DNA
Adenine (A) and thymine (T) can pair up because they make two hydrogen bonds, and cytosine (C) and guanine (G) pair up to make three hydrogen bonds. Although the bases are always in fixed pairs, the pairs can come in any order (A-T or T-A; similarly, C-G or G-C). This way, DNA can write 'codes' out of the 'letters' that are the bases. These codes contain the message that tells the cell what to do.This phenomenon is known under the name of Complementary base pairing or Watson-Crick base pairing that determines the essential forming of the helical structure of DNA as due to the base pairing, the DNA strands are complementary to each other, running in opposite directions, that forms anti-parallel strands,The main reason why DNA strands are anti-parallel is due to the high level of stability achieved in an anti-parallel configuration.. Moreover,it is also important in replication as it allows semi-conservative replication.