Reference no: EM132954030

In the Dec. 14, 2004, issue of Current Biology, Leocadia Paliulis and Bruce Nicklas report their progress in understanding how the pairs of chromosomes in each cell manage to balance their adhesion to one another and their release during cell division. Their work was sponsored by the National Institutes of Health. Chromosomes are the tiny fibre structures in the cell that house its genes. They replicate and separate in the process of cell division.

The exquisite management of adhesion properties between newly divided chromosomes, called chromatids, is crucial if the cells are to divide properly. In this process chromatids are drawn apart to separate poles of the dividing cell so that each new "daughter" cell contains a single copy of each. The same basic process operates in normal cell division, called mitosis, as well as the proliferation of sperm and egg cells called meiosis.

"Chromosomes in mitosis and meiosis have to be held together, because otherwise they don't attach to the apparatus called the spindle that distributes them to opposite poles," explained Nicklas, who is a Research Professor of Biology. "If they're held together, then one replicated chromatid can attach to one pole and the other to the opposite pole. But if they are not held together, they attach independently, and often both sister chromatids can go to the same pole rather than to opposite poles. This creates chromosome imbalances that can lead to cancer or chromosomal abnormalities that cause birth defects."

According to Nicklas, it was known that the two sister chromatids adhered to one another and released at the appropriate time during cell division. However, that understanding was based on biochemical experiments that revealed when the "glue" protein called cohesin that holds chromatids was degraded during cell division. Also, microscopic studies had shown that there appeared to be two separate chromatids during an early stage of cell division, so it was believed that they had detached from one another at that time.

"What hadn't been done was to attempt to separate chromatids to directly determine whether they, in fact, are held together or not," said Nicklas. "So, Leocadia set out to use micromanipulation to distinguish between visible separateness and physical separateness."

Q: Using the above information identify the phase or phases in mitosis where the chromosomes must be held together. Explain why it is important that the chromosomes be held together during this phase(s).