# Structural Biochemistry/Animal Cytokinesis
## What is Cytokinesis in an Animal Cell?

Cytokinesis is the final step in cell division in which a cell physically splits into two cells. In an animal cell, cytokinesis involves compressing the cytoplasm of the cell inward until the cytoplasm splits into two equal parts. Both the cytoplasm and the nucleus equally split into two parts.

## What is a Parts List for Cytokinesis and how is it determined?

A parts list for cytokinesis is an inventory of all the molecules involved in cytokinesis. To determine the genes involved in cytokinesis and their functions, RNA interference (RNAi), genome sequencing, and proteomics are used.

## What are the Subprocesses in Cytokinesis?

First (1), the cell goes into metaphase in which the microtubules of the mitotic spindle align chromosomes are the metaphase plate. Second (2), the cell in now in late anaphase in which the microtubules have extended and contacted the cortex. The region of bundled microtubules between the chromosomes and the midzone also rearranges. At the equator, components of the cleavage furrow are developments. Third (3), the cell is in early telophase in which the cleavage furrow enters by compressing the midzone. (4) As a result of this compression, a intercellular bridge that contains a microtubule midbody is created. The formation of two daughter cells from the intercellular bridge completes the process of cytokinesis.

## What is Cleavage Plane Specification?

To guarantee equal separation of the genome, the cleavage is present perpendicular to the axis of where the chromosome splits. The equal separation of the chromosome usually occurs at the middle of the cell. Unequal separation of the cleavage often results in spindle fibers incorrectly positioning during the embryonic and stem cell divisions in adult homeostasis. Research has shown involvement of the microtubles and the actin cortex in cleavage plane specification. A cleavage furrow is formed when proteins at the equator move and add towards the positive side of the equator. As a result, myosin II activation and actin polymerization are implicated by the Rho pathway. If there Rho pathway is not present in mammalian cells, cleaving can be done without the presence of myosin II.

## Why Does Failure of Cytokinesis Lead to Cancer Progression?

Many cancer cells have a chromosome number that is not an exact multiple of the haploid number. As a result, this allows a faster evolution of the cells genotype. The failures of mitosis and cytokinesis reveal polyploidy and single chromosome abnormalities in solid tumors. Researchers predict that cancer cells and solid tumors both arise from the defects of the spindle assembly and cytokinesis. Through further research of carcinogenic mice, researchers found a causal connection between the failure of cytokinesis and carcinogenesis. Further research is being done to develop therapeutic cancer drugs that selectively block cytokinesis in cancer cells without destroying the normal stem cells.

## Reference

Eggert, Ulrike S., Field, Christine M., Mitchison, Timothy J. Animal Cytokinesis:From Parts List to Mechanisms. Annual Review of Biochemistry. Web. 30 November 2011. <http://ucelinks.cdlib.org:8888/sfx_local?sid=Entrez:PubMed&id=pmid:16756502>