Correspondence

Date:
1980
Reference:
PP/CRI/H/6/13/4
Catalogue details

Licence: In copyright

Credit: Correspondence. Source: Wellcome Collection.

    34/92
    increase the nuclear surface area and the number of nuclear pores. The suggestion is therefore that excess DNA is used, not as has been incorrectly suggested [by us 4 and by Orgel and Crick 5 ] to slow development, but rather to prevent the excessive slowing of development which would otherwise be caused by large increases in cell size. This apparent indeci siveness as to which is cause and which effect aside, the notion that the only way in which rRNA transport can be increased is by increasing 'skeletal' (non-coding) DNA assumes that there is no easier way to increase the number of nuclear pores than to increase the surface area of the nucleus, and ignores the voluminous data which show that cells which need to produce more rRNA can easily do so by increasing the amount of chromosomal or extrachromosomal rDNA. 10 Cavalier-Smith also claims that the fact that certain species retain excess DNA in germ-line nuclei but eliminate it in somatic nuclei is consistent with the notion that excess DNA is advantageous in the former but disadvantageous in the latter, and that the idea that the extra DNA is retained merely because it cannot be gotten rid of is completely untenable in this case, for a mechanism clearly exists. Clearly it exists in somatic nuclei, but we don't know that it exists in germ-line nuclei, or even in any detail what it is. Such data are not incompatible with the notion of selfish DNA. Selfish DNA has been selected to ensure its own perpetuation in subsequent generations of its host; it has no need to be represented in somatic nuclei. We see no reason not to suppose that there is indeed sometimes selection for decreased developmental rate, increased cell volume and increased nuclear volume.** In cells so selected, excess DNA can accumulate. Whether it does so because large cells with large nuclei require such accumulation, or because they simply permit such accumulation (that is, because phenotypic selection against the proliferation of excess DNA is very weak), is a question not easily answered.