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Genetic Engineering
The genetic code is universal and a certain codon codes for the same amino acid in all
organisms. This has enabled scientists to transfer genetic material between species.
Consequently, if a certain human gene is transferred to a bacterium, the bacterium will make
the human protein coded for that by gene. Every gene codes for one specific protein.
The technique used in transfer of genes between organisms is carried out as shown in the
following steps:
1.
A plasmid (small circular DNA) is removed from a bacterium.
2.
It is cut by a restriction enzyme, which cuts the DNA between
specific nitrogenous bases and at a very specific nucleotide sequence that it recognizes.
3.
A gene is cut from the DNA of another organism using the
same restriction enzyme.
4.
This gene is then added to the cut plasmid and spliced (stuck)
to it by the enzyme ligase (ligase sticks pieces of DNA together).
5.
The plasmid of the DNA is now genetically engineered –
it is a recombinant DNA.
6.
The recombinant plasmid can then be inserted into another bacterium (host). This host
accepts the inserted recombinant DNA.
Recombinant
DNA
Spliced
gene
Plasmid
7.
The bacterium with the recombinant DNA divides making a clone of cells, all containing
the recombinant plasmid.
8.
This recombinant plasmid contains a gene form another species. Eg, taken from a
human cell and coding for insulin. The recombinant plasmids of these bacteria now
contain the gene that codes for human insulin.
9.
Protein synthesis in these bacteria will produce insulin as coded by the inserted gene.
10.
A large population of genetically engineered bacteria produces large amounts of insulin.
11.
Insulin is then extracted from the culture and purified, and used in the treatment of
diabetes. Other proteins such as growth hormone and vaccines can be manufactured in
the same way by genetic engineering.
Genetic engineering is also widely used in agriculture to improve crops and animal breeds.
Advantages include:
 Increased resistance to disease
 Tolerance to heavy metals
 Bigger yields
The DNA model was first constructed by Watson and Crick in 1953. Since then, this has been a
fast growing area of science, including the discovery of restriction enzymes in 1970. Genetic
engineering of food began in 1994 and continues today. Several of these issues are highly
controversial, eg, cloning.
Polymerase Chain Reaction
DNA polymerase is the enzyme needed for DNA replication. Scientists are now able to use this
enzyme to produce large amounts of DNA from tiny quantities.
A solution containing DNA polymerase, the DNA double helix, some of the 4 types of
nucleotides, and primers is made. Primers begin the process of replication. Slight heating
breaks the hydrogen bonds between the 2 strands of DNA, and cooling allows free nucleotides
to build a new complementary DNA chain.
Uses of PCR include: forensic science, matching of donors in transplants, evolutionary studies,
detection and diagnosis of disease, and in sequencing the human genome.
Gel electrophoresis
This technique allows for the separation of a mixture of DNA strands according to size and
charge.
The DNA mixture is placed on gel between two electrodes. The DNA strands begin to move
and settle in bands according to their charge and length. When certain nucleotides are labelled
it is possible to identify the sequence of DNA fragments.
DNA profiling
The process of identifying the sequence of nucleotide bases in a DNA segment. This can be
achieved by gel electrophoresis.
DNA profiling can be used in criminal investigations, including murders, rape and paternity suits.
DNA can be isolated from blood, semen or any other tissue available. DNA profiling is then
carried out on these specimens and on the suspect. The results obtained are very reliable,
though can be affected by contamination with bacteria or other DNA sources.
Gene Therapy
This involves the replacement of a defective gene (one that causes a certain genetic disease)
with a normal gene that produces the normal condition. One example of this is the treatment of
cystic fibrosis, a genetic disease that causes the abnormal production of mucus in the body,
including the respiratory passages.
Another example is in the treatment of an immunodeficiency disease (SCID) that results from
the absence of the enzyme adenosine deaminase (ADA). White blood cells are removed from
the patient and mixed with a virus that contains the normal gene which codes for the production
of this enzyme. The virus enters the white blood cells, carrying with it the normal gene. These
genetically engineered white blood cells are returned to the patient. This treatment serves only
temporarily since the white blood cells do not multiply to produce more of themselves. To
overcome this, cells are taken from the bone marrow of the patient and injected with the normal
gene as already described. These genetically engineered cells are then returned to the bone
marrow, so that they can begin to produce white blood cells capable of making ADA enzyme.
Normal genes can be introduced into cells of children, adults, embryos or the cells in testes and
ovaries that divide to form sex cells. This allows the treatment of genetic diseases (such as
thalassaemia and sickle cell anaemia) that were once considered untreatable before genetic
engineering.
Harmful results of genetic engineering
For all its advantages, genetic engineering has some harmful side effects. These include:

