Nucleotide has free –hydroxyl group (-OH) at 3´ carbon and a phosphate group at 5´ of sugar moiety. The first nucleotide has a free phosphate and the 3´ hydroxyl group makes bond with the phosphate group at 5´ of next nucleotide (Figure 26.3). The propagation of nucleotide along the length of chain give rise polynucleotide.  As a result of each polynucleotide chain has a free 5´ free phosphate group (on first nucleotide) and free 3´-hydroxyl group (on last nucleotide). It gives polarity to the polynucleotide chain and it runs in the direction from 5´-3´. DNA is a double stranded whereas RNA is single stranded. Both strand of DNA are held together by hydrogen bonding between bases attached to the sugar. Adenine of one chain is always making 2 hydrogen bondings with the thymine of next chain. Similarly, guanine of one chain is making 3 hydrogen bondings with the cytosine of next chain (Figure 26.4).

 

Figure 26.3: A small segment of DNA.