Ribonucleic Acid Polymerase Activity in Extended and Contracted Chromosomes

Abstract

VARIOUS reports have appeared in the literature concerning the absence of RNA synthesis in metaphase cells. An explanation commonly given is that the condensed or compacted state of chromosomes prohibits DNA transscription¹. This explanation is supported by evidence that regions of condensed chromatin in an interphase nucleus are sites of minimal RNA synthesis². We wish to report some results of recent studies on RNA metabolism during the meiotic cycle which provide additional evidence on the relationship between the physical state of the chromosomes and RNA synthesis. Two features of meiosis are relevant to this report: (1) Cells in early prophase of meiosis (leptotene) synthesize RNA at a much higher rate than those at metaphase³. Indeed, it is probable that no RNA is synthesized during metaphase. (2) At leptotene, chromosomes are in their most extended form; at metaphase, they are in their most contracted form. The biochemical and cytological data are thus consistent with the view that the capacities of chromosomes to transcribe DNA into RNA are inversely related to their degree of condensation. We addressed ourselves to the question of whether in vitro studies of isolated nuclei and chromosomes could yield information on the respective roles of RNA polymerase and template availability in accounting for established observations. Meiotic cells obtained from anthers of Tulipa gesneriana were used in these experiments.