explain

Restriction Fragment Length Polymorphism ee p. 431) to synthe naciously (see p. 550] and western if protein is being studied (see p. 551], nei- gg whites, ther of which relates to anyone's name or to points of the compass.) able opti- B. Mutation detection ayed, for tinylated Southern blotting can detect DNA mutations such as large insertions or deletions, trinucleotide repeat expansions, and rearrangements of nucleotides. It can also detect point mutations (replacement of one nucleotide by another; see p. 498) that cause the loss or gain of Ca restriction sites. Such mutations cause the pattern of bands to differ Sell or from those seen with a normal gene. Longer fragments are gener- . If the ated if a restriction site is lost. For example, in Figure 34.13, per- son 2 lacks a restriction site present in person 1. Alternatively, the point mutation may create a new cleavage site with the production of shorter fragments. (Note: Most sequence differences at restriction sites are harmless variations in the DNA.) e the RESTRICTION FRAGMENT LENGTH OLYMORPHISM Of It has been estimated that the genomes of any two unrelated people are 99.5% identical. With 6 billion bos in the diploid human genome, that represents variation in -30 million bos. These genome variations are the result of mutations that lead to polymorphisms. A polymorphism is tradi- TV tionally defined as a sequence variation at a given locus (allele) in 1% of a population. The change in genotype results in either no change in phe- notype or a harmless change in phenotype, causes increased suscepti- bility to a disease, or, rarely, causes a disease. Polymorphisms primarily occur in the 98% of the genome that does not encode proteins (i.e., in Fi introns and intergenic regions). A restriction fragment length polymor- All phism (RFLP) is a genetic variant that can be observed by cleaving the pr . 3 DNA into fragments (restriction fragments) with a restriction endonucle si ase. The length of the restriction fragments is altered if the variant alters the DNA sequence so as to create or abolish a restriction site. RFLP can be used to detect human genetic variations, for example, in prospective parents or in fetal tissue. A. DNA variations resulting in RFLP Two types of DNA variations commonly result in RFLP: single- base changes in the DNA sequence and tandem repeats of DNA sequences. 1. Single-base changes: About 90% of human genome variation comes in the form of single nucleotide polymorphisms (SNPS, pronounced "snips"), that is, variations that involve just one base (Fig. 34.14). The substitution of one nucleotide at a restriction site can render the site unrecognizable by a particular restriction endonuclease. A new restriction site can also be created by the same mechanism. In either case, cleavage with an endonuclease results in fragments of lengths that differ from the normal and can be detected by DNA hybridization (see Fig. 34.13). The altered restriction site can be either at the site of a disease-causing mutation (rare) or at a site some distance from the mutation