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DNA Fingerprinting.

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DNA Fingerprinting This is a way of making a pattern from pieces of DNA cut with restriction enzymes. Everybody has a different DNA, so the pattern or fingerprint is unique to each individual. DNA is a long chain, double-helix molecule composed of building units called nucleotides. Each nucleotide consists of a sugar, a phosphate and a nitrogenous base. There are four types of nitrogenous base: adenine, guanine, cytosine and thymine. The genetic information carried by a DNA molecule is contained in the sequence of these four bases. A gene is a length of DNA that codes for a specific protein or polypeptide. They are organised into chromosomes and located within the nucleus in human cells. Only 2% of the DNA of a human cell contains genes. The rest consists of non-coding sequences of bases called introns, which occur between or within a gene. ...read more.


The trick in DNA profiling is to choose mini-satellites that show the most variation between people. To make a DNA fingerprint, a very small sample tissue containing cells with a nucleus, such as, blood (0.5cm3), one hair root or semen (0.005cm3), is taken into the laboratory where the DNA is extracted. This is done by shaking the sample in a mixture of water-saturated phenol and chloroform. The proteins precipitate out leaving pure DNA dissolved in the water layer. Certain restriction enzymes are added to the DNA to cut it. These enzymes recognise specific base sequences and so cut at specific points close to but not within the mini-satellite region, leaving the mini-satellites intact. A number of DNA fragments are produced of different lengths, some of which contain the mini-satellite. The DNA fragments are separated according to size by electrophoresis. This involves placing the DNA fragments in wells at one end of a block of agarose gel. ...read more.


A radioactive DNA probe is used to bind onto and reveal the location of a certain type of mini-satellite. This involves immersing the nylon membrane in a solution containing the radioactive DNA probe. DNA probes consist of a single strand o a length of DNA made up of sequences of bases complementary to the core sequence. The probes used in forensic work are commonly of a type that will bind only at one specific site or locus and are known as single-locus probes. Excess robes are washed away. The process can be repeated with different radioactive probes, which bind to different core sequences and thus identify different mini-satellites. The mini-satellite regions that have been picked up by radioactive probes have to be made visible by putting an X-ray film over the nylon membrane. The places where radioactive probes have bound to DNA fragments will emit radiation which fogs the film. This creates a pattern of bands known as a DNA fingerprint. It looks like the barcode found on supermarket products. Aisha Hussain Page 1 08/05/2007 ...read more.

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