C—G base pair

A transversion is a point mutation that replaces a purine-pyrimidine base pair with a pyriinidine-purine base pair. For example, an A-T base pair becomes a T-A or a C-G base pair. 

The A T and C G base pairs are oriented in such a manner that the sugar phosphate backbones of the two twined chains are in opposite or antiparallel directions, with one end starting at the 5 and ending at the 3 and the other starting from the 3 and ending at the 5 so that opposite one 3 end is the other 5 end. Thus, the two chains run in opposite directions. 

Scheurer and Briischweiler71 calculated 2hJ(N,N) couplings in three nucleic acid base pairs, namely, Watson-Crick uracil-adenine (U A) [4a] and cytosine-guanine (C-G) base pairs [4b] and in the Hoogsteen adenine thymine (A-T) base pair [7]. 

Transversion mutations convert purines to pyrimidines or vice-versa. For example, conversion of an A-T base pair to a T-A or C-G base pair is a transversion. Ultraviolet light can cause transversions, although not exclusively. Frameshift mutations result from the insertion or deletion of a single base pair. These are often due to planar intercalating mutagens, which insert into the double helix. More extensive deletions and insertions are common events, as shown in Figure 8-13. 

In mammalian genomes, some cytosine residues of the CpG (cytosines adjacent to guanines) sequences in DNA are methylated, forming 5-methylcytosine. Deamination of 5-methylcytosine, however, yields thymine, a normal base component of DNA. In single-stranded DNA, this is a challenging problem as cells are not able to determine that this thymine is abnormal. In double-stranded DNA, however, deamination of the 5-methylcytosine in a methylated C G base pair yields a T G mismatch. Cells are therefore able to distinguish the thymine in a T G mismatch as... 

Figure 3 Normal C-G base pair and abnormal T-G base pairs...

The orientation of the amino proton attached to the N2 donor on G differentiates the G C and C G base pairs from each other as well as from the A-T and T-A pairs. The latter base pairs can be discriminated by small synthetic molecules that take advantage of both the asymmetric steric structure of the adenine C2-H and the capability of the thymine 02 (with two sets of lone pair electrons) to form an additional hydrogen bond not possible with the pseudo-symmetrically related adenine N3 (37, 38). It is not yet clear whether naturally occurring, DNA-binding proteins use similar principles to distinguish between A T and T A base pairs in the minor groove. 

Figure 2.9 The two H-bond configurations between A-T and C-G base pairs in DNA.

The conformationally-locked 3 -amino thymidine derivative (47) was prepared and incorporated into DNA as a dimer unit, attaching the amino group via a carbamate or phosphoramidate linkage.Both modifications were found to be destabilising towards both RNA and DNA targets. The hybridisation properties of an amino-modified bicyclic thymine nucleotide (48) have been exam-ined. Using alternate T and (48) residues there is an increase in duplex stability with d(A)io, with no pH dependence. When the amino group is acetylated, the duplex stability is decreased. The locked C-nucleoside oxazole derivative (49) has been incorporated into TFOs where it was found to selectively enhance the stability with a C.G base pair. ... 

Recognition of a C-G base pair by a parallel third strand was achieved using... 

An improved method for the incorporation of 2 -deoxyxanthosine (dX) has been reported. The stability of dX in duplexes opposite each of the natural bases is in the order T>G>A = Cat pH 7.5, and A T>C>Gat pH 5.5, which is in general agreement with a previous report. 2-Aza-dA (114) when incorporated into duplexes forms the most stable base pair with dG, almost equivalent to a C.G base pair. It also forms a more stable duplex when present as a dangling end than thymidine. 

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. 

The melting point of a DNA molecule is the temperature at which the double-helical strand breaks apart. Suppose you are given two DNA samples. One sample contains 45 percent C-G base pairs while the other contains 64 percent C-G base pairs. The total number of bases is the same in each sample. Which of the two samples has a higher melting point Why When fruits such as apples and pears are cut, the exposed parts begin to turn brown. This is the result of an oxidation reaction catalyzed by enzymes present in the fruit. Often the browning action can be prevented or slowed by adding a few drops of lemon juice to the exposed areas. What is the chemical basis for this treatment ... 

FIGURE 23-25 Formation of a spontaneous point mutation by deamination of 5-methyl cytosine (C) to form thymine (T). If the resulting T-G base pair is not restored to the normai C-G base pair by base excision-repair mechanisms (D), it will lead to a permanent change in sequence following DNA replication (i.e., a mutation) (B). After one round of replication, one daughter DNA molecule will have the mutant T-A base pair and the other will have the wild-type C-G base pair. 

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