3.1 Structure and Function of DNA

From chemical analysis it was shown that DNA is composed of units known as nucleotides. Each nucleotide consists of pentose sugar deoxyribose, phosphate group and nitrogenous bases. There are four types of nitrogenous bases, Adenine (A), Guanine (G), Cytosine (C) and Thymine (T). Watson and Crick proposed the double helix model for the DNA molecule, with two chains interwined running in opposite directions (Fig 3).

Figure 3 : Structure of DNA double helix

Each DNA strand is composed of nucleotides arranged in a random fashion. The two strands opposite to each are connected together by the hydrogen bonds between the opposite nitrogenous bases. The opposite pairs are arranged in a specific way, where the guanine always bonding with cytosine and adenine bonds with thymine. Thus the two strands are complementary to each other.

One of the important characteristics for the genetic material is that it forms copies of itself, so that it can be transmitted to the daughter cells. The double helix structure of DNA suggests a model for replication of genetic material.   This involves unwinding the double helix and each strand acts as a template to form a complementary strand depending upon the specificity of pairing between the nitrogenous bases, where A pairs with T and G pairs with C. Therefore, two daughter molecules are produced. Each daughter double helix molecule contains one parental strand and one newly synthesized strand (Fig 4).

Figure 4 : DNA Replication