The two chains are held together by hydrogen bonds between pairs of bases. The precise sequence of bases carries the genetic information. The most important aspect of the DNA double helix is the specificity of the pairing of bases. Watson and Crick deduced that adenine must pair with thymine, and guanine with cytosine, because of steric effects and hydrogen bondings. We turn now from the storage and transmission of genetic information to its expression.
Genes specify the kinds of proteins that are made by cells.
However, DNA is not the direct template for protein synthesis. The flow of genetic information in normal cells is from DNA to RNA to protein. RNA is a long, unbranched macromolecule consisting of nucleotides joined by 3', 5'-phosphodiester bonds. As the name indicates, the sugar unit in RNA is ribose. Adenine can pair with uracil, and guanine with cytosine.