Histones are a special group of proteins found in the nuclei of eukaryotic cells responsible for DNA folding and chromatin formation. This is important because without histones DNA would be extremely long in relation to the cell. It also allows for additional gene regulatory control, with the modification of DNA packing controlling access to transcriptional machinery. Eukaryotic chromatin structure consists of repeating units, known as nucleosomes, which resemble beads on a string connected by sequences of linker DNA. A nucleosome is formed of DNA sections (approximately 145-146 bp) wrapped around a core octamer of eight histones.
Each octamer has two pairs of different histone proteins: H2A, H2B, H3 and H4. They are arranged as a pair of H2A-H2B dimers, and an (H3)2(H4)2 tetramer. There is an additional type of histone, the linker histone H1, which is present in condensed DNA. Histone H1 is associated with the linker regions connecting core particles, and is required to obtain condensed 30-nm chromatin fibres from the smaller “beads on a string” fibres of approximately 10-11 nm . Although histone H1 is the most commonly studied linker histone there are others, for example histone H5 is the substitute for H1 in nucleated erythrocytes.
Histone packaging of DNA is a ubiquitous mechanism throughout species, and the structures of histones are highly conserved. They contain high amounts of two positively charged amino acids: lysine and arginine. Of course, there are some differences between the types of histone proteins:
The N-terminal and C-terminal ends of the histones have different functions:
Histones vary in their affinity for and interaction with each other:
Histones contain a large proportion of the positively charged (basic) amino acids, lyseine and arginine in their structure. DNA is negatively charged due to the phosphate groups on its backbone. These result of these opposite charges is strong attraction and therefore high binding affinity between histones and DNA. Hydrogen bonding involving hydroxyl amino acids in the histone peptide and the phosphodiester backbone of DNA and are also important in further stabilizing the structure. One of the advantages of histones interacting mainly with the backbone of DNA is it means the interaction is not sequence dependent. This means that despite an apparent preference of histones to some sequences of DNA, they are able to bind anywhere.
Despite their important role, histones aren’t found in all eukaryotic cells. Dinoflagellates were found to have completely different proteins which act in the same manner, and the spermatozoa makes use of protamines to package most of the DNA.
DNA packaging - http://www.youtube.com/watch?v=gbSIBhFwQ4s
Histone "beads on a string" appearance - http://www.youtube.com/watch?v=_5vzKDYgmys
Chromatin and histones - http://www.youtube.com/watch?v=eYrQ0EhVCYA
Schneider, R. et al Histone H3 lysine 4 methylation in higher eukaryotic genes. Nature Cell Biol, 6: 73-77
Jackson, J. et al (2004) Dimethylation of histone H3 lysine 9 is a critical marker for DNA methylation and gene silencing in Arabidoposis thaliana. Chromosoma, 112: 308-315
Herrman, H. (1989) Cell Biology: An Inquiry into the Nature of the Living State. New York: Harper & Row Publishers.
Hardin, J., Bertoni, G, Kleinsmith, L. (2011) Becker’s World of the Cell. 8th Edition. San Francisco: Pearson Education.
Alberts, B., Johnson, A., Lewis, J., et al. (2002) Molecular Biology of the Cell. 4th Edition. New York: Garland Science.
Epigenomics Help [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2010-. Epigenomics Scientific Background. 2010 Aug 31 [Updated 2011 Jan 20].
Wolffie, A. (1998) Chromatin. 3rd Edition. London: Academic Press.
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