c.
(1)
Name the circle of DNA used in stage D.
d. What is the role of the hamster cells in this process?
(2)
e.
Name two conditions needed by the hamster cells in the culture tank.
(2)
9. DNA is cut with using a restriction enzyme before transferring a gene from one
organism to another.
The table shows a sequence of bases found at the cut ends of DNA.
The cuts were made using two different restrictions enzymes.
a.
Tick two boxes in the table below to show which of the base sequences could join
together.
(2)
DNA Base Sequence
1+2
1+3
1+4
2+3
2+4
3+4
Tick Box
b. A piece of double stranded DNA was cut into fragments shown. Each end made by
using a restriction enzyme is known as a sticky end.
How many sticky ends are there on the DNA fragment above?
(1)
c.
The table below shows the stages in a gene transfer technique to enable bacteria to
produce human insulin. The stages are in the wrong order.
Write a number (1 – 7) in each empty box to show the correct order.
(6)
10. Soya bean plants have been genetically modified so that they are resistant to weed
killer.
a.
How can this help to produce a better yield of beans?
b. Suggest why this genetic modification is a potential threat to the environment.
(2)
(2)