vi. Complementary base pairing : In each pair, the two bases of the opposite strands are joined by hydrogen bonds. A and T are joined by two hydrogen bonds, while G and C are joined by three hydrogen bonds. This is called complementary base pairing. The two strands are thus held together all along their lengths by these hydrogen bonds.

v. Purine : Pyrimidine ratio : Because of the fixed or complementary base pairing in the DNA molecule, the total number of A is equal to the total number of T and the total number of G is equal to the total number of C. In other words, (A+G)= (T+C). Hence, purines: pyrimidines ratio is 1:1.

vi. C-3 and C-5 ends of the strand : In each strand one end of the strand has one free phosphate group on carbon-5 of the sugar molecule. This is the end of the strand is called C-5 (or 5') end. The other end of the strand has a free -OH on carbon-3 of the sugar molecule. This is called C-3 (or 3') end of the strand (Figure 8.4c).

vii. Antiparallel nature of strands: The two strands are oppositely oriented and hence are called antiparallel. This means, the 3' end of one strand is adjacent to the 5' end of the other strand (Fig. 8.4c) . This is because, the phosphate-sugar linkages run in opposite directions in the two strands.

viii. Dimensions: The diameter of the DNA double helix is 20 A^o. The length of each complete spiral (turn or pitch) of the molecule measures 34 A^o. 10 pairs of nucleotides are present in each complete spiral. Therefore, each nucleotide in the strand occupies a distance of 3.4A⁰.

(c) Circular DNA molecules : Chromosomes of most prokaryotes (e.g. bacteria, cyanobacteria,) are circular molecules of DNA.

In bacteria, there is no organized nucleus. The bacterial nucleoid consists of a single circular DNA molecule (bacterial chromosome). The molecule has two complementary strands forming a covalently closed circle. Generally, the circular molecule is present in a highly folded and suspercoiled state (Figure 8.5A). This is expected because the diameter of a bacterial cell (e.g. Escherichia coli) is about 1-2 microns while the total length of the circular DNA is about 1100 microns. The circular molecule has 40-50 folds or looped domains. These folds are held in position by RNA molecules (RNA connectors) and some non-histone proteins associated with the bacterial chromosome. (Histones are absent in bacteria). The DNA segment in each loop is supercoiled independently. Because of this characteristic formation of loops as well as the supercoils within the loops, the large circular DNA molecule can be packed into a small bacterial cell. Otherwise, the relaxed and fully expanded circular molecule (Figure 8.5) would be too large (about 350 microns diameter) to be contained in the bacterial cell. The coils of the supercoiled circle can be relaxed by treatment with enzymes such as RNAse or DNAse. The uncoiling occurs due to a break (nick) in one or both the strands of DNA.

In some viruses, e.g. certain bacteriophages, the circular DNA is single stranded. It becomes double stranded only during replication (replicatable form).

Figure 8.5

Table of Contents

8.0 Introduction
8.1 Packaging of Hereditary Material
8.2 The Structure of DNA
8.3 Replication Of DNA In Eukaryotes
8.4 Replicatin of Pokaryotic Chromosome
8.5 Plasmids
8.6 RNA: Structure and Types
8.7 The Genetic Code
8.8 The Central Theme of Protein Synthesis

Chapter 9

Follow @Pinkmonkey_com