Mitosis and its Regulation

There is an anaphase-promoting complex/cyclosome factor or APC/C. Its function is to break down M cyclins so the cell can begin to enter the G1 phase again. It destroys the proteins that hold the sister chromatids together so that they can separate. This is an enzyme that attaches ubiquitin to a target protein, making the target recycled by the proteasomes of the cell. This ubiquitin tagging triggers the separation of sister chromatids by destroying cohesin that normally binds chromatids.

The cyclin-dependent kinases and the cyclins are direct cell cycle regulators. They respond to cues that can be inside or outside the cell. There are positive cues, such as growth factors, and negative cues, such as DNA damage, that block activity of cell growth and division in the cell cycle.

The products of the gene p53 act as regulators of cell cycling. These products will stop the cycle at G1 by producing CDK inhibitor proteins. These will block the activity of CDK-cyclin complexes so that DNA repair can take place. If DNA damage is not fixable, apoptosis will happen so that damaged DNA does not get passed onto the next generation. In this way, p53 prevents the passage of mutations to the next cell generation. If insufficient, cancer can develop.

MITOSIS AND ITS REGULATION

Mitosis happens in eukaryotic cells in order to produce two daughter cells that are the same genetically. Chromosomes are copied in order to allow for their separation in the process of mitosis. The process takes about an hour in order to happen. There is a mitotic apparatus that aligns the chromosomes and separates the different sister chromatids. Karyokinesis is the mitotic nuclear division, while cytokinesis is the actual cell division process. Mitosis is continuous but it is typically divided into five separate stages.

The five stages of mitosis are prophase, prometaphase, metaphase, anaphase, and telophase. Figure 60 is a depiction of mitosis in the eukaryotic cells: