Kinetochore

Kinetochore
During cell division, the kinetochore and microtubule pull chromosome copies away from each other. Zina Deretsky, National Science Foundation

The kinetochore is a specialized region on the centromeres of chromosomes. The kinetochore is composed of a number of protein complexes and its purpose is to move chromosomes during mitosis and meiosis. Kinetochore fibers extend from the kinetochore region and attach chromosomes to microtubule spindle polar fibers. These fibers work together to separate chromosomes during cell division.

Kinetochores form ​in the central region of a duplicated chromosome known as the centromere.

The centromere joins two identical copies of a single chromosome known as sister chromatids together. A kinetochore consists of an inner region and an outer region. The inner region is bound to chromosomal DNA. The outer region connects to spindle fibers. Kinetochores also play an important role in the cell's spindle assembly checkpoint. During the cell cycle, checks are made at certain stages of the cycle in order to ensure that proper cell division takes place. One of the checks involves making sure that the spindle fibers are correctly attached to chromosomes at their kinetochores. If not, the dividing cell could end up with an incorrect number of chromosomes. When errors are detected, the cell cycle process is halted until corrections are made. If these errors or mutations can not be corrected, the cell will self-destruct in a process called apoptosis.

Mitosis

In metaphase of mitosis, kinetochores and spindle fibers help to position chromosomes along the central region of the cell called the metaphase plate.

During anaphase, polar fibers push cell poles further apart and kinetochore fibers shorten in length. Kinetochores tightly grip polar fibers as they are pulled toward the cell poles. As a result, sister chromatids are separated and pulled centromere first toward opposite cell poles. At the end of mitosis, two daughter cells are formed with the full complement of chromosomes.

Meiosis

In meiosis, a cell goes through the dividing process two times. In meiosis I, kinetochores are selectively attached to polar fibers extending from only one cell pole. This results in the separation of homologous chromosomes, but not sister chromatids during meiosis I. In meiosis II, kinetochores are attached to polar fibers extending from both cell poles. At the end of meiosis II, sister chromatids are separated and chromosomes are distributed among four daughter cells.