Trinity College (Dublin, Ireland). Department of Microbiology
Abstract
THESIS 8384The typical eukaryotic human diploid cell contains 3.2x10 9 base pairs of deoxyribonucleic
acid (DNA) which, if presented in an extended form, would measure 1.2m in length. The
large amount of DNA is tightly wrapped and compacted through its interaction with a set
of small basic proteins called histones (Alberts et al, 2002). Histones are positively
charged proteins that facilitate the folding of DNA. Four types of histones have been
described: H2A, H2B, H3 and H4 (Baxevanis and Landsman, 1997). A 147 base pair
segment of eukaryotic DNA associates with histones, forming an octameric protein
complex known as the nucleosome (Baxevanis and Landsman, 1998; Grunstein and Mann,
1992). The quaternary structure of the nucleosome is stabilised through an interaction with
an additional histone, H1. This structure is highly conserved. The level of compaction of
DNA varies during the life cycle of the cell. During Interphase, chromatin is relatively
decondensed and distributed throughout the nucleus. As cells enter mitosis, chromatin
condenses into chromosome structures, which are relatively transcriptionally silent. As the
cells undergo DNA replication, the newly formed DNA must be repackaged into
chromatin. Therefore the production of histones is tightly controlled to ensure that
maximum production occurs In the S-phase of the cell cycle (Xu et al, 1990)