G:C and A:T base pairs

This is because the DNAs have different amounts of G C and A T base pairs, and the former confer the greater stability to the helix, perhaps through the presence of three H bonds per base pair rather than two (Fig. 7-1). Thus, the higher the GC content, the higher is the Tm. The value of Tm under standard conditions can be used, therefore, to obtain an estimate of the G + C content of an unknown DNA. This is obvious from Fig. 7-9, which shows a plot of Tm versus G + C content for a number of DNAs. 

Under physiological conditions DNA exists predominantly as a duplex formed between complementary anti-parallel strands of DNA in which the purines of one strand are hydrogen bonded to the pyrimidines of the complementary strand and vice versa to form G C and A T base pairs. The Watson-Crick G C base pairs (Figure 2) possess three hydrogen bonds and the A T base pairs two hydrogen bonds, each hydrogen bond contributing approximately 2 kcal mol to the stabil-... 

Figure 1. Numbering scheme of nucleotide bases of DNA, showing hydrogen-bonding interactions between G-C and A-T base pairs. As depicted in the diagrams, the top of each base pair points to the major groove, and the bottom, to the minor groove. Reproduced with permission from Ref. 116 (copyright 1987, American Chemical Society).

This occurs because the DNA molecules have different proportions of G C and A T base pairs. G C base pairs confer a greater stability to double-stranded DNA because of the presence of three (rather than two) hydrogen bonds per base (Fig. 3-37). Thus, the higher the G C content, the higher the T. ... 

By contrast, the proportion of G C versus A T base pairs is highly variable in the DNA from different organisms, with over-representation and under-representation of residues foimd at dimeric, i.e., adjacent base-pair step, and higher levels (5). Factors, which may underlie the observed compositional patterns, are not yet understood. 

The replication process requires that each double-helical molecule of DNA produce two identical molecules of DNA. This means that wherever a G-C or A-T base pair occurs in the parental molecule, the identical base pair must occur in the progeny molecules. However, many factors interfere with accurate replication of DNA. If an A should pair with C or G with T as a result of tautomeriza-tion (Chapter 23), a point mutation (a change in one base pair) will result. Occasionally, a segment of DNA will be replicated more than once (duplication) or a segment may fail to be replicated (deletion). These and other aberrations in DNA replication do occur, but the mechanism of replication has evolved to minimize such mistakes. 

Base pairing between adenine and thymine (A-T) and between guanine and cytosine (G-C). An A-T base pair is held by two hydrogen bonds, whereas a G-C base pair is held by three hydrogen bonds. 

The paper by Montroll and Yu makes use of a general formalism for the investigation of the properties of lattices with two types of component, type I and type II. The formalism is in the spirit of the Mayer cluster integral approach. Let us consider a pure type 1 component lattice with the effect of the type II components imposed upon it. Type II components are considered first as single units, then as pairs, triples, etc. For the DNA problem, the pure lattice is a one-dimensional string of A-T (or G-C) base pairs, witli the G-C (or A-T) base pairs acting as the type II components. 

DNA contains only deoxyribose sugars while RNA contains only ribose.The major bases found in DNA are adenine, guanine, cytidine, and thymine. RNA contains mostly adenine, guanine, cytidine, and uracil. The bases pair with one another to form A-T, G-C, or A-U base pairs. 

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