Histones

=Histones=

 DNA winds around histone proteins like threads around a spool to produce structures called **nucleosomes **; further condensation of the nucleosomes enables a large amount of genetic material to be compacted into a small space such as the cell nucleus.
 * Histones **are the proteins that, together with DNA, make up the majority of chromatin. Histone is a strongly alkaline (positively charged) protein, hence its ability to interact strongly with negatively charged DNA. Other mechanisms for its interaction with DNA include:
 * hydrogen bonds between amide groups on histone proteins and the sugar-phosphate backbone of DNA;
 * non-polar interactions between histone proteins and deoxyribose sugars
 * salt bridges between the R-groups of basic amino acids in the histone protein, and the oxygen in phosphate groups of DNA

As well as being involved in the packaging of DNA, histone proteins play an important role in regulating gene expression. **Histone acetylation ** involves the addition of an acetyl group to a conserved lysine residue on the N-terminus of the protein. This neutralizes the positive charge of the histone protein, lowering its ability to interact with DNA and enabling transcriptional proteins, such as transcription factors and RNA polymerase, to access the DNA. Histone acetylation therefore enhances the rate of transcription, while the removal of these acetyl groups ( **histone deacetylation **) silences gene transcription. Enzymes called histone acetyltransferases and histone deacetylases are responsible for catalysing these processes.