eukaryotic genes and gene expression 2 Flashcards
3 RNA process mechanisms
all occur while RNA pol 2 is actively elongating the RNA chain
capping
polyadenylation
splicing
Capping what is used and what does it do
7 methylguanosine
protects mRNA from degradation and increases translation efficiency
capping what happens
5’ end capped
terminal Phosphate of 5’ (usually on A or G) is cleaved and 5’ terminal G added in the reverse orientation
the G is connected to 5’ end using 5’ to 5’ triphosphate bridge
G is then methylated at the 7th position to get m7G cap
polyadenylation what is the signal and what happens
polyadenylation signal (aauaaa) this is recognised and mRNA cleaved 11-30 nb downstream multiple A are added to the end importance is for mRNA stability and efficiency of translation
splicing
introns recognised by splicing machinery (sliceosome)
joining of exon
forms mRNA which contains all the exon
splicing processes 1
what do introns have
they have conserved 5’donor site, 3’ acceptor site and interval branch site
U1 binds to donor site (AG/GU)
U2 binds to branch sequence (CUR(A)YY)
slicing processes
what is a spliceosome
what happens to branch point
U4, U5, U6 come in (spliceosome)
branch point brought closer to donor site
splicing processes 3
what happens to introns and exons
introns are excised as a lariet
exon are ligated together
alternative splicing what is it and what does it do
and problems
greatly increases protein diversity
choice of exons used regulated again by tissue specific or developmental stage specific manner
problems are commonly atypical cystic fibrosis and retinitis pigmeatosa
negative control of alternative RNA splicing
and what introns cause
repressor protein may bind to the pre mRNA
this blocks access of the splicing machinery
intron will be retained
introns may contain stop codon and cause frame shift causing degradation
positive control of alternative RNA splicing
activator proteins may bind to pre mRNA
recruit splicing machinery
introns will be spliced out
binding sites for activator proteins can be remote from the splice site (splicing enhancer)
what does mRNA contain
a cap made from mod of G nucleotide and a poly A tail
untranslatedregion UTRs at 5’ and 3’ end
open reading frame ORF,
containing triplet code used for assembly of polypeptide
how does the ORF begin and end
usually starts with AUG
ends with either UAG
UAA
UGA
elF
eukaryotic initiation factor
PABP
poly a binding protein
what is requires for transcriptional initiation
PABP
elF4G
elF4E
formation of the transcriptional initiation complex
requires poly a tail and a cap
3 steps of translation
initiation elFs
elongation eEFs
termination eEFs
initiation elFs what happens
scanning of the small ribosomal subunit from cap to initiation codon
joining of large ribosomal subunit to make whole 80s ribosome
elongation eEFs
ensures correct AA is added in correct protein chain
termination eEFs
releases a complete polypeptide and recycle of ribosomal subunits
what is the limiting step of translation
initiation
transcriptional control mechanisms
global control
specific control
global control
changes in TF and Ribosomal activity
kinase signalling via p38 MAPK
cleavage of initiation factor during viral infection and apoptosis
specific control
signals in mRNA define the Efficiency of their translation
cap independent translation
multiple product from same mRNA
what do viruses do to elF4G
they cleave it disabaling translation of host cell mRNA
how does a virus initiate translation
IRES
internal ribosomal entry site
alternative initiation codon usage (leaky scanning)
isoforms with different n-termini can be produced by leaky scanning
from alternative AUG and near cognate initiation codons
mRNA degradation in 2 phases
deadenylation and 3’-5’ decay
decapping and 5’-3’ decay
AREs what are they
AU rich elements
more vulnerable to degradation
AREs ate typified by
and they can control
single or tandem repeatsof AUUUA pentamer
they can control transcriptional efficiancy as well as deadenylation and decapping
NMD
nonsense mediated mRNA decay
NMD is what
a survalence mechanism to detect and destroy aberrupt mRNA containing premature terminal codons PTCs before multiple rounds of translation
what does NMD prevent
accumalation of proteins with C-terminal trucations which create incatve or even dominant negative versions
splicing and the exon junction complex what is it
protein complex which is deposeted during splicing
20-24 nucleotide upstream
NMD has a recruitment of what
surf complex between pioneer ribosome and EJC
NMD has rapid degradation via
deadenylation
decapping
endonuclease attack near the ptc
MircoRNA what is it
small regulatory RNA
targeted to 3’ UTRs
if imperfect match downregulate translation
If perfect, mRNA cleaved
