DNA FINGERPRINTING
On some human chromosomes, a short sequence of DNA has been repeated a number of times.In any particular chromosomes the repeat number may vary from one to thirty repeats.
Since these repeat regions are usually bounded by specific restriction enzyme sites, it is possible to cut out the segment of the chromosome containing this variable number of tandem repeatsor VNTR’s, run the total DNA on a gel, and identify the VNTR’s by hybridization with a probe
specific for the DNA sequence of the repeat. VNTR occurs in minisatellite DNA concentrated around Telomere
It is widely known that each individual has a DNA profile as unique as a fingerprint.
Actually, over 99% of all 3 billion nucleotides in human DNA, which we inherit from each parent, are identical among all individuals. However, for every 1000 nucleotides that we inherit there is 1 site of variation or polymorphism, in the population. These DNA polymorphisms
change the length of the DNA fragments produced by the digestion of restriction enzymes.
The resulting fragments are called restriction fragments length polymorphisms (RFLP’s—“riflips”).
Gel electrophoresis can be used to separate and determine the size of the RFLPs. The exact number and size of fragments produced by a specific restriction enzyme digestion varies from individual to individual. The fundamental techniques involved in genetic fingerprinting were
discovered in 1984 by Alec J. Jeffreys.
Steps to Producing a Genetic Fingerprint
hair roots, or saliva. Using newly developed biochemical techniques to multiply the
amount of DNA present, researchers can work with as small a sample as one hair root.
• The individual cells from the sample are split open, and the DNA is separated from
the rest of the cellular debris.
• The DNA is then treated with specialized proteins called restriction enzymes, which
cleave the DNA into smaller fragments by cutting at specific sites. Since the
minisatellites from any two individuals have different compositions, they are cleaved at different sites, producing fragments of different lengths.
• The DNA fragments are then applied to one end of a thin, jellylike substance called
an agarose gel, and an electric current is passed through the gel. The negatively
charged DNA fragments will migrate across the surface of the gel in response to the
current, with the smaller, more mobile pieces traveling farther. The DNA is thus
separated into individual bands, with the fragments in each one progressively smaller in size.
• Because the gel cannot be easily handled, a thin nylon membrane is laid over its
surface and covered by a layer of paper towels. As the towels draw moisture from the gel, the DNA is transferred onto the surface of the nylon membrane, a process called blotting.
• The DNA bands are still invisible to the eye, and there are too many to be useful.
Therefore, a solution of the radioactive probes made from minisatellites is washed
over the surface of the membrane.
• If any of the probes have the same composition as a part of a DNA fragment, they will bind to it. The probes will ingnore the vast majority of the hundreds of bands present and will bind anywhere from 6 to 20.
• To see the pattern of bands, the researchers place a sheet of photographic film on top of the membrane.
• The radioactive labels will expose the film, ultimately producing a pattern of thick-
and-thin dark bands.
• This pattern of bars is the GENETIC FINGERPRINT.
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