DNA molecules are fundamentally like two long strings of beads, each of which consists in irregular alternation of four types of beads – nucleotides, twisted in a helix, one around the other (Fig. 3.2 in the book). At the site where nucleotide A is present in one chain, nucleotide T is present in the second chain and where nucleotide G is present in one chain, nucleotide C is present in the other chain. If the two chains are separated, which can be achieved, e.g., by heating, the appropriate enzyme and all four nucleotides and a few other things are added, the appropriate complementary chain is formed according to the sequence in each chain so that, finally, instead of one DNA double chain, two identical DNA double chains are obtained (Fig. 3.3 in the book). This is essentially the basis for heredity of genetic information.