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Chapter 21
Genetic Engineering
Genetic Engineering (Recombinant DNA Technology) involves the
transfer of genes from one organism (e.g. man) to another (e.g.
bacterium). It provides man with unlimited opportunities to create new
combinations of genes from more than one species which would not
occur naturally.
Particular gene for a desired characteristice (e.g. gene for human insulin
production) is spliced into the DNA of a Vector (Plasmid from a
bacterial cell). This vector is injected into a host cell (e.g. bacterium Ecoli). When the reprogrammed host cell is propagated, the new gene
produces it's produt (e.g. insulin) which is identical to human insulin.
Bacteria cells are chosen as host cells because of their speed of
reproduction.
Genetic engineering requires 2 special Enzymes.
1) Endonuclease to cleave open the plasmid and to cut out required
DNA fragment so that the ends are complementary.
2) Ligase to seal the two together.
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Altering the Genome
The single haploid set of chromosomes typical of a species is called it's
Genome. Genetic engineering involves the transfer of one or more genes
from the genome of one organism (e.g. human) to the genome of another
organism (e.g. bacterium).
Location of Genes
In genetic engineering, it is essential to be able to locate the required
gene(s). This can be done by the following methods.
1) Recognition of Characteristic Banding Patterns on
Chromosomes.
Giant chromosomes are found in salivary gland cells of fruir fly larvae.
When stained, distict bands varying in thickness appear. This particular
pattern of banding is a Constant Characteristic of each type of
chromosome.
It is possible to relate the location of an individual gene to a particular
band on a chromosome.
E.g. Locating the gene for red/white eye colour in fruit fly.
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The gene for red/white eye colour in fruitflies is sex-linked. To find the
location of the gene, re eyed males are first irradicated to induce
mutations. They are then crossed with white-eyed females. Almost all
the females produced are found to be re-eyed as expected. However a
few are white-eyed because they have received an X chromosome which
has suffered a Deletion at the locus of the R allele.
If the X chromosomes from the salivary glands of larvae produced by
these white eyed females are examined, some of them are found to be
unusual. Instead of matching one another band for band, one x
chromosome is found to have a band missing. It is therefore concluded
that this is the location of the gene for red/white eye colour.
Other mutations also lead to gross
chromosomal abnormalities. For
example, inversion causes a section of
a chromosome to become reversed (see
page 84). Homologous chromosomes
are only able to pair up by forming a
loop as shown in figure 20.5. If this
structural abnormality can be
connected with unusual results in
breeding experiments, then it must be
the site of the affected gene(s).
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2) Gene Probes
A Gene Probe is a short length of a single-stranded DNA (or RNA)
containing an easily identified Chemical Label e.g. radioactive
phosphate. The probe is complementary to a single-stranded DNA
segment from the gene that the genetic engineers wish to locate.
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Advantages of Genetic Engineering
Genetic engineering has been successfully employed to produce Insulin,
Interferon and Human Growth Hormone. Since each of these products
is identical to the human type, it does not cause side effects when put to
use in the human body.
Production of New Plants by Soatic Fusion
Two different species cannot interbreed successfully, at best, they would
produce a sterile hybrid.
Scientists are attempting to overcome this problem by Somatic Fusion.
1) Unspecialised cells of 2 different plant species are selected.
2) Their cell walls are digested away using the enzyme Cellulase.
3) Protoplasts are left. They consist of the cell's living contents
(nucleus and cytoplasm) surrounded by the cell membrane.
4) Protoplasts are fused together to form a Hybrid Protoplast. This
process is called somatic Fusion.
5) The Hybrid Protoplast is induced to form a cell wall and to divide
into an undiffentiated cell mass (callus).
6) In the presence of hormones, calluses develop into hybrid plants
containing a mixture of the parents' genetic traits.
(See diagram overleaf).
Resistance to Potato Leaf Roll:
Complete:
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