The kinetochore is a protein structure specialized in moving chromosomes - the filaments that contain genetic material - in a cell that is going to divide by either of the two processes of cell division (mitosis or meiosis).
Kinetochores are formed by the assembly of various proteins in an area called the centromere, which is located in the center of a duplicated chromosome. The centromere is the main connection point between the microtubules of the spindle and the chromosomes, in such a way that these can be distributed equally among the resulting cells.
Some organisms have only this central region where the centromere is located. These organisms are called "monocentric" and include vertebrates, a large part of plants, and fungi.
On the contrary, there are some organisms such as nematodes (flat worms) and some plants that assemble kinetochore in a diffuse centromere along the chromosome, these organisms are called "holocentric".
Structure of the kinetochore
A kinetochore consists of an inner region and an outer region. The inner region is connected to the centromere through highly repetitive DNA called "centromeric DNA". This material assembles into a specialized form of chromatin.
The outer region of the kinetochore is rich in proteins, which serve to connect to the microtubules that make up the spindle fibers at each end of the poles of a cell about to divide. These dynamic components work only during mitosis.
A third region called fibrous crown has been described, which is located between the internal and external parts. The fibrous crown is created from a network of permanent and temporary proteins, and its function is to help regulate the attachment of microtubules to the outer plate.
Each region works in a particular way to aid in the separation of sister chromatids. Their activities and relationships only occur during cell division and are essential as they help separate chromatids. Each chromatid has its own kinetochore.
Kinetochore functions
The kinetochore performs many important functions for the dividing cell, including the following:
-The binding of the ends of microtubules to chromosomes
-Checking these unions before cell division
-The activation of a checkpoint to delay the progression of the cell cycle (if defects are detected)
-The generation of the force necessary to mobilize the chromosomes towards the poles.
Importance in cell division
During the cell cycle, checks are performed at certain stages to ensure that cell division occurs properly and without errors.
One of the checks involves making sure that the spindle fibers are correctly attached to the chromosomes in their kinetochores. If not, the cell could end up with the wrong number of chromosomes.
When errors are detected, the cell cycle process stops until corrections are made. If these errors cannot be corrected, the cell will self-destruct through a process called apoptosis.
Finally, the kinetochore is an essential molecular machine that drives chromosome segregation during mitosis and meiosis. Some 100 proteins with a wide range of functions important for proper cell division have been identified.
References
- Albertson, DG, & Thomson, JN (1993). Segregation of holocentric chromosomes at meiosis in the nematode, Caenorhabditis elegans. Chromosome Research, 1 (1), 15–26.
- Chan, GK, Liu, ST, & Yen, TJ (2005). Kinetochore structure and function. Trends in Cell Biology, 15 (11), 589–598.
- Cheeseman, IM (2016). The Kinetochore. Cold Spring Harbor Perspectives in Biology, 6 (7), 1–19.
- Cleveland, DW, Mao, Y., & Sullivan, KF (2003). Centromeres and kinetochores: From epigenetics to mitotic checkpoint signaling. Cell, 112 (4), 407-421.
- Johnson, MK, & Wise, DA (2009). The Kinetochore Moves Ahead: Contributions of Molecular and Genetic Techniques to Our Understanding of Mitosis. BioScience, 59 (11), 933-943.
- Lodish, H., Berk, A., Kaiser, C., Krieger, M., Bretscher, A., Ploegh, H., Amon, A. & Martin, K. (2016). Molecular Cell Biology (8th ed.). WH Freeman and Company.
- Maiato, H. (2004). The dynamic kinetochore-microtubule interface. Journal of Cell Science, 117 (23), 5461–5477.
- van Hooff, JJ, Tromer, E., van Wijk, LM, Snel, B., & Kops, GJ (2017). Evolutionary dynamics of the kinetochore network in eukaryotes as revealed by comparative genomics. EMBO Reports, 1–13.