Figure 26.5: Packing of purine and pyrimidine in DNA double helix.

What is complementarity means to you????

It means, if I will provide you the sequence of nucleotide on one strand, it will let you to predict very precisely the sequence of nucleotide on other strand. For example as given in Figure 26.6, every appearance of “A” will give “T” and “G” will give “C” on the second strand.   

Figure 26.6: Both strands in DNA are complementary in DNA double helix.

As discussed earlier, individual strand of DNA runs in the direction of 5´-3´ and the other strand run in the direction of 3´-5´. Hence, both strands are running in the antiparellel direction to maintain base complementarity. The presence of complementarity in base pairing and running of strand in the antiparellel direction allow precise duplication of DNA through replication (Figure 26.7). Understanding the base pairing required, chargaff’s has proposed rule about composition of DNA. The summary of this rule is as follows :