Chromosomes function/replication

A FIGURE 10-27 Microscopic appearance of typical metaphase chromosome. Each chromosome has replicated and comprises two chromatids, each containing one of two identical DNA molecules. The centromere, where the chromatids are attached, is required for their separation late in mitosis. Special telomere sequences at the ends function in preventing chromosome shortening. 

Although chromosomes differ In length and number between species, cytogenetic studies have shown that they all behave similarly at the time of cell division. Moreover, any eukaryotic chromosome must contain three functional elements In order to replicate and segregate correctly (1) replication origins at which DNA polymerases and other proteins initiate synthesis of DNA (see Figures 4-34 and 4-36), (2) the centromere, and (3) the two ends, or telomeres. The yeast transformation studies depicted In Figure 10-32 demonstrated the functions of these three chromosomal elements and established their importance for chromosome function. 

The basis for selective inhibition of chromosome replication and function... 

While some histone variants can become deposited during DNA replication, certain variants also are assembled into chromatin in a replication-independent manner (reviewed in Jin et al. 2005). This allows the incorporation of histone variant into chromosomal regions with high levels of histone turnover. Histone variants can distinguish the affected nucleosomes from their canonical counterparts and are likely to have important function in the specialization of chromatin domains and their epigenetic maintenance. 

Telomeres are r etitive sequences at the ends of linear DNA molecules in eukaryotic chromosomes. With each round of replication in most normal cells, the telomeres are shortened because DNA polymerase cannot complete synthesis of the 5 end of each strand. This contributes to the aging of cells, because eventually the telomeres become so short that the chromosomes cannot function properly and the cells die. 

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