Chromosome

A chromosome and its packaged long strand of DNA unraveled. The DNA's base pairs encode genes, which provide functions. A human DNA can have up to 500 million base pairs with thousands of genes.

Diagram of a replicated and condensed metaphase eukaryotic chromosome:
Chromatid
Centromere
Short arm
Long arm

A chromosome is a package of DNA with part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells the most important of these proteins are the histones. These proteins, aided by chaperone proteins, bind to and condense the DNA molecule to maintain its integrity.^[1]^[2] These chromosomes display a complex three-dimensional structure, which plays a significant role in transcriptional regulation.^[3]

Chromosomes are normally visible under a light microscope only during the metaphase of cell division (where all chromosomes are aligned in the center of the cell in their condensed form).^[4] Before this happens, each chromosome is duplicated (S phase), and both copies are joined by a centromere, resulting either in an X-shaped structure (pictured above), if the centromere is located equatorially, or a two-arm structure, if the centromere is located distally. The joined copies are now called sister chromatids. During metaphase, the X-shaped structure is called a metaphase chromosome, which is highly condensed and thus easiest to distinguish and study.^[5] In animal cells, chromosomes reach their highest compaction level in anaphase during chromosome segregation.^[6]

Chromosomal recombination during meiosis and subsequent sexual reproduction play a significant role in genetic diversity. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe. Usually, this will make the cell initiate apoptosis leading to its own death, but sometimes mutations in the cell hamper this process and thus cause progression of cancer.

Some use the term chromosome in a wider sense, to refer to the individualized portions of chromatin in cells, either visible or not under light microscopy. Others use the concept in a narrower sense, to refer to the individualized portions of chromatin during cell division, visible under light microscopy due to high condensation.

^ Hammond CM, Strømme CB, Huang H, Patel DJ, Groth A (March 2017). "Histone chaperone networks shaping chromatin function". Nature Reviews. Molecular Cell Biology. 18 (3): 141–158. doi:10.1038/nrm.2016.159. PMC 5319910. PMID 28053344.
^ Wilson, John (2002). Molecular biology of the cell : a problems approach. New York: Garland Science. ISBN 978-0-8153-3577-1.
^ Bonev, Boyan; Cavalli, Giacomo (14 October 2016). "Organization and function of the 3D genome". Nature Reviews Genetics. 17 (11): 661–678. doi:10.1038/nrg.2016.112. hdl:2027.42/151884. PMID 27739532. S2CID 31259189.
^ Alberts B, Bray D, Hopkin K, Johnson A, Lewis J, Raff M, Roberts K, Walter P (2014). Essential Cell Biology (Fourth ed.). New York, New York, US: Garland Science. pp. 621–626. ISBN 978-0-8153-4454-4.
^ Schleyden, M. J. (1847). Microscopical researches into the accordance in the structure and growth of animals and plants. Printed for the Sydenham Society.
^ Antonin W, Neumann H (June 2016). "Chromosome condensation and decondensation during mitosis". Current Opinion in Cell Biology. 40: 15–22. doi:10.1016/j.ceb.2016.01.013. PMID 26895139.

[HammondStrømme2017-1] Hammond CM, Strømme CB, Huang H, Patel DJ, Groth A (March 2017). "Histone chaperone networks shaping chromatin function". Nature Reviews. Molecular Cell Biology. 18 (3): 141–158. doi:10.1038/nrm.2016.159. PMC 5319910. PMID 28053344.

[2] Wilson, John (2002). Molecular biology of the cell : a problems approach. New York: Garland Science. ISBN 978-0-8153-3577-1.

[3] Bonev, Boyan; Cavalli, Giacomo (14 October 2016). "Organization and function of the 3D genome". Nature Reviews Genetics. 17 (11): 661–678. doi:10.1038/nrg.2016.112. hdl:2027.42/151884. PMID 27739532. S2CID 31259189.

[4] Alberts B, Bray D, Hopkin K, Johnson A, Lewis J, Raff M, Roberts K, Walter P (2014). Essential Cell Biology (Fourth ed.). New York, New York, US: Garland Science. pp. 621–626. ISBN 978-0-8153-4454-4.

[vlp.mpiwg-berlin.mpg.de-5] Schleyden, M. J. (1847). Microscopical researches into the accordance in the structure and growth of animals and plants. Printed for the Sydenham Society.

[6] Antonin W, Neumann H (June 2016). "Chromosome condensation and decondensation during mitosis". Current Opinion in Cell Biology. 40: 15–22. doi:10.1016/j.ceb.2016.01.013. PMID 26895139.

[1]

[2]

[3]

[4]

[5]

[6]