Improper cleavage furrow formation in cytokinesis can produce cells with abnormal chromosome numbers, which can lead to the development of cancer cells or birth defects. Cleavage furrow formation and cytokinesis are essential for proper development of cells, tissues, and for proper development of an organism as a whole. The cell cortex is responsible for producing the contractile ring that constricts the cell and "pinches" it into two cells. Asters help to position cleavage furrows so that cytoplasmic division results in two evenly divided cells. It is this contact with the cortex that induces the formation of a cleavage furrow. Some aster microtubules continue to extend until contact is made with the cortex. Microtubules from nearby asters interconnect, which helps to limit expansion and cell size. During the course of cell division, asters growing from centrioles extend their microtublules toward one another. The cell cortex is found directly beneath the plasma membrane and consists of actin filaments and associated proteins. How Asters Induce Cleavage Furrow FormationĪsters induce cleavage furrow formation due to interactions with the cell cortex. The position of the cleavage furrow is determined by the asters. In animal cells, a contractile ring of microfilaments forms a cleavage furrow that pinches the cell in two. The final step of cell division is cytokinesis. Cytokinesis involves the division of the cytoplasm, which separates the dividing cell into two new daughter cells.In telophase, spindle fibers break down and separated chromosomes are enveloped within their own nuclear envelope.This separation is accomplished as spindle fibers shorten, pulling attached chromatids along with them. Duplicated chromosomes ( sister chromatids) separate and are pulled toward opposite ends of the cell during anaphase.Polar fibers extending from the poles interlock like the fingers of folded hands. Chromosomes are kept in place at the metaphase plate by the equal forces of the spindle fibers pushing on the centromeres of the chromosomes. The spindle fibers will move the chromosomes until they are lined up at the spindle equator. They attach at a point called the kinetochore, which is a disk or protein that is on each side of the centromere. Spindle fibers move chromosomes to the center of the cell during metaphase. The spindle fibers from the other side of the cell attach to the other sister chromatids in the chromosome.Asters organize spindle fibers that extend from the cell poles (polar fibers) and fibers that attach to chromosomes at their kinetochores.
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