Oocytes Without Lampbrush Chromosomes

Abstract

In order to discuss this topic, the first requirement is a definition of what is intended by the term lampbrush chromosome. Lampbrush chromosomes are restricted to the diplotene stage of meiosis. They consist of intermittently distributed compact regions of chromatin, the chromomeres, that are for the most part inactive in RNA synthesis, and likewise intermittently distributed regions of extended DNP, the lateral loops, on which RNA transcription occurs. In organisms with relatively high C-values the lateral loops are visible in the light microscope because in them the units involved in transcription are unusually long, and the RNA transcripts with their associated ubiquitous “core” proteins are either exceptionally densely packed or, if the transcripts are relatively sparse, bulky aggregates of accessory proteins have accumulated amongst them. As will become apparent later in this chapter, though this may appear to be an acceptable working definition, and it is certainly valid for the lampbrush chromosomes of Amphibia, it is not entirely satisfactory when other organisms are being considered. I will return to the vexing definition question in Chap. 7.