Guanine base pairing with cytosine

As adenine undergoes a tautomeric shift, its imino form can base pair with cytosine. The transition shows up in the second generation of DNA replication when cytosine base pairs with guanine. In this manner an A-T base pair is replaced by a C-G base pair. 

DNA wound around one another. The sugar-phosphate backbone is on the outside of the helix, and complementary pairs of bases extend into the center of the helix. The base pairs are held together by hydrogen bonds. Adenine base pairs with thymine, and cytosine base pairs with guanine. The two strands of DNA in the helix are antiparallel to one another. RNA is single stranded. 

DNA, adenine (A) pairs with thymine (T) and cytosine (C) pairs with guanine (G). Therefore, when there is an A in one strand of the double-stranded DNA molecule, there is a T in the other strand. When the genetic code is copied from DNA to RNA, the two strands of DNA molecule separate, and the RNA nucleotides pair with nucleotides on each strand of DNA. In this case, the nucleotide that pairs with adenine (A) on the DNA is uracil (U) because RNA does not contain thymine (T). Because of the exact nature of base pairing, the genetic code can be transmitted accurately at each stage of the process. 

Figure 6.2. Molecular architecture of DNA. Each strand of DNA is composed of alternating pentose sugar (deoxyribose) and phosphate moieties linked to each other via phosphodiester linkage. The first carbon position of the sugar is attached to one of the four nitrogenous bases (A, T, G, or C). The two strands are in opposite orientation to each other with respect to a 5 or 3 phosphate group attached to the sugar moiety. Cytosine (C) pairs with guanine (G) via three hydrogen bonds, and adenine (A) pairs with thymine (T). (Reproduced from Textbook of Biochemistry with Clinical Correlations, T. M. Devlin, ed., Wiley, New York, 1982.)...

Figure 3. (a) Similarities in the shapes of BP, DMBA, a base pair of guanine and cytosine, and the steroid, estradiol. In (b) the shapes of BD and DMBA (filled bonds) are compared with that of aflatoxin B] (open bonds) and the sites of activation of each to an epoxide is indicated by the arrows. 

The rules of base pairing (or nucleotide pairing) in DNA are adenine (A) always pairs with thymine (T) cytosine (C) always pairs with guanine (G). 

Guanine-Cytosine Base-Pair Interacting with Metal Cations... 

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. 

In molecular biology, a set of two hydrogen-bonded nucleotides on opposite complementary nucleic acid strands is called a base pair. In the classical Watson-Crick base pairing in DNA, adenine (A) always forms a base pair with thymine (T) and guanine (G) always forms a base pair with cytosine (C). In RNA, thymine is replaced by uracil (U). 

Figure 8.2 Base pairing of cytosine or methylcytosine with guanine in DNA. R Phosphodeoxyribose of the DNA backbone.

Alkylating agents can alter certain bases of DNA For example, the highly reactive chemical dimethylsul-fate (Fig. 8—35b) can methylate a guanine to yield O6-methylguanine, which cannot base-pair with cytosine. 

DNA isolated from cells (Kessel et al., 2002) and tissues and urine of animals (Vijayaraghavan et al., 2001 Yamanaka et al., 2001) treated with arsenic show lesions induced by oxidative stress. These lesions include 8-oxo-2 -deoxyguanosine and 8-hydroxy-2 -deoxyguanosine. These DNA lesions may lead to base-pair substitutions (guanine to thymidine and adenine to cytosine) during DNA synthesis, which could lead to altered gene products. 

Dactinomycin is an antitumor antibiotic isolated from a Streptomyces organism. It binds tightly to double-stranded DNA through intercalation between adjacent guanine-cytosine base pairs and inhibits all forms of DNA-dependent RNA synthesis, with ribosomal RNA formation being most sensitive to drug action. 

S. Metzger, B. Lippert, A metalated guanine,cytosine base quartet with a novel GC pairing pattern involving h(5) of c. J. Am. Chem. Soc. 118, 12467-12468 (1996)... 

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