Lecture 10: Regulation of eukaryotic Gene Expression Flashcards
Chromatin modification –>
transcription –> RNA processing –>
mRNA degradation OR translation –> protein processing and degradation
DNA is wrapped around __ that make up ___
histones
chromatin
Nucleosome:
8 histones + 146 base pairs of DNA
tightly packed chromatin :
- heterochromatin
- associated with methylated DNA
- INACCESSIBLE for transcription
lightly packed chromatin :
- euchromatin
- associated with un-methylated DNA
- ACCESIBLE for transcription
tightly packed chromatin to lightly packed chromatin –>
- histone acetyl transferases
- histone methyl transferases
lightly packed chromatin to tightly packed chromatin
- histone deacetylase
- histone demethylase
the ‘inactive’ state of RNA polymerase ll
- RNA polymerase is NOT bound to gene promoter
- Gene + promoter are typically cytosine-methylated and associated with tightly packed chromatin
the ‘paused’ or ‘stalled’ state of RNA polymerase ll
- Some transcription factors bind to regulatory elements in promoter
- RNA polymerase ll binds to promoter and is phosphorylated at Serine 5, but remains inactive
the ‘elongation’ or ‘active’ state of RNA polymerase ll
- More transcription factors bind to regulatory elements in promoter
- RNA polymerase ll becomes phosphorylated at Serines 2 & 5, which triggers the transcription progress
RNA processing: splicing and alteration of mRNA ends
- transcription forming pre-mRNA
- pre-mRNA converted to Mature mRNA as Adenine added to ends and removal of introns (non coding DNA)
RNA processing: ‘alternative’ splicing
- transcription to produce pre-mRNA
- pre-mRNA converted to either mRNA variant A (added A and full removal of introns) or mRNA variant B (added A and partial removal of introns) = two different tissues
small RNAs two types:
- microRNA (miRNA)
- small interfering RNA (siRNA)
small RNAs are
double stranded and between 20 and 30 nucleotides long
Small RNAs repress (prevent)
gene expression via 3 mechanisms
three mechanisms of small RNAs repressing gene expression:
1) Repressor of gene translation: miRNA
2) promote mRNA degradation: miRNA
( both 1&2 = ‘post-transcriptional gene silencing’)
3) Repession of gene transcription via chromatin remodelling and DNA methylation: siena & piRNAs
(‘transcriptional gene silencing’
post-transcirptional gene silencing by miRNAs
- pre-miRNA = hairpin shaped
- miRNA double stranded by H-bonds
- Dicer comes along breaks end hairpin joint
- forms miRNA-proteni complex
- either mRNA degraded OR translation is blocked
the proteasome degrades
ubiquitin-tagged proteins
Ubiquitin is a
small (8.5kDa) protein regulating turn-over of other proteins
Addition of multiple Ubiquitins targets the protein for
DEGRADATION
1) Multiple ubiquitin molecules are attached to a protein by enzymes in the cytosol
2) the ubiquitin-tagged protein is recognised by a proteasome, which unfolds the protein and sequesters it within a central cavity
3) Enzymatic components of the proteasome cut the protein into small peptides, which can be further degraded by the other enzymes in the cytosol
