Watson-Crick base pairs G«C and

Fig. 9. Base triplets formed between a Watson-Crick base pair G C and a third base G or C Hoogsteen-pairing) (left) or a transplatin-modified G or C (right)...

Figure 1.80 Illustrations of the specific RNA equivalent Watson-Crick base pairings, G.C and A.U involving complementary nucleoside residues. Overlay structure provides visual demonstration of the G.C/A.U isomorphous geometry.

The Watson-Crick base pairs G-C and A-T and the dinucleotide ASP-TSP as unit cell of the macromolecule have been chosen to build up the models of the DNA double helices. 

Figure 4.15 The classic Watson-Crick base-pairing between A and T, and between G and C in DNA.

Fig. 1 Watson-Crick guanine-cytosine (G-C) and adenine-thymine (A-T) base-pairs.

Figure 2.31 Watson-Crick base pairing, (a) C-G and (b) A-T. The presence of three hydrogen bonds in the G-C pair makes it more stable than A-T.

Figure 2. The complementary Watson-Crick base pairs, A-T and G-C.

Figure 1. The G C and A T Watson-Crick base pairs. In this and all subsequent figures, dashed lines represent hydrogen bonding interactions.

The strands must be antiparallel (one strand is 3 - 5 while the other is as described in Chapter 28) to maximize hydrogen bonding because the stereo-genic nature of the ribofuranose and deoxyribofuranose units leads to a twist in the polynucleotide backbone. The term stereogenic is described in Chapter 9, Section 9.1. These hydrogen bonding base pairs are called Watson-Crick base pairs the C-G pair is shown in 97 and the A-T base pair is shown in 98. The inherent chirality (see Chapter 9) of the D-ribofuranose and the D-deoxyribofura-nose leads the P-form of DNA to adopt a right-handed helix (see 99). 

Fig. 8. Non-Watson-Crick base pairs occurring in double-stranded RNA where — represents the site of attachment to the sugar (a) A—U reverse-Watson-Crick (b) G—C reverse-Watson-Crick (c) A—U Hoogsteen (d) A—U reverse-Hoogsteen (e) G—U wobble and (f) G—U reverse-wobble.

Fluorescence probes possessing the PyU base 46 selectively emit fluorescence only when the complementary base is adenine. In this case, the chromophore of is extruded to the outside of the duplex because of Watson-Crick base pair formation, and exposed to a highly polar aqueous phase. On the contrary, the duplex containing a PyU/N (N = G, C and T) mismatched base pair shows a structure in which the glycosyl bond of uridine is rotated to the syn conformation. In this conformation, the fluorophore is located at a hydrophobic site of the duplex. The control of base-specific fluorescence emission is based on the polarity change in the microenvironment where the fluorophore locates are dependent on the l>yU/A base-pair formation. 

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 20.4 The bases present in RNA and DNA and the Watson-Crick base pairing relationships. Uracil is present in RNA but is replaced by thymine in DNA that is, the pairs C-G and T-A are found in DNA the pairs C-G but U-A are found in RNA. The pairing is brought about by hydrogen bonding, indicated by a broken line.

Fig. 1.11. H -bond donors (D) and H-bond acceptors (A) in A T and G C base pairs. Schematic display of the differing pattern of H-bond acceptors and donors in the Watson-Crick base pairs. The groups above the base pairs (above the line) are accessible in the major groove, and those below the line are accessible from the minor groove.

Scheme 2 Rearrangement of hydrogen bonding induced by hole localization at the G-C Watson-Crick base pair, a Proton shift from the G radical cation to cytosine, b Dissociation of the N2 hydrogen bond in the G radical cation (adapted from [9] and [48])...

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