[Meaning in] molecular biology

DNA sequences of organisms can be thought of as artifacts created by biological evolution, and current data suggests that they contain some long-range correlations not present in typical random sequences. Most likely, however, these have fairly simple origins, perhaps being associated with iterative splicing of subsequences. And in the few thousand proteins currently known, standard statistical tests reveal no significant overall regularities in their sequences of up to a few thousand amino acids. (Some of the 20 standard amino acids do however occur more frequently than others.) Nevertheless, if one looks at overall shapes into which these proteins fold, there is some evidence that the same patterns of behavior are often seen. But probably such patterns would also occur in purely random proteins—at least if their folding happened in the same cellular apparatus. (See page 1003.) Note that the antibodies of the immune system are much like short random proteins—whose range of shapes must be sufficient to match any antigen. (See also page 1194.)