GENE PROBES

Gene probes are short lengths of single-stranded DNA or RNA capable of binding to complementary single-stranded DNA segments which are used to pinpoint the regions on chromosomes where the defective genes are located using recombinant DNA technology. Gene probes have provided a specific test for single-gene disorders which can be used to treat these disorders in the embryo in womb and in young children. Gene probes are also used to detect the presence of DNA from pathogens, as in the diagnosis of AIDS and other viral conditions. In the future DNA probes may become available for the diagnosis of cancer. Cells become cancerous only after certain genes, called oncogenes, have been altered. Once altered, some cancerous cells produce a specific type of mRNA, not found in healthy cells. It should soon be possible to detect this mRNA by using a probe.

Procedure for the identification of gene fragments with gene probes:

a) The fragments are transferred to a well in a flat plate of agarose gel, contained in a box.

b) The fragments are separated by electrophoresis. The fragments move through the gel at different rates, depending on their length. Small fragments travel faster than larger ones.

c) After several hours, the fragments become arranged in parallel bands, each band containing many DNA fragments of uniform length.

d) The pattern is then plotted onto a nitrocellulose filter.

e) Heating binds the DNA to the filter and also separates each DNA fragment into its two single strands of DNA that contain complementary base sequences. Such binding is detected by placing an X-ray film over the nitrocellulose filter.

f) When the film is developed, a radioactive region shows up as a dark band.

Using this technique, the DNA sample of sickle cell carrier was studied. Sickle cell anaemia is caused by a mutation in a region of DNA that codes for the protein part of a haemoglobin. One of the bases in DNA molecule, thymine is replaced by adenine. That, in turn, leads to an amino acid sequence in which glutamate is replaced by valine. Due to this defect, the restriction endonulcease M st II cuts the DNA of people with normal haemoglobin (HbA) and sickle cell haemoglobin (HbS) into fragments of different lengths.

GENE THERAPY

A possible cure for single-gene disorders, such as sickle cell anaemia, lies in gene therapy.

Gene Enhancement therapy is an attempt to boost the effect of a gene that is generally thought to be desirable. Height, intelligence and athleticism are traits resulting from complex gene interactions. Once the individual genes that contribute to these attributes have been identified, attempts could be made to add additional copies to the genome. Somatic gene replacement therapy is an attempt to replace defective genes with normal genes.

The Therapy involves adding normal genes to the genome so that the recipient gains a normal gene, while still retaining the defective ones specially in single-gene disorders in which homozygous recessive (aa) individuals show symptoms of the disease.
For example to cure sickle cell anaemia, the process of therapy would involve insertion of a single copy of the normal gene [HbA] followed by multiplication of transformed cells in a suitable growth medium. At an appropriate time, the bone marrow cells of the donor are destroyed using radiation and drugs. These would be replaced by an injection of the donor’s own genetically engineered bone marrow cells. Further treatment would ensure that the engineered cells continued to grow and multiply. At a point where the transplanted cells produced enough haemoglobin [HbA] to meet the individual’s needs, a ‘cure’ would have been affected.

Two important aspects related to gene replacement therapy are:

The first is that treated individuals continue to pass on defective genes in their sperm and eggs.
The second point is that the treated individuals may carry more number of genes controlling a particular trait and any harmful effects of this combination may take place several years before they begin to show up.

In future, gene therapy could be used for finding a permanent cure for single gene disorder by inserting normal genes in to sperm and eggs. This is Germ-line gene replacement therapy (eugenic gene therapy). This would enable a cure to be passed to the offspring of the treated individuals.

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