Genetic screening in humans might raise ethical issues such as genetic discrimination
between people in job interviews, health insurance, etc.

Genetically engineered foods might contain some toxic or allergy causing proteins.

Genetic engineering in plants might produce weed varieties that are resistant to pesticides
and other pest control methods. These might compete with crops and other plants in the
environment causing an imbalance in ecosystems and extinction of certain species.
Genetic Screening
A process carried out to identify the genotype of an individual or identify genetic diseases.
Advantages include identification of genetic diseases, such as Down’s syndrome, in the baby
before birth. Methods used include:
1.
Karyotyping – The chromosomes of the baby are examined by amniocentesis to detect
any chromosomal abnormality. White blood cells of individuals of any age can be
karyotyped to identify chromosomal abnormalities. Chromosomes are stained using
specific chemicals to show up dark lines or bands across them. Individual chromosomes
can be recognised by their banding pattern.
2.
Pedigree charts – The pedigree chart of a family can be studied to detect any
individuals heterozygous for a certain genetic disease.
Human Genome Project
This is a project being carried out to sequence the complete human genome – the complete
nucleotide sequence of the DNA of all the genes of a human cell. This involves the
collaborative work of scientists from all over the world, since there are 23 pairs of chromosomes
and each contains huge numbers of genes. It is believed that this project will help in the
identification, treatment and prevention of genetic diseases.
Cloning
A clone is a genetically identical copy of a cell, tissue, organ or even a whole organism. Cloning
can be used to produce farm animals with certain favourable characteristics. The following
steps are carried out:
1.
2.
3.
An egg is fertilized by a sperm in vitro. The 8-cell stage embryo resulting from this
fertilization is divided into separate cells.
Each cell is grown into a new embryo before being transferred to surrogate mothers such
as cattle or sheep.
If this process is repeated several times, a line of offspring are produced that are all
genetically identical. They are clones of the original embryo.
This technique allows the production of cattle and other food animals with strong characteristics,
such as resistance to disease and the production of low fat milk and meat.
The cloned cell can be genetically engineered to contain DNA from another organism that
codes for a certain protein. The organisms containing this transferred DNA are called
transgenic organisms. Transgenic animals include dairy cattle, beef, sheep and fish.
Using this technique fish eggs were injected with DNA that codes for growth hormone. The
eggs were cloned and allowed to grow and produce fish three times the size of ordinary fish.
In humans cloning occurs naturally in the case of identical (monozygotic) twins. Artificially, an
embryo can be separated into cells that can all grow into babies that are all identical. This
obviously has ethical implications, and many people are worried about it leading to the selection
of the fittest and the production of a super race.
Crop and Animal Breeding
Selective breeding of plants and animals has enabled the production of varieties with favourable
characteristics. Cabbage, cauliflower, broccoli and brussel sprouts were all produced from
mustard – the wild variety of this species. Parents with the desired characteristics are allowed
to mate in order to produce a variety that combines strong characteristics from both parents.
Selective breeding has enabled the production of farm animals with high yields of milk, meat,
wool, proteins, etc, as well as high resistance to disease.