[Proofs]

Date:
1977
Reference:
PP/CRI/H/6/2/1
Catalogue details

Licence: In copyright

Credit: [Proofs]. Source: Wellcome Collection.

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    from two or more chromatids joined together. There is no evidence for this and we consider it unlikely. The Unit Fiber as a Supersolenoid . The above arguments are independent of the exact arrangement of DNA and protein in the unit fiber. We now assume that the DNA in one chromatid consists of one long DNA molecule as suggested by recent work (see for example ref. 10), that the nucleofilament — the string of closely apposed nucleosomes — is indeed folded into a solenoid of about 300 A diameter (3,4), and that the solenoid is folded once more into a further helix, or supersolenoid, of a diameter about 4000 A (= 0.4 jam) to give the unit fiber. Since the contraction ratio for the solenoid itself is believed to be about 40 the calculated contraction ratio for the unit fiber comes to some figure in the region of 1300 to 1500 depending on the exact parameters assumed, in very good agreement with the estimate given earlier. We have looked to see whether we could observe the cross- section of such a structure by examining thin sections of the fibers in the electron microscope. Several cross-sections of the predicted dimensions have been found, some of which are illustrated in Fig. 3. These pictures show a fiber, often somewhat irregular, of a diameter of 300 A coiled to form a hollow structure of about 4000 A diameter. In some cases as in Fig. 3D, the helical character of the supersolenoid was apparent in the 600 to 800 A sections. This finding agrees with the hollowness of the structure suggested by the