That's because you have to satisfy this base pair matching in order to have hydrogen bonding in each of the struts of the ladder in order to form a stable structure.

NA双链要满足互补配对原则,以使碱基对之间,形成氢键,从而形成稳定的双链结构

耶鲁公开课 - 生物医学工程探索课程节选

Well you do that by this Watson-Crick base pairing, so I know if I have I know what messenger RNA to make from that because I have to satisfy these base pairing rules.

依据的是沃森克里克碱基互补配对原则,如果给我一个ACGCGA的序列,就能知道转录出的mRNA会是什么,因为我必须满足碱基配对原则

耶鲁公开课 - 生物医学工程探索课程节选