H2 Biology 13 - Regulation of GE in eukaryotes

ensures appropriate genes expressed @ appropriate times to synthesise right type & amount of protein/RNA products for proper functioning of cell

chromatin level mechanisms control transcription by modifying DNA between euchromatin(loosely packed chromatin) and heterochromatin(highly condensed chromatin);

euchromatin; DNA associates < tightly w/ histones, promotes access of RNA pol, TF to promoters; genes expressed

heterochromatin; DNA associates > tightly w/ histones, limits access of RNA pol, TF to promoters; genes not expressed

chromatin level;

(histone tails +vely charged, interact strongly w/ -vely charged phosphate group of s-p backbone of DNA)

histone acetylation > t/c

histone acetyltransferase(HATs) adds acetyl groups to lysine residues on histone tails >

removes +ve charge on histones, < histone affinity for DNA >

DNA coils < tightly around nucleosomes, chromatin decondenses -> euchromatin >

gene promoter > accessible to GTF & RNA pol

histone deacetylation < t/c

histone deacetylase(HDACs) removes acetyl groups to lysine residues on histone tails >

restore +ve charge on histones, > DNA affinity for nucleosome >

DNA coils > tightly around nucleosomes, chromatin condenses -> heterochromatin >

gene promoter < accessible to GTF & RNA pol

chromatin level;

DNA methylation < t/c

DNA methyltransferase adds -CH₃ to C residues @ CpG islands; physically blocks GTF, RNA pol binding to promoter >

methylated DNA recruits DNA-binding proteins(repressors, histone deacetylase), forms > compact, inactive heterochromatin >

promoter inaccessible to GTF, RNA pol; formation of TIC prevented

DNA demethylation > t/c

DNA demethylase removes -CH₃ from C residues @ CpG islands, forms < tightly coiled euchromatin >

allows GTF, RNA pol r&b to promoter, formation of TIC allowed

transcriptional level;

activators r&b to enhancer sequences >

DNA bends >

brings enhancer in contact w/ GTF, RNA pol via protein-protein interactions; facilitate binding of GTF, RNA pol to promoter >

recruits, positions, modifies t/c machinery(GTF+RNA pol) @ promoter, forms stable TIC, > rate of initiation of t/c >

assist in HATs recruitment, increase promoter accessibility to t/c machinery >

activator to enhancer binding accelerates TIC assembly @ promoter, > rate of t/c

transcriptional level;

repressors r&b to silencer sequences >

DNA looping pattern changed, activators bound to enhancers prevented from binding to GTF, RNA pol @ promoter >

TIC assembly @ promoter inhibited >

HDACs recruited, makes promoter inaccessible to GTF, RNA pol >

repressor to silencer binding inhibit t/c OR < rate of t/c

transcriptional level;

GTF r&b to promoter sequences(TATA box) >

recruits RNA pol to r&b to core promoter >

forms TIC, initiates t/c @ t/c start site >

help stabilise TIC(RNA pol bind > securely), progression of RNA pol to elongation stage; allows basal level t/c

post-transcriptional level;

small nuclear RNAs(snRNAs)+proteins=small nuclear ribonucleoprotein particles(snRNPs); several snRNPs join w/ additional proteins, forms larger assembly called spliceosome >

snRNPs recognise splice sites @ each end of intron >

spliceosome interacts w/ splice sites @ ends of intron, cuts @ specific pts to release intron, joins 2 exons tgt >

pre-mRNA introns excised, exons flanking introns spliced tgt, forms continuous coding seq, ensures synthesis of correct polypeptide

post-transcriptional level;

pre-mRNA contains polyadenylation signal seq @ 3' end >

ribonuclease recognises seq, cleaves mRNA @ 3' end; poly-A polymerase adds A nucleotides to 3' end of mRNA, forming poly-A tail >

poly-A tail acts as signal to export mature mRNA out of nucleus,

poly-A tail recognised by poly(A)-binding protein; binding enhances mature mRNA stability, prevents mature mRNA degradation by ribonucleases

post-transcriptional level;

7-methylguanosine cap added to 5' end of pre-mRNA molecule >

5' cap protect mRNA from degradation by ribonucleases,

acts as signal; facilitate transport of mature mRNA out of nucleus,

promote initiation of t/l; TIC bind to 5' cap, recruits SRS to bind to 5' cap

post-transcriptional level;

involves specific regulatory proteins controlling intron-exon choices by binding to regulatory seq in pre-mRNA >

enables 1 gene to code for > than 1 polypeptide, depending which exons spliced tgt to form continuous coding seq

translational level;

stability of mRNA determined by length of poly-A tail; longer tail, > stable mRNA, longer mRNA can be used as template for t/l >

mRNA degradation - ribonucleases remove poly-A tail from 3'->5' until critical length reached; triggers 5' cap removal; triggers mRNA degradation from 5' end >

controlling lifespan, stability of mature mRNA determines duration t/l occurs >

enables control of no. of protein molecules t/l from mature mRNA

translational level;

t/l initiation factors(TIF) bind to SRS; facilitate SRS binding to 5' cap of mature mRNA, initiation of t/l occurs >

TIFs involved in locating 5' cap of mature mRNA, start codon AUG & binding site for initiator tRNA to AUG >

phosphorylation of TIF activate t/l, dephosphorylation of TIF repress t/l

t/l repressors bind to mature mRNA @ specific seq(5' untranslated region) >

block binding of ribosomes, prevents t/l

post-translational level;

glycosylation; targets protein for association w/ membrane >

phosphorylation; activates protein/yields functional protein(dephosphorylation is opposite) >

removal of a.a. from precursor protein to yield mature protein

post-translational level;

proteins marked for degradation; ubiquitin ligase adds ubiquitin to protein to mark for degradation >

ubiquitin-tagged proteins recognised by proteasome, cleaves protein into smaller peptides, further degraded by enzymes in the cytoplasm >

for degradation of proteins not required