Eukaryotic Chromosome

The compact nature of chromatin structure presents a barrier to processes that require access to DNA, such as replication and transcription. It seems likely that the separation of parental DNA strands during replication must disrupt higher-order chromatin structure to at least some degree. In the Drosophila polytene chromosomes, this disruption can be seen, at least in part, as puffs at sites of DNA replication. It is unclear, however, what happens to the nucleosomes during this process. If the nucleosomes are removed during replication, they are quickly reassembled, for there is no time period during which microscopic analysis shows the DNA to be free of nucleosomes.

Transcriptional activation of eukaryotic genes requires that the machinery responsible for synthesizing the RNA gain access to the regulatory regions of the DNA that control gene expression. This again necessitates some decondensation of the chromatin structure. Decondensation can be facilitated by protein complexes known as chromatin-remodeling enzymes. Chromatin-remodeling enzymes alter the structure of chromatin in such a way that regulatory factors can gain access to the DNA. These enzymes are divided into two groups, those that chemically modify histones and those that use the energy derived from ATP hydrolysis to alter histone-DNA linkages.