Crick here elaborated on his idea that chromatin, the assembly of DNA and histones (proteins) that is found on the chromosomes
of higher organisms, had a solenoid, or pipe-shaped, coiled form. In his letter he pointed out the limits of this idea, especially
with regard to the irregular arrangement of the nucleosomes, distinct complexes of histone and DNA in the cells of higher
organisms which are part of chromatin and which under the electron microscope appear as bead-like bodies on a string of DNA.
Item is a photocopy.
Number of Image Pages:
1 (93,011 Bytes)
1979-12-10 (December 10, 1979)
Thomas, Charles A. Jr
Original Repository: Wellcome Library for the History and Understanding of Medicine. Francis Harry Compton Crick Papers
I think I should make more explicit my objection to your chromosome model. This is that there is no evidence to suggest that
the "solenoid" type of nucleosome assembly is highly regular. Nothing as regular as TMV, for example, has ever been
seen in chromatin and there are very sound reasons to suspect that it cannot be regular. This is because if there has to
be this high degree of regularity the axis of the solenoid must be straight. If the solenoid axis were bent or helical then
necessarily the packing between nucleosomes in a solenoid will not repeat exactly at each nucleosome. The further the deviation
from the straight, the greater the disturbance to the packing.
No one has ever succeeded in proposing successfully a model of a chromosome consisting almost entirely of straight solenoids.
The most likely models are those in which the solenoid axis is either bent in a circle or coiled into another helix at a higher
level or coiled in an irregular way. The data provided by Laemmli and other evidence suggesting domains in chromosomes, almost
demands such a type of super-folding.
For these reasons I think that the packing of nucleosomes into a solenoid is not a highly precise one and the variation in
the length of DNA per nucleosome also suggests that this is the case. I think it most unlikely that, with this situation,
you can have the sort of process you have suggested.
For these reasons I do not merely think that your model may perhaps be wrong. I feel rather strongly that it is highly likely
to be wrong! Of course if you can persuade me to change my opinion I would be only too delighted.