Adenine/thymine, hydrogen bonding

Fig. 12.3. Adenine-thymine hydrogen-bonded pairs (a) found by Hoogsteen in a 1 1 complex of 3-N-methyllhymine and 9-Af-methyladenine (b) postulated by Watson and Crick (current version refined by Arnott) for part of the structure of DNA. Guanine-cytosine pairs (c) Hoogsteen type (d) Watson-Crick type (Wilkins and Arnott, 1965). (From Mahler and Cordes, 1966.)...

The Watson and Crick (interstrand) adenine-thymine hydrogen-bonded base pair... 

Despite these similarities, there are some very important differences between the reactions carried out by DNA polymerase and RNA polymerase. Remember that DNA is synthesized with the use of four deoxyn bon ucl cosidc triphosphates (dATP, dGTP, dTTP, and dCTP). In the double-stranded DNA helix, adenine bases hydrogen-bond with thymine bases, and guanine bases hydrogen-bond with cytosine bases. RNA, however, is synthesized with the use of four ribonucleoside triphosphates (ATP, GTP, CTP, and UTP). Adenine and guanine bases in the DNA template will direct the addition of uracil and cytosine bases to the growing RNA molecule, respectively, while... 

Adenine and thymine hydrogen-bond with each other, and cytosine and guanine hydrogen-bond with each other. 

Fig. 11. Scheme for the attachment of zeolite crystals by thymine/adenine-mediated hydrogen-bonding (left). The thymine and the adenine were tethered to the respective surfaces through 11 -trimethoxysilyl- -undecyl-linkers. [81] SEM image of a monolayer of cubic thymine-zeolite-A crystals assembled on adenine-glass at 50 °C (right). 

Hydrogen bonds hold the base pairs of double-stranded DNA together (see Section 25.4). Thymine hydrogen bonds with adenine. Cytosine hydrogen bonds with guanine. 

Double-stranded helix structure of DNA. Adenine is hydrogen-bonded to thymine (A T), and cytosine is hydrogen-bonded to guanine... 

The DNA base pairs guanine (G), cytosine (C), adenine (A) and thymine (T). The uracil-2,6-diaminopyridine pair can also form three hydrogen bonds but has a much lower association constant than G-C. 

Like Thr 124 and Thr 215, the Asn 69 and Asn 159 residues occupy equivalent positions in the two homologous motifs of TBP. By analogy with the symmetric binding of a dimeric repressor molecule to a palindromic sequence described in Chapter 8, the two motifs of TBP form symmetric sequence-specific hydrogen bonds to the quasi-palindromic DNA sequence at the center of the TATA box. The consensus TATA-box sequence has an A-T base pair at position 4, but either a T-A or an A-T base pair at the symmetry-related position 5, and the sequence is, therefore, not strictly palindromic. However, the hydrogen bonds in the minor groove can be formed equally well to an A-T base pair or to a T-A base pair, because 02 of thymine and N3 of adenine occupy nearly stereochemically equivalent positions, and it is sufficient, therefore, for the consensus sequence of the TATA box to be quasi-palindromic. 

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. 

Examine AT pair and GC pair, adenine-thymine and guanine-cytosine base pairs, respectively. Identify the hydrogen bonds in each. Are they the same as those you sketched Are the base pairs flat as normally drawn in textbooks, or are they significantly puckered or twisted ... 

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