Hydrogen bonding adenine-thymine pairs

There is a large variability possible in the structures of double stranded DNA due to the fact that (compared to polypeptides) many more bonds can be rotated in the backbone of each monomer (Scheme 14). The most common and physiologically most important structure is the B-DNA helix. It consists of two polynucleotide chains running in opposite direction which coil around a common axis to form a right-handed double helix. In the helix, the phosphate and deoxyribose units of each strand are on the outside, and the purine and pyrimidine bases on the inside. The purine and pyrimidine bases are paired by selective hydrogen bonds adenine is paired with thymine, and guanine with cytosine (Scheme 15). The structure is very flexible and can form a supercoil with itself, or around proteins. It can form a left-handed supercoil around histones to form nucleosomes which assemble in yet another helical structure to form chromatin. ... 

DNA wound around one another. The sugar-phosphate backbone is on the outside of the helix, and complementary pairs of bases extend into the center of the helix. The base pairs are held together by hydrogen bonds. Adenine base pairs with thymine, and cytosine base pairs with guanine. The two strands of DNA in the helix are antiparallel to one another. RNA is single stranded. 

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. 

The double-helical strand of a DNA molecule. The diagram at the left shows the hydrogen bonding between base pairs adenine-thymine and cystosine-guanine that hold the strands together. 

The ability of DNA to replicate lies in its double-helical structure. There is a precise correspondence between the bases in the two strands. Adenine in one strand always forms two hydrogen bonds to thymine in the other, and guanine always forms three hydrogen bonds to cytosine so, across the helix, the base pairs are always AT and GC (Fig. 19.29). Any other combination would not be held together as well. During replication of the DNA, the hydrogen bonds, which are... 

The DNA structure involves two polyanionic phosphodiester strands linked together by hydrogen bonding of base pairs. The strands can be separated by a denaturation process (melting). The melting temperatnre increases with an increase in guanine (G)-cytosine (C) content, since this base pair possess three hydrogen bonds as compared to just two for the adenine (A)-thymine (T) pair. 

The structures of RNA molecules consist of a single polymer chain of nucleotides with the same bases as DNA, with the exception of thymine, which is replaced by uracil, which forms a complementary base pair with adenine (Figure 1.33(a)). These chains often form single stranded hairpin loops separated by short sections of a distorted double helix formed by hydrogen bonded complementary base pairs (Figure 1.33(b)). 

The sugar phosphate backbone is on the outside, and the bases point inward. These bases are paired so that for each adenine (A) on one chain a thymine (T) is aligned opposite it on the other chain. Each cytosine (C) on one chain has a guanine (G) aligned with it on the other chain. The AT and GC base pairs form hydrogen bonds with each other. The AT pair has two hydrogen bonds the GC pair has three hydrogen bonds (Fig. 47.1). 

Each chain of double-helical DNA is bound to the other through complementary base pairs, with adenine (A) in one being hydrogen-bonded to thymine (T) in the other, and guanine (G) to cytosine (C). Watson and Crick proposed that, to achieve precise copying of a nucleotide (base) sequence, the two chains of the DNA must unwind from one another to allow each single chain to act as a template for the synthesis of a new one. Thus, the assembly of the sequence in the newly synthesized... 

Tautomerization plays a vital role in biological systems as well. For example, multi-hydrogen bonds between base pairs construct the DNA double helix. When the adenine (A) base, for example, is UV irradiated, A is converted to its tautomer (A ), and the resultant tautomer A is misread as guanine (G) and is paired with cytosine (C) (Fig. 1) instead of thymine. Such a miscoding can become the origin of mutation (Watson and Crick, 1953 Goodman, 1995). 

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... 

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