Reverse Watson-Crick base pairs

Tautomerism in 5,8-diaza-7,9-dicarbaguanine (alloguanine) has been studied An X-ray structure analysis of the title compound revealed that this purinoid exists in the crystal as the two tautomers which interact with each other in the mode of a reverse-Watson-Crick base pair <05HCA1960>. 

Shows reverse Watson-Crick base pairing behaviour. 

FIGURE 1.7 Base pairing schemes of Watson-Crick and non-Watson-Crick types. Among the non-Watson-Crick base pairs, wobble pairs are formed by H-bonds between tautomeric forms of hydroxyl (OH) and imino (NH) groups. Non-Watson-Crick base pairs also comprise Hoogsteen base pairs and reverse Watson-Crick base pairs which form the basal structural units in triplex DNA and In parallel-stranded DNA (psDNA), respectively. 

The non-Watson-Crick type of base pairing also comprises the base pairs formed by inosine (I) with C, T, or A, purine hase pairs A G(or I), A - A, and G C, and the reverse Watson—Crick base pair A T. In the reverse Watson—Crick A T pairing, the T ring is rotated 180° around the N-3—C-6 axis from the normal Watson-Crick pair. One peculiarity of the reverse Watson—Crick hase pairing is that the two pairing strands can form an as yet unnatural para//e/-stranded, right-handed double helix (2, 3). 

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.

A logical extension of the work on phosphoryl cleavage reactions is the study of the reverse, ligation reaction. The principle of microscopic reversibility offers the comforting thought that the requirements for catalysis are basically the same, and Watson-Crick base-pairing provides a simple... 

The antisense approach is use of nucleic acids to reduce the expression of a specific target gene. As shown in Figure 58.2, a small piece of DNA, an oligodeoxynu-cleotide that is in the reverse orientation (antisense) to a portion of a target messenger RNA (mRNA) species, is introduced into a cell and a DNA-RNA duplex is formed by complementary Watson-Crick base pairing. Cessation of protein synthesis then may result from the rapid... 

Hoogsteen pairs were first observed in nature in transfer RNA molecules (Fig. 5-31). These molecules contain mostly Watson-Crick base pairs but there are also two reversed Hoogsteen pairs. One of them, between U8 and A14, is invariant in all tRNAs studied. Hoogsteen pairing also occurs in four-stranded DNA, which has important biological functions. A G quartet from a DNA tetraplex held together by Hoogsteen base pairs is shown in Fig. 5-8. 

All the modifications described above for RNA interfere with Watson-Crick base pairing and thus impede the progress of reverse transcriptase... 

Combination AU62 is the Watson-Crick base pair, AUS2 is called reversed Watson-Crick (Fig. 16.10, left). AU32 is the base pair discovered by Hoogsteen, and AU22 is the reversed Hoogsteen base pair (Fig. 16.10, right). 

Mature mRNA transcripts (sense strand) from eukaryotic cells can be purified and then reverse transcribed, with the assistance of a reverse transcriptase enzyme (from Moloney murine leukemia virus, MMLV), into complementary DNAs (cDNAs) that will anneal with the mRNA transcripts by Watson-Crick base pairing to give anti-parallel DNA/RNA duplexes or double helices. The poly(A) tail in each mature mRNA transcript is actually a usefiil handle for each reverse transcriptase reaction. Thereafter, DNA/RNA duplexes must be broken down with the assistance ofRNAse enzymes (specific for the hydrolysis of RNA phospho diester links) and a sense strand of DNA constructed instead on each cDNA single strand so that equivalent, more stable antiparallel DNA/DNA duplexes are generated instead, with the assistance of a DNA polymerase enzyme. In this instance, the poly(T) tail in each cDNA molecule turns out to be important for the DNA polymerase reaction ... 

Fig. 4 Scheme of electroactive sites in NA bases. Adenine (A) and cytosine (C) are reducible at mercury electrodes. Guanine (G) undergoes a chemically reversible reduction at the mercury electrodes, yielding an anodic peak G due to oxidation of its reduction product. A and G can be oxidized at carbon electrodes. Watson-Crick base pairs TA and CG are shown without the reduction sites involved in hydrogen bonding. Arrows... 

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