"General model for the chromosomes of higher organisms"

Date:
1971
Reference:
PP/CRI/I/1/26
Catalogue details

Licence: In copyright

Credit: "General model for the chromosomes of higher organisms". Source: Wellcome Collection.

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    (.Reprinted from Nature, Vol. 234, No. 5323, pp. 25-27, November 5, 1971) flu, CapVf fkob _ General Model for the Chromosomes of Higher Organisms FRANCIS CRICK The model suggests that chromosomal DNA falls into two classes: globular DNA (containing unpaired regions for control) and a much smaller fraction consisting of fibrous DNA which alone codes for proteins. I wish to propose a general model for the structure of the chromosomes of higher organisms *. This model is derived from ideas and data from many sources t- Because I have found it impossible to set out my ideas and the supporting evidence in a short space, I merely summarize here my conclu sions. A much fuller account is in preparation and will be submitted for publication in the near future. The model assumes that the DNA in a chromatid is a very long mononeme (see the review by Prescott 2 and the recent careful work by Laird 3 ), which probably runs continuously from one end of the chromatid to the other. * I have used the term higher organisms rather than eukaryotes because I want to avoid having to discuss, at this stage, the chromo somes of various lower eukaryotes such as the dinoflagellates and the fungi. t Proper acknowledgments will be given in the fuller paper, but I cannot refrain from mentioning here the very stimulating theo retical paper by R. J. Britten and E. H. Davidson 1 , which the reader is strongly recommended to read in parallel with this one. It contains extensive references to the earlier literature. The model has three basic features. (1) The coding sequences of the DNA (that is, all those sequences which code for poly peptide chains) are postulated to be mainly, if not entirely, in the interbands (as visualized in the giant polytene chromo somes of the Diptera 4 ). The bands, which contain all but a few per cent of the DNA, are identified as the control elements. This is illustrated diagrammatically in Fig. 1. A genetic com plementation group is usually contained in a band plus an interband. Thus on this view most of the DNA in higher organisms does not code for proteins but is used for control purposes, as al ready suggested by F. Vogel 6 in 1964. I have calculated that the average^ amount of DNA (per mononeme) in an interband of Drosophila 7 •* is enough to code for an average protein of molecular weight 30,000-40,000. I have, therefore, adopted this speculation as a good working hypothesis. (2) The central idea is that the recognition sites, needed for control purposes in higher organisms, are mainly unpaired single stranded stretches of double stranded DNA. I call this the Unpairing Postulate and it is set out diagramma tically in Fig. 2. It has been derived from a theoretical con sideration of the general nature of protein molecules and the probable variety and length of the base sequences which need to be recognized in higher organisms. A particular type of example of this postulate has already been put forward by Gierer 9 . The argument I give is a general one. It springs from the Î The average amount of DNA in the mononeme of an interband had been estimated previously by Beermann 4 .