Normal base pairs

Mismatch Repair. Mispairs that break the normal base-pairing rules can arise spontaneously due to DNA biosynthetic errors, events associated with genetic recombination and the deamination of methylated cytosine (Modrich, 1987). With the latter, when cytosine deaminates to uracil, an endonuclease enzyme, /V-uracil-DNA glycosylase (Lindahl, 1979), excises the uracil residue before it can pair with adenine at the next replication. However, 5-methyl cytosine deaminates to form thymine and will not be excised by a glycosylase. As a result, thymine exits on one strand paired with guanine on the sister strand, that is, a mismatch. This will result in a spontaneous point mutation if left unrepaired. For this reason, methylated cytosines form spontaneous mutation hot-spots (Miller, 1985). The cell is able to repair mismatches by being able to distinguish between the DNA strand that exists before replication and a newly synthesized strand. 

From the point of view of this review, a mutation is a change in the phenotype of an organism resulting from a change in one of the purine or pyrimidine bases which alters the normal base pairing sequence in the... 

Figure 7.4 The formation of a benzo[a]pyrene adduct alters the normal base pairing of guanosine with cytidine. The adduct pairs instead with the imine form of adenosine, leading to a base pair transversion after replication.

The normal base pairs in DNA. Adenine in one polynucleotide chain pairs with thymine in the complementary chain guanine pairs with cytosine. A-T base pairs are joined by two hydrogen bonds G-C base pairs are Joined by three hydrogen bonds. 

FIGURE 10.12 UV irradiation causes dimerization of adjacent thymine bases. A cyclobutyl ring is formed between carbons 5 and 6 of the pyrimidine rings. Normal base pairing is disrupted by the presence of such dimers. 

Single-base mismatches refer to single non-Watson-Crick base opposition flanked by normal base pairing. The dismption to the overall geometry of the double helix is quite... 

Figure 1. Normal base pairing found in DNA between adenine and thymine and between cytosine and guanine...

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