Abstract
In this paper we review recent work on the structure of chromomeres and loops in lampbrush chromosomes, and we discuss several of the hypotheses that attempt to explain lampbrush function. Regarding lampbrush structure the following conclusions are reached: many, but not all loops probably represent transcription units, involving both unique and repeated sequences. Lampbrush chromomeres evidently contain more than one cistron and cannot be equated with polytene bands. The questions now raised address the functional or developmental relationships that may occur among the cistrons in individual chromomeres. In addition, each chromomere could conceivably contain similar repetitious DNA sequences as previously proposed by others or merely sequences with similar base ratios.
Concerning lampbrush chromosome function, we discuss Master-Slave, several reprogramming hypotheses and Cavalier-Smith's recent proposal. In situ hybridization autoradiographs of labelled DNA to nascent RNA in lampbrush loops indicate that loops are static structures with at least one promoter and that processing begins within the loop. This observation, along with the predominance of single copy sequences in transcripts, precludes the need for a corrector mechanism.
Reprogramming hypotheses implicitly view development as an algorithm with a temporal component which requires an initial reprogramming event or preprogramming. The mechanism mar involve the transfer of information that later participates in celular differentiation to localized regions in the cytoplasm. Changes at the level of the chromatin fiber remain a possibility, but under a static loop model could only involve a small fraction of the total chromatin. The Cavalier-Smith proposal makes some interesting predictions that may resolve the C-value paradox. Concerning lampbrush chromosome structure the requirement of nuc\eoskeletal RNA predicts the intense transcription of untranslatable sequences but does not c\early tie this meiotic stage with embryogenesis.
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