Figure 1. Information transfer between DNA, RNA and protein macromolecules. (a) Complementary base pairing in a short segment of DNA. The four types of nucleotides are represented by the bars (solid or open, long or short), and each nucleotide can pair with only one complement. (b) Information transfer between DNA (drawn as in part a) and RNA (green). RNA can also be used to specify DNA by an analogous mechanism. (c) Information transfer from RNA to protein. The 20 types of amino acids (the four oval shapes represent four of these) are linked according to the sequence of nucleotides in the messenger RNA, which in turn was copied from the gene in part b. The next amino acid which will be added is shown with its adaptor transfer RNA (tRNA) attached. (d) The central dogma of molecular biology specifies the forbidden information transfers.