Question

Please briefly explain the Regulation of Gene Expression in Eukaryotes at various levels (Please explain EACH):

- Chromatin Structure/ Epigenetics

- Transcriptional control

- Processing and Stability of mRNA

- Translational Control

- Posttranslational control of protein function

- Eliminating the Protein

Answer 1

1. Chromatin structure/epigentics: DNA is present in condensed form due to the basic positively charged proteins called histones or protamines in case of sperm. The chromatin is basically DNA + histones. This structure has different levels condensation- from nucleosomes to 30nm fibre to solenoid form to chromosomes which is the most condensed form. Chromatin is of two types- heterochromatin and euchromatin. In heterochromatin, the DNA is tighly interwoven and the genes are not expressed in this region whereas euchromatin is loosely wound and the genes are active in this region and can be replicated or transcribed. These states of chromatin are usually interchanged depending on the need for proteins encoded by the genes in question. This is possible due to histones side chain which are subject to various modifications like methylation, acetylation, phophorylation etc on different residues of the protein. These modifications lead to opening or condensation of the chromatin changing the level of genetic expression. For examply methylation of lysine at the 9th position leads to condensation and thus transcriptional inactivation.

2. Transcriptional control: The above discussed mechanisms of chromatin remodelling is an important mechanism for increasing or decreasing transcription. Besides the chromatin remodelling, activators and repressors that bind to the promoter regions of the genes also activate and inhibit transcription respectively. The binding of these proteins aids in or restrict in assimble of transcription factors that initiate transcription. Other proteins called transcription factors are sequence specific which as a result of a tight regulation among cellular processes bind the DNA and may result in increase or decreased rated of transcription.

3. Processing and stability of mRNA: mRNA is the messenger RNA, a result of transcription. DNA has both introns and exons- exons are the coding regions and introns are non coding regions whereas mRNA has just the exons, introns are excised from the transcript in a process called RNA processing. RNA processing takes place inside the nucleus. snRNP are small nuclear ribonuclearproteins that take part in this processing as a bunch of them make splicesoome. Each intron at 5' has GU and AG on 3' and there is a branch point in the middle of the intron with the base "A". The hydroxyl group of A attacks the phosphodiester bond at 5' G- First transesterification reaction. In the second transesterification reaction the OH now formed at the 3' of the exon attacks the 3' intron, circularizing the intron into a lariat structure removing it from the transcript.

mRNA is stabilized by adding a polyadenylation at 3' end and addition of 5'mGuanosine cap at 5'. This keeps rNA from degrading and is important for translation.

Translational control: translation can be regulated at the level of mRNA. Increase or decrease in mRNA level controls the translation. certain RNAs like microRNAs, siRNAs are responsible for degrading the mRNA decreasing the level and thus decreasing the translation level. Level of translation related proteins- like ribosomes, tRNAs, amino acids also influence the translation process. Various regulators which may bind to ribosome of tRNAs to increase or decrease the level of transcription. The stability of the mRNA also is involved in the translational intiation.

Posttranslational control of protein function and Eliminating the Protein: after synthesis, the polypeptide chain collapses into secondary, tertiary and then the quarternary structure. This for major proteins happens in cellular chaperons which provide a sub-environment to the proteins for appropriate folding. The proteins that are not folded properly are marked for ubiquinylation which leads to the degradation of the protein.