Entanglement and crosslinking of chromatin in the mitotic chromosome

During mitosis chromosomes are condensed about 10,000 fold compared to the naked DNA length. Eight histone proteins make a nucleosome core. Nucleosomes bind DNA and make chromatin fiber, and chromatin fiber is compacted about 30 fold compared to the naked DNA length. The mechanism how chromatin fiber is further compacted is not fully understood. Previous results showed that a metaphase chromosome is a crosslinked network of proteins and DNA. We have further studied the condensation mechanism by micromanipulation techniques.;We have shown that DNA entanglement plays a major structural role in metaphase chromosomes in experiments using topoisomerase II. Topoisomerase II is a large dimeric ATPases which catalyzes transient dsDNA cleavage and passes another DNA strand.;The interiors of cells are protected from oxidation and disulfide bonds are rarely formed in them. However, we found that disulfide bonds play a major structural role in metaphase chromosomes.;CALI is a technique developed to inactivate specific proteins using fluorescently labeled antibodies. We planned to use this technique to cleave specific proteins. However, we found that CALI crosslinks proteins rather than cleaving proteins. Use of fluorescence DNA dyes leads to strong stiffening of metaphase chromosomes, in accord with this observation.;Meiosis is another type of cell division. We have studied the basic mechanical structures of mouse meiotic metaphase I chromosomes.