The genes coding for the various chorion proteins are clustered within the genome, each gene present in single-copy number in germ-line cells. Amplification result in a 15-to 50 fold increase in the number of chorion gene copies per haploid genome. Chorion gene amplification appears to enable the genes to produces the amounts of mRNA necessary to build the egg shell. Although the mechanism of chorion gene amplification has not been completely elucidated, the data suggest that each chorion gene cluster contains a specific origin of DNA replication.

Multiple rounds of replication beginning t this origin give rise to a multiforked structure in which one strand branches into two, each of these branch again and again, and the branched structures contain copies of the chorion gene. Another single-copy gene which appears to be amplified during development is one of the genes coding for the muscle protein actin during development of muscle cell (myogenesis) in the chick. The actin genes constitute a small family of genes which are present in single or very low copy numbers in the germ line. During myogenesis (formation of skeletal muscle cells)the actin genes coding for skeletal muscle cell actin amplified approximately 85-fold.

Of course, not all single-copy genes which are responsible  for the synthesis of a large amount of cell product at a particular stage of development are amplified The silk gland of the larval silk moth, Bombyx mori, for instance, produces copious amounts of silk fibroin protein. Analysis of DNA from silk glands indicates that the silk fibroin protein. Analysis of DNA from silk glands indicates that the silk fibroin genes are not amplified above the level of ploidy expressed by the silk glands. Similarly, amplification of globin genes is not a factor in the ability of the reticulocyte (precursor cell of the red blood cell) to synthesize large amounts of the protein hemoglobin. Other examples

In the above-mentioned examples of gene amplification the amplification phenomenon appears to e developmentally regulated, and the amplified copies of the gene are subsequently lost from the cell. Studies on cells in culture have demonstrated "amplification" of genes involved in resistance to specific drugs. Resistance to the chemotherapeuti drug methotrexate by cultured mouse cells is associated with amplification of the dihydrofolate reductase gene. In this case the amplified are passed to daughter cells at the time cell division. Such gene amplification can be either retained by cells or lost from cells when they are grown in the absence of methotrexate. Polymerase chain reaction

The polymerase chain reaction (PCR) is a technique amplifies DNA sequences in laboratory cultures. Beginning with a ample that may contain only one DNA molecule, a selected sequence within that DNA molecule can be amplified millions or even billions of times.To amplify a given segment of DNA, its entire sequence need not be known, but only short sequence near the ends of the segment. Short oligonucleotides, complementary to the end sequences and designed so that their 3 end orient toward the interior of the segment, are synthesized by conventional chemical means.

The DNA in the sample is denatured by heating and then is cooled in the presence of the oligonucleotides, which anneal to the separated DNA strands. The annealed oligonucleotides are then utilized by DNA polymerase as primers for DNA synthesis, replicating the target DNA segment. These simple steps are repeated over and over again, doubling the amount of the target segment in each cycle (see illus). The use of thermostable DNA polymerase (isolated from bacteria that live naturally in hot springs) is an important refinement. Since the polymerase is not denatured when the DNA I heated, it need not to be added at every cycle.

Amplification of DNA by the polymerase chain reaction is carried out conveniently by inexpensive machines that produces the amplified product DNA in a few hours. Target sequences between a few hundred and 10,000 base pairs in length are readily amplified. After 20 cycles, the target DNA is amplified more than a millionfold; after 30 cycle the amplification is over a billionfold.
The utility of polymers chain reaction can be illustrated with a few examples. (1) For molecular biologists, this technique simplified the cloning of known genes, and led to the development of powerful methods to alter DNA sequences laboratory cultures and detect alterations in DNA in organisms. (2) Certain parts of the human genome are sufficiently variable that they can be used to identify a particular individual unambiguously. With the aid of the polymerase chain reaction, DNA from samples as small as a single human hair can be amplified and analyzed to place an individual at the scene of crime. Similar techniques are used to accurately establish paternity. Humans DNA extracted from centuries-old burial sites and amplified by this techniques is being used to trace prehistoric human migrations.

Fig Amplification of DNA using the polymerase chain reaction. The colored region is the target DNA to be amplified. The short arrow represent the oligonucleotide primers

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(3) As the molecular genetic basis for more genetic diseases becomes known, polymerase chain reaction-based diagnostic tests for these diseases are possible. For example, prenatal diagnosis of genetic diseases can be carried out with minute samples of fetal tissue. (4) Polymerase chain reaction has also been used to amplify and study DNA from extinct species such as the quagga and the woolly mammoth. In addition, very small amounts of DNA added to the cargo hold of an oil tanker can be used to trace the origins of an oil spill.