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Organisation & Control of Eukaryotic Genome (2) - Coggle Diagram
Organisation & Control of Eukaryotic Genome (2)
1) control of eukaryotic gene expression
(a) chromatin structure
(i) histone modification
Histone acetylation involves the covalent addition of acetyl groups (-COCH3) to positively charged lysine residues in the tails of histone proteins -> +ve charge is neutralised & does not bind to negatively charged DNA
reduces interaction btw histones & DNA -> less compact & more accessible to the t/c machinery for initiation of t/c
histone acetyltransferase (HATs) catalyse the addition of acetyl groups & promote t/c
histone deacetylases (HDACs) catalyse the removal of an acetyl group & inhibit t/c -> histone methyltransferases (HMTs) catalyse the adition of methyl groups to cytosine of DNA
(ii) DNA modification
gene silencing : addition of methyl groups on certain bases (usually cytosine) of DNA / DNA methylation
DNA methyltransferases catalyses the transfer of a methyl group (-CH3) to the carbon-5 of the cytosine ring
methylated DNA attracts other proteins which in turn recruits histone deacetylation enzymes
association of these proteins & HDACs makes DNA more compact & inhibits t/c
(b) transcriptional control
(i) transcriptional initiation
presence of general t/c factors lead to a low/basal rate of t/c & RNA synthesis
(ii) effects of enhancers & silencers
(c) post-transcriptional control
(i) RNA processing
5' capping
splicing
addition of a poly-A tail to the 3' end of mRNA (polyadenylation)
(ii) RNA transport
mature mRNA is recognised by their processed modifications & then exported through the nuclear pore by chaperone proteins
unprocessed RNA / incompletely processed RNA is degraded in the nucleus -> gene is effectively not expressed
(d) translational control
(i) translation initiation
requires t/l initiation factors
phosphorylation & dephosphorylation can activate / inactivate t/l initiation factors needed to initiate ribosomal binding
(ii) translation repressors
proteins which bind to mRNA
they bind at 5' UTR -> preventing initiation / prevent ribosome from progressing
(iii) stability of mRNA (half-life of mRNA)
the more stable an mRNA molecule is, the longer it is translated, more polyp. are produced
stability of mRNA depends on length of poly-A tail
there are specific sequences in the 3' UTR which increases mRNA longevity
(iv) cytoplasmic polyadenylation
some mRNA are synthesised to deliberately contain a short poly-A tail
accumulated in the cytoplasm of unfertilised oocytes -> not translated as t/s cannot be initiated due to short poly-A tails
only at specific times of oocyte maturation & post-fertilisation, cytoplasmic polymerase catalyses the cytoplasmic polyadenylation of the 3' end
(v) localisation of mRNA
concentrate high mRNA levels to ensure a high level of polyp. to be synthesised at a specific location where polyp. is needed the most
(e) post-translational control
(i) biochemical modification
biochemical functional groups such as lipids, carbohydrates & phosphate groups attach to protein
protein is activated / becomes functional
(ii) structural modification
removal of a.a from the protein may occur to produce the mature protein
many proteins are synthesised as a pro-protein & sections of a.a are cleaved to produce the functional protein
(iii) protein degradation
excess / misfolded proteins can be degraded by proteasomes
small proteins called ubiquitin are covalently attached to unwanted proteins
proteasome recognises & binds to ubiquitin-tagged proteins
proteasome unfolds the tagged protein & injects it into the core of the proteasome while ubiquitin is released during the entry
within the core, the protein is degraded into short peptides / a.a which are recycled back into cytoplasm