Adenine pairs

As indicated in Chapter 11, the base pairing in DNA is very specific the purine adenine pairs with the pyrimidine thymine the purine guanine pairs with the pyrimidine cytosine. Further, the A T pair and G C pair have virtually identical dimensions (Figure 12.10). Watson and Crick realized that units of such similarity could serve as spatially invariant substructures to build a polymer whose exterior dimensions would be uniform along its length, regardless of the sequence of bases. 

Figure 3.10 Stniciural details of the bridging units between pairs of bases in separate strands of the double helix of DNA (a) the thymine-adenine pair (b) the cytosine-guanine pair.

The two strands which make up DNA are held together by hydrogen bonds between complementary pairs of bases adenine paired with thymine and guanine paired with cytosine. The integrity of the genetic code (and of life as we know it) depends on error-free transmission of base-pairing information. 

One way in which chemical compounds can induce base substitution mutation is through their incorporation into the structure of DNA itself. Thus, 5-bromodeoxyuridine (or bromouracil) can replace thymidine in DNA, where it serves as an efficient mutagenic agent.767 2-Aminopurine, an analog of adenine, pairs with thymine, just as does adenine when incorporated into DNA. 

Guckian, K.M., Krugh, T.R. and Kool, E.T. (1998) Solution structure of a DNA duplex containing a replicable difluorotoluene-adenine pair. Nat. Struct. Biol., 5, 954-959. 

In some extreme cases, such stabilization in the biological systems may have significant consequences. For example, the tautomeric imine-enol form of thymine enables it to form H-bonding with guanine (26, dR = deoxyribose) instead of the normal adenine pair (27), wherein the enol is stabilized via H-bonding with the keto group on guanine which causes mismatch in the structure of the DNA double helix. 

Figure 1.3. Watson-Crick Base Pairs. Adenine pairs with thymine (A-T), and guanine with cytosine (G-C). The dashed lines represent hydrogen bonds.

According to base-pairing rules, adenine pairs with ... 

In DNA, two antiparallel polynucleotide chains are joined by pairing between their bases and are twisted to form a double helix. Adenine pairs with thymine, and guanine pairs with cytosine. One chain runs in a 5 to 3 direction and the other runs 3 to 5. ... 

The structure relies crucially on the pairing up of nucleic acid bases between the two chains. Adenine pairs only with thymine via two hydrogen bonds, whereas guanine pairs only with cytosine via three hydrogen bonds. Thus, a bicyclic purine base is always linked with a smaller monocyclic pyrimidine base to allow the constant diameter of the double helix. The double helix is further stabilized by the fact that the base pairs are stacked one on top of each other, allowing hydrophobic interactions... 

Base pairing between nucleic acid bases can occur in RNA with adenine pairing with uracil, and cytosine pairing with guanine. However, the pairing is between bases within the same chain, and it does not occur for the whole length of the molecule (e.g. Fig. 6.18). Therefore, RNA is not a double helix, but it does have regions of helical secondary structure. 

The most stable DNA stracture is formed when two polynucleotide chains are joined by hydrogen bonding between the side chain bases. The base pairing is specific in that adenine pairs with thymine and guanine pairs with cytosine (A-T G-C)... 

Purines are bases found in the nucleosides and nucleotides that make up nucleic acids. In nucleic acids, the purines match up with specific pyrimidine bases. The matching between purines and pyrimidines forms "base-pairs" in which adenine pairs with thymine (in DNA) or uracil (in RNA). Guanine pairs with the base cytosine in either nucleic acid. 

DNA Structure. Genetic information is encoded by the sequence of different nucleotide bases in DNA. DNA is double-stranded it contains two antiparallel polynucleotide strands The two strands are joined by hydrogen bonding between their bases to form base-pairs Adenine pairs with thymine, and guanine pairs with cytosine The two DNA strands run in opposite directions. One strand runs 5 to 3, and the other strand runs 3 to 5. The two DNA strands wind around each other, forming a double helix... 

RNA chains are usually single-stranded and lack the continuous helical structure of double-stranded DNA. However, RNA still has considerable secondary and tertiary structure because base pairs can form in regions where the strand loops back on itself. As in DNA, pairing between the bases is complementary and antiparallel. But in RNA, adenine pairs with uracil rather than thymine (Fig. 12.18). Basepairing in RNA can be extensive, and the irregular looped structures generated are... 

The X-ray diffraction pattern of DNA demonstrated the helical structure and the diameter. The combination of evidence from X-ray diffraction and chemical analysis led to the conclusion that the base pairing is complementary, meaning that adenine pairs with thymine and that guanine pairs with cytosine. Because complementary base pairing occurs along the entire double helix, the two chains are also referred to as complementary strands. By 1953, studies of the base composition of DNA from many species had already shown that, to within experimental... 

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