Takousis, P.; (2007) DNA replication in the human major histocompatibility complex. Doctoral thesis , University of London.

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Abstract

DNA replication is a vital component of the eukaryotic cell cycle. During the course of S-phase, numerous origins of replication become activated along each chromosome. Several adjacent origins fire synchronously to replicate large sections of a chromosome at specific times. Early studies identified a relationship between cytogenetic bands and replication timing: GC-rich R-bands replicate early while AT-rich and gene poor G-bands replicate late, apparently regardless of differentiation and developmental status. Subsequent studies revealed that other factors such as transcriptional status also influence the replication programme. The aim of this thesis is to examine the organisation of DNA replication in the human Major Histocompatibility Complex (MHC) on chromosome 6, and understand how it relates to gene expression and inherent genomic properties. A previous investigation from the Human Cytogenetics Laboratory using fluorescence in situ hybridisation (FISH) suggested that replication timing of the MHC is organised into distinct zones, with the MHC class II region, an AT-rich isochore, replicating later than neighbouring regions. Using a biochemical approach, the entire MHC was found to replicate within the first half of S-phase in cell lines derived from different tissues. Subsequent analysis of a B-lymphoblastoid cell line using a high resolution tiling path array for the MHC confirmed that a large proportion of the MHC class II replicates later than its neighbours. The data suggested the existence within the MHC of replication origins that fire at distinct times in S-phase. An investigation of replication initiation in the MHC revealed the presence of several potential initiation sites, which were further analysed by quantitative PCR. The gene-rich MHC class III was found to have a relatively large number of replication initiation sites. Overall, these results suggest that either specific origins of replication or zones of initiation can fulfill the replication requirements of a region.

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