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Title: The cytology and biochemistry of DNA amplification in the ovary of Xenopus laevis
Author: Bird, Adrian
ISNI:       0000 0001 2416 9933
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1971
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Amplification of the ribosomal RNA genes is a feature of early oogenesis in many organisms. This thesis is concerned with some biochemical and cytological aspects of the process, as it occurs in the anuran, Xenopus laevis. Using the technique of (³H) thymidine autoradiography, meiotic amplification has been timed at 24 ± 3 days. First signs of nucleolar DNA synthesis during meiosis are seen in zygotene nuclei - often at several sites on the nuclear periphery. In pachytene nuclei, all parts of the amplified DNA cap are active in rDNA synthesis. Autoradiography of ovaries after (³H) uridine incubation has provided general information about RNA synthesis at this time. Chromosomal RNA synthesis increases dramatically from leptotene, through pachytene, to diplotene. It is accompanied by a similar increase in nucleolar DNA synthesis. Thus, throughout amplification, a site within the replicating nucleolar DNA remains active in RNA transcription. Towards the and of pachytene, more than one site of RNA synthesis in the amplified DNA is visible. The number of sites increases further during early diplotene. Biochemical investigations have shown that meiotic amplification is intimately dependent upon concurrent protein synthesis. Partial inhibition of ovarian protein synthesis leads to an equivalent inhibition of nucleolar DNA synthesis. This implies that the rate of amplification is normally limited by the rate of protein synthesis in the oocyte. Similar studies using an inhibitor of RNA synthesis produced less clear cut results, though it is likely that RNA synthesis is less intimately linked with the amplification process. The possibility that amplification occurs by an RNA-dependent mechanism of DNA synthesis has been investigated. Whilst not conclusively discounting such a mechanism, the results do not confirm any of the predictions of this hypothesis. The exact molecular mechanism for amplification remains unknown. Various proposals are discussed, and an extrachromosomal mechanism is found to be most likely.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available