Cytosine and guanine residues

In Z-DNA, guanine residues are syn, whereas cytosine and thymine residues are anti. Eukaryotic DNA contains several alternating purine-pyrimidine sequences consistent with die Z-DNA conformation however, the biologic significance of Z-DNA is still unclear. 

Several nucleotide bases undergo spontaneous loss of their exocyclic amino groups (deamination) (Fig. 8-33a). For example, under typical cellular conditions, deamination of cytosine (in DNA) to uracil occurs in about one of every 107 cytidine residues in 24 hours. This corresponds to about 100 spontaneous events per day, on average, in a mammalian cell. Deamination of adenine and guanine occurs at about l/100th this rate. 

Each of the sugar residues of DNA is bonded at the 1-position to one of four bases cytosine, 15 thymine, 16 adenine, 17 and guanine, 18. The four... 

Oligonucleotides containing intrastrand or interstrand cross-links are easily separated by this technique. Over a period of 50 h, in the platinated duplex 1, only interstrand cross-links between complementary guanine and cytosine residues in agreement with previous results [64], are detected. In the platinated duplex 2, which contains a [Pt(dien)(dG)]2+ adduct in the lower strand three base pairs away from the trans-[Pt(NH3)2(dG)Cl]+ adduct and on its 5 -side, both intrastrand and interstrand cross-links are detected. In the platinated duplex 3, which contains a [Pt(dien)(dG)]2+ adduct in the lower strand two base pairs away from the /ra v-[Pt(NH3)2(dG)Cl]+ adduct and on its 3 -side, mainly two kinds of interstrand cross-links are detected. In the platinated duplex 4, which contains two [Pt(dien)(dG)]2+ adducts, two kinds of interstrand crosslinks are formed, but in a different ratio to that in duplex 3. In duplexes 3 and 4, there is one interstrand cross-link between the complementary guanine and cytosine residues, and the other is between the guanine and an adenine residue located four or five base pairs away on the 3 -side of the guanine residue. 

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