Question

3. (3 marks) Name and briefly describe the three different ways that chromatin can be modified to change gene expression.

Answer 1

Three different ways that chromatin can be modofied to change gene expression are :

1. Covalent histone modification such as methylation : Modification of histone proteins that are bound to the DNA determine whether DNA is loosely or tighlty associated with them that in turn affect the packaging of   the chromatin.This packaging /remodelling of chromatin affects the gene expression.For example. enzymatic addition of methylation of lysine residues in histone 3 and 4 results in tight association of histones with DNA as a result of which chromatin is tightly condensed and DNA is not accesible for binding of transcription factors , causing gene repression.
2. Covalent histone modification such as acetylation: Acetylation of the histone tails result in chromatin relaxation and gene expression takes place via binding of transcription factors to DNA.
3. Chromatin remodellinG complexes that are ATP dependent: Chromatin remodelling complexes SWI/SNF, ISWI restructure the nucleosomes.They consist of an ATPase domain through which they utilise energy from ATP and remodel the nucleosomes i.e twisting and sliding that either removes histones from DNA or exchanges histone with other proteins.This results in exposing the DNA for gene expression to take place.