Repeating sequences and gene duplication in proteins

The theory that proteins have evolved by repeated internal duplication of short segments of polypeptide chains has been tested by looking for repeats and near repeats in over 50 different proteins, many of them of known structure. The probability that the observed repeats could arise by chance has been calculated. The search does not yield a single new example where the evidence for gene duplication is compelling. No protein shows a unique internally consistent pattern of repeats which both correlates with repeats in the structure and cannot be explained by chance. The evidence is discussed in detail for haemoglobin, chymotrypsin, subtilisin and carboxypeptidase. The evolution of complex large proteins from simple small ones has probably been a process of continuous growth in which chains have been gradually added to the outer surface surrounding an invariable core near the active centre.