Polynucleotides guanine-cytosine base pairs

The bases of one strand are paired with the bases of the second strand so that an adenine is always paired with a thymine, and a cytosine is always paired with a guanine. Therefore, one polynucleotide chain of the DNA double helix is always the complement of the other. Base pairs are held together by hydrogen bonds. 

DNA is a duplex molecule in which two polynucleotide chains (or strands) are linked to one another through specific base pairing (Fig. 7-1). Adenine in one strand is paired to thymine in the other, and guanine is paired to cytosine. The two chains are said to be complementary. This was one of the essential features of Watson and Crick s proposal regarding the structure of DNA. Hydrogen bonds form between the opposing bases within a pair. In the structure proposed by Watson and Crick, A T and G C base pairs are roughly planar, with H bonds (dotted lines), as shown in Fig. 7-1. Note that two H bonds form in an A T pair and three in a G C pair. 

For poly(dG-dC) with alternating guanine and cytosine sequence, a right-handed B-DNA form prevails at low salt conditions, whereas at higher salt concentrations a change occurs to a left-handed form called Z-DNA [661, 662]. In Z-DNA, the G C base pairs are again of the Watson-Crick type and the two polynucleotide strands are antiparallel, but the nucleotide conformations are different (Box 20.1). 

The normal base pairs in DNA. Adenine in one polynucleotide chain pairs with thymine in the complementary chain guanine pairs with cytosine. A-T base pairs are joined by two hydrogen bonds G-C base pairs are Joined by three hydrogen bonds. 

Base pairing The hydrogen bonds formed between complementary bases that are part of the polynucleotide chains of nucleic acids. The base pairing is specific in that adenine will base pair with thymine (uracil in RNA) and guanine will pair with cytosine. 

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)... 

The important features of the Watson and Crick model are as follows. The DNA consists of a double helix whereby two polynucleotide chains are coiled around a common axis (figure 3.18). The bases are on the inside of the helix whereby a base on one chain hydrogen bonds with a base on the other chain. There is a very specific pairing of bases (figure 3.17) adenine (A) must pair with thymine (T) whereas guanine (G) must pair with cytosine (C). These pairs fit perfectly into the space available on the inside of this helix whose dimensions are consistent with the X-ray fibre diffraction pattern. 

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... 

As proposed by Watson and Crick, each DNA molecule consists of two polynucleotide chains joined by hydrogen bonds between the bases. In each base pair, a purine on one strand forms hydrogen bonds with a pyrimidine on the other strand. In one type of base pair, adenine on one strand pairs with thymine on the other strand (Fig. 12.6). This base pair is stabilized by two hydrogen bonds. The other base pair, formed between guanine and cytosine, is stabilized by three hydrogen bonds. As a consequence of base-pairing, the two strands of DNA are complementary, that is, adenine on one strand corresponds to thymine on the other strand, and guanine corresponds to cytosine. 

Complementary structures two structures which define one another, e.g. the 2 polynucleotide chains in the DNA duplex. The base pairs adenine/thymine (or adenine/uracil in RNA) and guanine/cytosine are complementary, so that by base pairing the nucleotide sequence of one polynucleotide chain defines a unique sequence in the complementary strand. 

In Zubay s model, the polynucleotide sequence is bent in the middle. Under conditions in which all except five bases are paired to form hydrogen bonds of the same type as those found in Watson and Crick s model for DNA, the base pair sequence is twisted to form a regular helix. At the acceptor end of the molecule, the pCpA sequence is free and can therefore accept the amino acid, and the first cytosine of the pCpCpA sequence forms hydrogen bonds with a complementary guanine. The loop of the helix is formed by three free bases and plays an important role in connecting the tRNA molecule with messenger RNA. Zubay s model was soon superceded by new and more sophisticated models that were proposed when the base sequence of transfer RNA became known. 

Non-covalent interactions play a leading role in controlling the secondary and tertiary structures of natural macromolecules such as peptides, polynucleotides and polysaccarides or, for example, to provide the double helix structure of DNA where the base pairing between guanine and cytosine takes place by means of a threefold H-bonding. However, it is only relatively recently that such interactions have been exploited in the molecular self-assembly of well-defined S5mthetic supramolecular structures and materials. 

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