Question

describe Regulation of Gene Expression in Eukaryotes (Chapter 17). Describe 4 ways gene expression can be regulated in Eukaryotes. (USE SENTENCES)

Answer 1

Gene expression means all transcription and translation events .Gene expression in eukaryotes occurs at 2 levels :- First one at the level of transcription ( DNA to RNA) and Second post transcriptionally ( RNA processing, RNA transport , translation and post translational modification of protein)

Ways by which gene expression in Eukaryotes can be regulated:-

1)-Covalent Histone modification :- These are mediated by Chromatin modification complexes. These modifications mainly include acetylation , Phosphorylation , methylation .

In acetylation acetyl group are added to lysine amino acid in histone tail by histone acetylase. Acetylation reduce the affinity for DNA , so that DNA can easily open and become transcriptionally active.

In methylation , methyl group is added by Histone methyltransferase.Methylation can be activating and repressing depending on the site and degree for example H3K4 ( lysine at 4th position of H3 ) is found at transcriptionally active region and H3K9 ( Lysine at 9 th position of H3) and H3K27 ( lysine at 27 th position of H3 ) are for repressed state

In Phosphorylation , transfer of phosphate group from ATP to target amino acid is mediacted by activity of Kinases and phosphatases .It also serves a site for DNA damage repair proteins.

Dna acetylation and Phosphorylation reduce the positive cahrge of histone, so that interaction between DNA and histone weakens , that facilitates DNA for transcription.

2). Nucleosome remodeling :- It involeves change in structure of nucleosome and change it's position ( energy dependent process ), so that interactions weakens between DNA and nucleosome. Two main classes of these are swi/and family and ISWI based family.

3) CpG islands :- p represents phosphodiester bond linking cytosine and Guanine, CpG islands are often found at transcription start site and associates with long term silencing of gene. In Eukaryotes DNA methylation predominantly at CPG dinucleotide sequence, It switch off eukaryotic gene expression.(One should notice here DNA Methylation switch off and histone methylation switch on.)

4)Post transcriptional gene regulation :-Response of mRNA to iron is an example of translational repression. Production of Ferreting and transferrin receptor is regulated at level of mRNA by iron regulatory proteins ( IRP) which binds to (IRE) on 5' and 3' untranslated regions of mRNA

In iron deficiency, IRP binds to IRE, protecting transferrin receptor mRNA from nuclease digestion, transferrin receptor synthesis can occur and prevent synthesis of Ferretin

When iron conc. is high IRP no longer binds to IRE , transferrin receptor mRNA to be destroyed and allowing expression of Ferretin.