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Dissociation Model And The Dynamics Of Histone Methylation Mitotic Cell Cycle In Nucleosome Maintain

Posted on:2012-12-26Degree:DoctorType:Dissertation
Country:ChinaCandidate:M XuFull Text:PDF
GTID:1110330335981968Subject:Biochemistry and Molecular Biology
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In eukaryotic cells, histones are packaged into octameric core particles with DNA wrapping around to form nucleosomes, the basic units of chromatin (Kornberg and Thomas,1974). An adult animal contains over 200 different cell types, each of them has specialized structure and distinct physiological function (Allis et al.,2006). With a few exceptions, all of these cells carry the same genetic information encoded by DNA. Multicellular organisms utilise chromatin marks to translate one single genome into hundreds of epigenomes for their corresponding cell types. During development and adult homeostasis, it is important to appropriately maintain the cromatin state after each cell division, as unscheduled compromise might lead to developmental disorder or disease. Thus, the molecular mechanism underlying epigenetic inheritance is highly interesting. Furthermore, unlike genetic information, which is a means of highly stable, epigenetic information stands for a certain level of plasticity and reversibility. The inheritance of epigenetic state may not be as faithful as that of genetic information.Histone and DNA modifications provide key epigenetic information. Newly synthesized DNA strand acquires DNA methylation pattern by copying the pre-existing DNA methylation signature from the template strand (Klose and Bird,2006). However, the mechanism by which patterns of histone modifications are passed on to daughter cells through mitotic divisions remains enigmatic. To understand this, the DNA replication-dependent nucleosome partition pattern must be unveiled first. Mono-nucleosomes containing either existing Flag-H3 from parental cells or newly deposited Flag-H3 were selectively purified. Stable isotope labeling-based quantitative mass spectrometry generated high-resolution, quantitative profiles for all co-purified individual native core histones. Surprisingly, significant amounts of H3.3/H4 tetramers split in vivo, while a vast majority of the H3.1/H4 tetramers remained intact. Inhibiting DNA replication-dependent deposition significantly reduced the level of splitting events, which suggest:1. the replication-independent H3.3 deposition pathway proceeds largely by cooperatively incorporating two new H3.3-H4 dimers; 2. detectable amounts of splitting events were seen during replication-independent deposition, but the majority of splitting events occurred during replication-dependent deposition.Histone lysine methylation has been implicated in transcription activation, gene silencing, heterochromatin formation and DNA repair. However, a thorough understanding of the dynamics of global methylation levels remains to be established. Using stable isotope labeling and quantitative mass spectrometry, we analyzed re-establishment of histone lysine methylation in dividing cells, and maintenance of methylation pattern in non-cycling cells experiencing an extended G1/S. Here, we report that:1) histone methylation levels are disturbed during S phase, and become gradually re-established at subsequent cell cycle stages; 2) despite the recovery of overall methylation levels before the next S phase, the methylation levels on parental and newly incorporated histones differ significantly; 3) in non-cycling cells, histone methylation levels eventually reach a plateau stage; 4) both restriction of methyltransferase activity and active turn over of methyl groups contribute to long-term equilibrium of methylation level in non-cycling cells arrested at G1/S. Taken together, we propose that the levels of epigenetic modifications are maintained in a flexible way, rather than in a local, precise manner.
Keywords/Search Tags:epigenetic inheritance, nucleosome split, histone methylation
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