Histones
In order for DNA to be packaged tightly enough to fit into a chromosome, it wraps around proteins called Histones, located in the nucleus of a cell[1].
There are two major classes of histones; Core and Linker.
These can, between them, be divided into 5 further sub-classes, as follows;
1. Core Histones - H3, H4, H2B and H2A.
2. Linker Histone - H1.
The histone octomer- made up of two dimers of H4/H3 and surrounded on either side by a H2A/H2B dimer - is capable of winding 147 base pairs of DNA around itself, in two left-handed loops, forming a structure known as a nucleosome. Each nucleosome is seperated by 20-30 base pairs of DNA, which can also be referred to as 'linker DNA'. [2]
The H1 linker is reqruited once the DNA has been wound around the nucleosome and it's affinity is increased. This linker protein allows for the 10nm strand to be condensed further into a 30nm strand[3] ( the structure is not 100% known, but it is believed to be a solanoid).
Without histones, DNA would be too long to fit within the nucleus.
Some other functions of histones include acetlyation, methylation, ubiquitination, and phosphorylation [4].
References
- ↑ http://en.citizendium.org/wiki/Histone
- ↑ White, Robert J. (2000). Gene transcription: mechanisms and control. Malden, MA: Blackwell Science.
- ↑ http://www.annualreviews.org/doi/full/10.1146/annurev.biophys.26.1.83
- ↑ http://www.iscid.org/encyclopedia/Histones