Studies on the loops of amphibian lampbrush chromosomes during the phase of retraction

Abstract

Flannery, A.V. and Hill, R.S. 1987. Studies on the loops of amphibian chromosomes during the phase of retraction. —Herediras 106: 209–218. Lund, Sweden. ISSN 0018–0661. Received August 8, 1986 The process of loop retraction in lampbrush chromosomes of Triturus cristastus carnifex, during oocyte maturation, has been investigated by studying the morphology of the loops isolated under different monovalent cation conditions, using a combination of light and electron microscopy. When lampbrush chromosomes bearing retracted loops were prepared in buffer containing reduced levels of monovalent cations (0.7 mM), the mean axial loop length was increased by at least 2.5 times over the mean length of loops prepared under ionic conditions which stabilize normal lampbrush structure (i.e., 70 mM cations). This cation-dependent change in loop structure indicates that the retracted appearance of the loops in maturing oocytes involves the folding of the loop axis. RNA transcription appears to continue during retraction since ribonucleo-protein transcripts are clearly present on the loop axes at all ionic concentrations used and in Miller spreading studies. In Miller spreading preparations, however, the density of transcripts is clearly less in transcription units from retracted loops. Direct evidence concerning the structure of the loop axis at cation concentrations of 0.7 mM and above has not been obtained because the axis remains obscured by RNP transcript under these conditions. However, a model is proposed which accounts for the folding of the loop axis and its simultaneous transcription by RNA polymerases.