Working Drafts: "Selfish DNA : the ultimate parasite"

Date:
[1980]
Reference:
PP/CRI/H/6/13/2
Catalogue details

Licence: In copyright

Credit: Working Drafts: "Selfish DNA : the ultimate parasite". Source: Wellcome Collection.

    4/184
    simple sequences, an extreme example of which is the very large amounts of fairly pure poly d(AT) found in certain crabs. Simple sequences, which are situated in chromosomes largely but not entirely in the heterochromatin, are usually not transcribed. Another class of repetitive sequences, the so-called intermediate repetitive, have much longer and less regular repeats. Such sequences are interspersed with unique DNA at many places in the chromosome, the precise pattern of interspersion being to some extent different in different species. Leaving aside genes which code for structural RNA of one sort or another (such as transfer RNA, ribosomal RNA, etc.) which would be expected to occur in multiple copies (since, unlike protein, their final products are the result of only one stage of magnification, not two) the majority of genes coding for proteins appear to exist in single copies, meaning here one or a few. A typical example would be the genes for a-globin, which occur in one to three copies and the human ß-like globins, of which there are four main types, all related to each other but used for slightly different purposes. Notable exceptions are the proteins of the immune system, and probably those of the histocompatibility and related systems. Another exception is the genes for the five major types of histone which also occur in multiple copies. Even allowing for all such special cases, the estimated number of genes 9 in the human genome appears too few to account for the 3x10 base-pairs found per haploid set of DNA, though it must be admitted that all such arguments are very far from conclusive.