The Evaluation of Forensic DNA Evidence (1996) / Chapter Skim
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2 Genetic and Molecular Basis of DNA Typing
2 Genetic and Molecular Basis of DNA Typing
Pages 60-74
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From page 60... ...
A fertilized human egg has 46 chromosomes (23 pairs) , which, with appropriate staining and microscopic techniques, are visible in the cell nucleus.
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From page 61... ...
The DNA thread is not visible in an ordinary microscope, and a stained chromosome is more rod-like than thread-like during the mitotic stages when it is most visible. The DNA thread is actually double two strands coiled around each other like a twisted rope ladder with stiff wooden steps (Figure 2.1~.
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From page 62... ...
A gene is a segment of DNA, ranging from a few thousand to more than a hundred thousand nucleotide pairs, that contains the information for the structure of a functional product, usually a protein. The specific sequence of nucleotides in a gene acts as an encoded message that is translated into the specific amino acid sequence of a polypeptide or protein.
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From page 63... ...
Nevertheless, the number of differences in the functional regions is large enough to account for the genetic diversity in the human population that is so apparent in such things as body shape, hair color, and facial appearance. Before a cell divides, each chromosome is copied.
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From page 64... ...
Such so-called somatic mutations play an important role in the causation of cancer, but they are not a problem in forensic testing because the tiny fraction of mutant cells in a tissue sample are swamped by the much larger number of nonmutant cells. There is a remote possibility that a mutation might occur so early in embryonic development that DNA in eggs or sperm might differ from that in blood from the same person.
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From page 65... ...
We therefore refer to them as markers. In these regions, usually ranging from 500 to 10,000 nucleotide pairs, a core sequence of some 15-35 base pairs is repeated many times consecutively along the chromosome.
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From page 66... ...
The next step involves cutting the DNA into small fragments. This is done with a restriction enzyme that recognizes a specific short DNA sequence and cuts the molecule at that point.
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From page 67... ...
The process requires several days for sufficient radioactive decay to produce a visible band on the film. Corresponding fragments from different persons differ in the number of repeat units; hence, the sizes of the fragments vary.
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From page 68... ...
standard DNA fragment sizing ladder; (2) K562, a standard cell line with two bands of known molecular weight; (3)
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From page 69... ...
K562 is from a human cell line and is a widely used laboratory standard. Lanes 1, 4, 6, 9, and 13 show standard DNA fragments used as a molecular-weight sizing ladder.
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From page 70... ...
The technique extends DNA typing to evidence samples that at present cannot be typed with other approaches. Moreover, the small amount of DNA required for PCR analysis makes it easier to set aside portions of samples for repeat testing in the same or another laboratory.
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From page 71... ...
This locus is part of the histocompatibility complex, a group of highly variable genes responsible for recognizing foreign tissue. Eight alleles at the DQA locus have been identified, although only six are commonly used in forensic work.
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From page 72... ...
Therefore, siblings, maternal half-siblings, and others related through female lines are as much alike in their mitochondrial DNA as identical twins. Mitochondrial DN is particularly useful for associating persons related through their maternal lineage, for example, for associating skeletal remains to a family.
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From page 73... ...
Since mitochondria are inherited independently of the chromosomes, mitochondrial information can be combined with nuclear data to yield probabilities of a random match (see Chapter 4~. A promising technique is minisatellite repeat mapping, or digital typing, which, apart from length variation, detects sequence differences within the base sequences repeated in VNTRs (Jeffreys et al.
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From page 74... ...
. bVNTR loci contain repeated core sequence elements, typically 15-35 bp in length.
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