John H. Frenster,
Laboratory of Cell Biology
Rockefeller Institute
New York, NY 10021

The chromosomal karyotype and gene-linkage pattern within a cell normally remain constant from one mitosis to the next (8), indicating that the structural integrity of each chromosome is maintained throughout interphase (9). This implies that the repressed and active portions of a chromosome in favourable material might display structural continuity during cell interphase (10).

In the course of the isolation of repressed and active chromatin from interphase calf thymus lymphocytes (6), a stage is reached in which the nuclei swell to twice their normal size, offering a favourable material for the examination of the structural relations between the active chromatin microfibrils and the repressed chromatin masses. Such swollen nuclei were prepared and examined as described previously (6).

The condensed masses of repressed chromatin tend to be arrayed at the periphery of the swollen nucleus (Fig. 1), while the extended microfibrils of active chromatin are dispersed between the masses within the interior of the nucleus (Fig. 1).

Fig. 1. Swollen nuclei displaying condensed masses of repressed chromatin and extended microfibrils of active chromatin (x 16,875).
Higher magnifications reveal the active chromatin microfibrils to be of 100 A. diameter (Fig. 2), and these microfibrils can be traced for up to 1.0 um. of their length (Fig. 2).

Fig. 2. Detail of swollen nucleus displaying the structural continuity of active microfibrils with repressed masses of chromatin (x 45,000).
These extended microfibrils of active chromatin are seen to be structually continuous with a dense reticulum of fibers within the condensed masses of repressed chromatin (Fig. 2). The zone of transition between the extended microfibrils and the condensed masses is sharp, occurring within less than 100 A. of the length of the microfibrils. 

The molecular basis of these structural transitions may lie in the excess of such polyanions as phosphoproteins, RNA, and phospholipids recently found within extended active chromatin as compared to condensed repressed chromatin (11). These chromatin polyanions can function as de-repressors of RNA synthesis within repressed chromatin by antagonizing the electrostatic interaction between the deoxyribonucleic acid (DNA) and the polycationic repressor histones of repressed chromatin (11).

I thank Drs. A.E. Mirsky, V.G. Allfrey, V.C. Littau, and T.A. Langan for their advice.

This work was supported by a research career development award from U.S. Public Health Service.

References:

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6. Frenster JH, Allfrey VG, and Mirsky AE, Proc. U.S. Nat. Acad. Sci. 50: 1026 (1963).

7. Littau VC, Allfrey VG, Frenster JH, and Mirsky AE, Proc. U.S. Nat. Acad. Sci. 52: 93 (1964).

8. Wilson EB, "The Cell in Development and Heredity", third ed. (New York: Macmillan Co., 1928).

9. Ris H, and Mirsky AE, J. Gen. Physiol. 32: 489 (1949).

10. Frenster JH, J. Cell Biol. 23: 117 A (1964).

11. Frenster JH, J. Cell Biol. 23: 32 A (1964). 

1. Electron Microscopy of Human Lymphocytes before and after Activation by PHA (Busch H, 1974).

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

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