Other Ways of Manipulating Genes

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Other Ways of Manipulating Genes

Viruses and some bacteria are known to transfer some of their DNA or RNA into the cells of a host and this genetic material integrates into the host genome, causing the production of disease or deformity such as Galls.

Galls are large tumour-like growths in plants, usually produced as a response by the plant to some invasion by microbe or insects.

The Ti plasmid of a particular bacterium called Agrobacterium tumefaciens has been used to insert DNA into plant genomes.

Genes coding for the tolerance to herbicides, resistance to insect pests, viral disease and other factors have been introduced to various plant species.

Cells can be taken from the meristem of a plant of the species in question and kept in a sterile nutrient medium so that the cells divide and form a tissue. This can be split several times and each group of cells can go on to form an individual plant (meristem tissue culture or micropropagation).

Genetic engineering in agriculture

If the cells are infected with the Ti plasmid at an early stage, all resulting cells will carry the gene for resistance to an herbicide for example.

The results of these transformations are called transgenic plants.

Genetic engineering in animals

This is relatively rare and more difficult to achieve than in plants. The desired gene is inserted into a fertilised egg so that all body cells will carry a copy of the gene.

Gene therapy

Treating a genetic disease by altering an individuals natural genotype by:

  1. Germ cell therapy of sperm, egg or early embryo.
  2. Somatic cell therapy.

Number 1 is technically possible but ethically unacceptable.

There are 3 ways in which gene therapy can work:

  1. Repair of the defective gene.
  2. Replacement of the faulty gene with a normal one.
  3. Addition of a normal gene, leaving the defective one in position.

Only number 3 is feasible after a zygote has started to develop and can only work when the disorder is caused by recessive alleles.

Example:

Cystic fibrosis

Cause: the inheritance of two recessive alleles for cystic fibrosis.

Symptoms: the transport of chloride ions and water by the cells in the airways, lungs and gut is disrupted. This causes thick mucus to line the lungs and ducts in the gut. Because the thick mucus is not shifted easily, it is more likely that a sufferer will pick up a bacterial infection

Gene therapy treatment: copies of the DNA for the normal allele were inserted into other loops of DNA, which were then attached to liposomes. The liposome complexes were then sprayed as an aerosol of fine droplets into the patients' noses. The DNA is then taken up by some of the cells lining the airways.

Fortunately only about 10% of these cells need to take up the DNA for symptoms to be relieved.

Much of your DNA is non - coding (i.e. it doesn't code for a protein), but rather it contains regions of highly repetitive sequences of bases (Variable Number Tandem Repeats - VNTRs)

The number of repeats, and hence the size of the VNTRs, varies markedly between individuals. Some will come from one parent; the rest will come from the other.

Only identical twins have the same number of VNTRs. Genetic fingerprinting reveals the differences in the size of the VNTRs in different individuals.

Genetic fingerprinting
  • DNA is obtained from the white blood cells, mixed with restriction endonuclease, which 'cuts' the DNA into millions of fragments, but not the repeated regions, which retain their original length.
  • The DNA fragments are then loaded into a slot at the end of an agarose gel.
  • An electric current is passed across the gel, with the positive electrode at the furthest end from the fragments.
  • Since DNA molecules have a negative charge, they will migrate through the gel towards the positive electrode. Larger fragments move through more slowly that the smaller ones. The result is a series of bands down the gel, but these are invisible at this stage.
  • Since the gel is difficult to keep, the DNA bands are then transferred to a nylon membrane, which is incubated overnight with radioactive probes.
  • The radioactive probes bind with the DNA that has repeating regions.
  • A sheet of X-Ray photographic film is laid over the membrane in total darkness. The radiation will affect the film, which is later developed, as a photograph.
  • The developed film reveals the series of DNA bands, which are unique to an individual.

Uses:

  • Identifying particular plants or animals with particular alleles of a gene for selective breeding.
  • Identifying a particular strain of microbe so that correct treatment can be given.
  • Establishing paternity.
  • Confirming animal pedigrees.
  • Establishing genetic diversity for gene banking.

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