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Figure 8.25 Action of Restriction Endonucleases Cleaving of double-stranded DNA at specific sites indicated by the red arrows.The few hydrogen bonds between the two cleavage sites cannot hold the strands together. Consequently, the DNA splits into two molecules frequently with overlapping ends. (a) Site of cleavage by a restriction enzyme, isolated from E. coli. (b) Site of cleavage by a restriction enzyme isolated from Bacillus amyloliquefaciens.

as a substrate and so do not cleave the DNA. For example, if cytosine has a methyl group, then it is not a substrate for a restriction enzyme. Since cytosine and methylcytosine have the same hydrogen-bonding characteristics, this alteration does not induce mutations. The pattern of methylation of DNA defines the DNA as self or foreign which determines whether or not the restriction enzyme will degrade it.

When foreign DNA enters a cell, a race begins between methylation and restriction. If the DNA is methylated before the restriction enzyme has a chance to degrade it, it becomes self and will not be degraded (figure 8.26a). In most cases, however, it is degraded before it has a chance to be methylated (figure 8.26b). The two enzymes, restriction enzyme and methylase comprise the restriction-modification system. ■ restriction-modification system, p. 335

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