The place where two chromosomes (each known as a chromatid before the cell splits) are joined before they split into two is called the centromere. A kinetochore is the patch of protein found on the centromere of each chromatid. It is where the chromatids are tightly connected. When it's time, at the appropriate phase of cell division, the kinetochore's ultimate goal is move chromosomes during mitosis and meiosis.
You can think of a kinetochore as the knot or central point in a game of tug-of-war. Each tugging side is a chromatid getting ready to break away and become part of a new cell.
The word "kinetochore" tells you what it does. The prefix "kineto-" means "move," and the suffix "-chore" also means "move or spread." Each chromosome has two kinetochores. Microtubules that bind a chromosome are called kinetochore microtubules. 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.
Location and Checks and Balances
Kinetochores form in the central region, or centromere, of a duplicated chromosome. 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. The two kinetochores of each chromosome should be attached to microtubules from opposite spindle poles. 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 cannot be corrected, the cell will self-destruct in a process called apoptosis.
In cell division, there are several phases that involve the cell's structures working together to ensure a good split. In the 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, much like the children's toy, a Chinese finger trap. Kinetochores tightly grip polar fibers as they are pulled toward the cell poles. Then, the kinetochore proteins that are holding the sister chromatids together are broken down allowing them to separate. In the Chinese finger trap analogy, it would be as if someone took a scissor and cut the trap at the center releasing both sides. As a result, in cellular biology, sister chromatids are pulled toward opposite cell poles. At the end of mitosis, two daughter cells are formed with the full complement of chromosomes.
In meiosis, a cell goes through the dividing process two times. In part one of the process, meiosis I, kinetochores are selectively attached to polar fibers extending from only one cell pole. This results in the separation of homologous chromosomes (chromosome pairs), but not sister chromatids during meiosis I.
In the next part of the process, 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.