Are lampbrush chromosomes unique to meiotic cells?
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Are lampbrush chromosomes unique to meiotic cells? Joseph G. Gall
Published online: 22 December 2012 # Springer Science+Business Media Dordrecht 2012
Abstract Lampbrush chromosomes (LBCs) are transcriptionally active chromosomes found in the germinal vesicle (GV) of large oocytes of many vertebrate and invertebrate animals and also in the giant singlecelled alga Acetabularia. These cells are all in prophase of the first meiotic division. Nevertheless, many meiotic cells do not develop LBCs, arguing that LBCs are not an essential feature of meiosis. LBCs probably represent the most active transcriptional state that can be attained by cells that must give rise to diploid progeny. Polyploidy permits cells to reach higher rates of transcription per nucleus but precludes a return to diploidy. In this sense, LBCs represent a relatively inefficient transcriptional compromise employed by large meiotic cells. These considerations help to explain why transcriptionally active GVs develop LBCs, but they do not explain why LBCs have never been seen in somatic cells, diploid or otherwise. If LBCs are truly limited to germ cells, then some of their unusual features may reflect reprogramming of the genome. If this is the case, LBCs provide unique opportunities to study reprogramming at the level of the individual transcription unit. Responsible Editor: Herbert Macgregor. J. G. Gall (*) Department of Embryology, Carnegie Institution for Science, 3520 San Martin Drive, Baltimore, MD 21218, USA e-mail: [email protected]
Keywords lampbrush chromosome . meiosis . oocyte . oogenesis . polyploidy . reprogramming . transcription Abbreviations GV Germinal vesicle LBC Lampbrush chromosome Pol II Polymerase II TU Transcription unit
Introduction When Herbert Macgregor asked me to write an introductory essay on LBCs for this volume of Chromosome Research, my original response was to outline the major structural features of these remarkable chromosomes, discuss briefly where they occur, stress their unusually high rate of transcription, and dodge the issue of why they have been found only in meiotic cells. The focus of this essay began to shift, however, when members of my lab and I struggled to understand the results of RNAseq experiments that we carried out during the past 2 years. In hopes of learning more about LBC transcription, we analyzed RNA from germinal vesicles (GVs) and oocyte cytoplasm of Xenopus tropicalis (Gardner et al. 2012). The results for the cytoplasm were not exceptional. The cytoplasm contains spliced transcripts derived from thousands of genes, as described earlier by John Gurdon's group (Simeoni et al. 2012). However,
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J.G. Gall
A major problem for the Xenopus oocyte is to produce the transcripts needed for protein synthesis during oogenesis and early embryogenesis. The strategy used by Xenopus and many other organisms with large oocytes is to develop transcriptionally active LBCs, which produce much more RNA per chromosome than the chromosomes of small somatic cells. This is the “standard” explanation for the oc
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