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| DNA Finger Prints |
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| The technique of finger printing was perfected by British genetist Dr. Alec Jeffreys in 1984. |
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| fig. 25.9 Identical Twins |
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| No two people (other than identical twins) have exactly the same sequence of bases in their DNA. By detecting these, the differences in DNA sequences, it is possible to distinguish one person from another. As you know, each human has a unique set of fingerprints, a marker of his or her identity. Like other sexually reproducing species, each human also has a DNA fingerprint a unique array of DNA fragments, inherited from each parent in a Mendelian pattern. Forensic scientists can use DNA fingerprinting to identify criminals and crime victims from the DNA in blood or small amounts of tissue left at a crime scene. Because DNA fragments that make up a DNA fingerprint are inherited, scientists can use them to resolve questions of paternity. DNA fingerprints are so accurate that even full siblings are readily distinguished from one another. |
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| More than 99 percent of the DNA is exactly the same in all humans. But DNA fingerprinting focuses only on the part that tends to differ from one person to the other. Throughout the human genome are present tandem repeats short regions of repeated DNA that differ substantially among people. For example, the five bases TTTTC are repeated anywhere from four to fifteen times in tandem in different people, and three bases (CCG) are repeated five to fifty time in tandem. |
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| During DNA replication, the number of repeats may increase or decrease. Mutation rates in tandem repeats are much higher that the rates at most other sites in DNA. Because such mutations occurred over many generations in many different family lineages, each person is usually heterozygous for a repeat number at any given tandem-repeat locus. By examining many tandem-repeat sites, researchers found out that each person carries a unique combination of repeat numbers. |
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| It is possible to detect the differences at tandem-repeat sites with gel electrophoresis, a laboratory technique that uses an electric field to force molecules through a viscous gel. In this case, it separates the DNA fragments according to their length. Size alone dictates how far each fragment moves through the gel, so the tandem repeats of different sizes migrate at different rates through the gel. |
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| A gel is immersed in a buffered solution, and then DNA fragments from individuals are added to the gel. When an electric current is applied to the solution, one end of the gel takes on a negative charge, and the other end a positive charge. DNA molecules carry a negative charge (because of the negatively charged phosphate groups), so they migrate through the gel toward the positively charged pole. They do so at different rates, and so they separate into bands according to length. The smaller the fragment, the farther it will migrate through the gel. After a set time, researchers identify fragments of different lengths by staining the gel or by specifically highlighting fragments that contain tandem repeats. |
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| fig. 25.10 - DNA Fingerprinting Technique |
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| The variation in tandem repeats can also be detected as restriction fragment length polymorphisms, or RFLPs (DNA fragments of different sizes that had been cleaved by restriction enzymes). In the case of tandem repeats, a restriction enzyme cleaves the DNA flanking the repeat. Alternatively, researchers might use PCR to amplify the tandem repeat region. Either way, differences in the size of the fragments, which reveal genetic differences, can be detected with electrophoresis. |
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| When DNA fingerprinting was first used as evidence in court, attorneys challenged conclusions based upon it. But DNA fingerprinting is now firmly established as an accurate and unambiguous way to identify individuals and paternity. It is widely used to convict the guilty and exonerate innocent suspects. For example, in 1998, and eleven-year-old girl was raped, stabbed, and strangled to death in Elisabethfehn, Germany. In the largest mass screening to date, police requested blood samples from 16,400 males in and around the vicinity. A single DNA fingerprint from a thirty-year-old mechanic matched the evidence, and he confessed to the crime. |
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| a. To identify criminals in forensic science for solving cases such as murders. |
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| b. To identify the true biological father to determine paternity |
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| c. To identify racial groups to rewrite biological evolution. |
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| d. To verify claims of being relatives to established residents in case of immigrants. |
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