The arrangement and transcription of “middle repetitive” DNA sequences on lampbrush chromosomes of Triturus

Abstract

Nick-translated ³H middle repetitive DNA (C₀t 0.2–50) from Triturus cristatus carnifex was in situ hybridized to RNA transcripts on the loops of lampbrush chromosomes and to the denatured DNA in mitotic chromosomes from intestinal epithelium of the same species. In autoradiographs of lampbrush chromosomes only 20–40 pairs of loops were distinctly and in some cases heavily labelled after exposures of 4–14 d. Most of these fast labelling loops were located on the heteromorphic arms of bivalent I. After longer exposures, certain other loops appeared labelled, but few were so heavily labelled as those on chromosome I. There were fast labelling loops in positions corresponding to those of the nucleolus organizers on chromosomes VI and IX. The pattern of distribution of fast labelling loops was different on each of the heteromorphic arms of bivalent I. Some fast labelling loops were present on chromosome I of larger oocytes but not evident in smaller oocytes from the same animal. Some of the fast labelling loops were labelled only over part of their lengths. Partial labelling was seen over thick and thin halves of asymmetrical polarized loops and over short intermediate segments of some loops. — Autoradiographs of mitotic chromosomes showed patchy labelling of all chromosomes, but no distinctive pattern of labelling on chromosomes I. — These observations are discussed in relation to current ideas about the organization of DNA sequences in chromomeres and loops of lampbrush chromosomes, and it is proposed that the abundance of fast labelling loops on the heteromorphic arms of bivalent I may somehow be related to the absence of meiotic crossing over in these regions.