The kinetochore is a structure in a cell’s nucleus that is involved in mitosis and meiosis, the two processes of cell division. Kinetochores are specialized regions located on the centromeres of chromosomes, and they are made up of many types of proteins. The main function of kinetochores is to aid in the movement of chromosomes during cell division. During mitosis, kinetochores act like handles on the chromosomes. Fibers attach to and pull on these “handles” in order to move the chromosomes within the dividing cell.
Mitosis is the process of cell division in somatic or non-sex cells. Meiosis is the process in which the sex cells, sperm and eggs, divide. There are four phases of mitosis: prophase, metaphase, anaphase and telophase. Kinetochores play a role in the first three phases.
Prior to mitosis, the cell makes a copy of all of its DNA in the nucleus as well as a second pair of centrioles. During prophase, the newly copied material condenses into chromosomes. There are 46 chromosomes, each with two matching or sister chromatids held together in the middle by a centromere. The kinetochores are located on either side of the centromere.
The nuclear envelope then breaks down, allowing the cell’s structures to move freely within the entire cell. Then each pair of centrioles moves to one end, or pole, of the cell. As they move, they sprout elongated microtubules, or spindle fibers, that help push them along. Some of the spindle fibers extend out and connect to a kinetochore on either side of the centromere. The spindle fibers pull and tug on the kinetochores until all the chromosomes are lined up across the center of the cell.
During metaphase, the spindle fibers form a lemon-shaped array. This is called the mitotic spindle. The fibers are connected to the kinetochores along the midline of the cell and are anchored to the plasma membrane at each end of the cell.
The sister chromatids are split in two at the centromere during anaphase. They divide into individual daughter chromosomes, and each one is still attached to the mitotic spindle at the kinetochore. One daughter chromosome migrates to each pole of the cell. Migration is achieved by means of a motor protein in the kinetochore moving down the spindle fibers. The fibers are then disassembled or “chewed up” at the pole.
In the final phase of mitosis, telophase, the cell reorganizes around both sets of chromosomes. It prepares to divide into two like cells. The chromosomes uncoil, and the kinetochores and spindle break down and vanish. The cell then goes through a process of cytokinesis, in which it completely divides into two daughter cells.