Presented at the Fourth Annual Meeting of The American Society for Cell Biology, Cleveland, Ohio, November 11, 1964, and published in: J. Cell Biol. vol. 23, no. 3, p. 117a (1964):

"Structural Continuity between Active and Repressed Chromatin within Interphase Lymphocytes".

John H. Frenster
The Rockefeller Institute, New York, New York


Abstract:

Prelabeling of RNA by uridine-2-C14 before the isolation of chromatin fractions from interphase calf thymus lymphocytes has demonstrated that repressed chromatin is visible as condensed masses of heterochromatin, while active chromatin is visible as extended microfibrils of euchromatin (Frenster, Allfrey, and Mirsky, Proc. Natl. Acad. Sci., 1963, 50, 1026). These results have recently been confirmed by electron microscope radioautography (Littau et al., Proc. Natl. Acad. Sci., 1964).

When isolated extracted nuclei of such lymphocytes are suspended in cation-free isotonic sucrose, they swell to twice their normal size, spreading the extended euchromatin microfibrils, and permitting a better visualization of  the relations between active and repressed chromatin. Electron microscopy of such preparations reveals that the 100-A euchromatic microfibrils are structurally continuous with 250-A fibrils densely packed within the heterochromatin masses. The euchromatic microfibrils extend radially from the masses for distances up to 0.25 micron, and individual microfibrils can be traced for distances up to 1.0 micron.

The structural continuity and the sharp transition from an extended to a condensed state suggest a high degree of localized repression or activity within the interphase chromatin of lymphocytes, and are compatible with a lateral loop model of chromosome structure as recently proposed by Nebel, Callan, and Gall; Moses; and Painter.

Supported by an award (CA-17857) from the USPHS.


Additional References:

1. Frenster JH, "Repression and De-repression within Interphase Chromatin of Lymphocytes", J. Cell Biol. vol. 23, no. 3, p. 32a (1964).

2. Frenster JH, "Ultrastructural Continuity between Active and Repressed Chromatin" Nature 205: 1341 (1965).

3. Frenster JH, "Nuclear Polyanions as De-repressors of Synthesis of Ribonucleic Acid", Nature 206: 680 (1965).

4. Frenster JH, "Mechanisms of Repression and De-Repression within Interphase Chromatin", In Vitro, vol. 1, pp. 78-101 (1965).


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euchromatin:  "the most active portion of the genome within the cell nucleus".