Translation lecture 2 Flashcards
what is bound to the p site at the initiation of transcription
in prokaryotes, methyl-tRNA
what is the sequence of the codon that codes for methonine ? sequence of the complementary anti-codon ?
AUG, UAC
whats the second step in the elongation phase ?
an aminoacyl-tRNA binds to the A site and a peptide bond is formed
what provides the energy for the formation of the peptide bond ?
hydrolysis of the peptidyl-tRNA linkage
when does translation initiation occur in bacteria
as soon as the 5’ end of the mRNA is synthesised. transcription is coupled with translation in prokaryotes
how can large mRNA molecules be translated simultaneously >
by polyribosomes or polysomes
describe the elongation phase
- aminoacyl-tRNA is positioned in the A site
- following the synthesis of the peptide bond, the newly formed peptide-tRNA is partly in the A site and partly in the P site
- translocation shifts the peptide-tRNA completely into the P site, leaving the A site empty and ejecting the deamino-acylated tRNA from the E site.
what are the three release factors in e.coli ? are they proteins
yes, RF1 and RF2 - bind to stop codons in the A site.
RF3-GTP - enhances effect of RF1 and RF2. proofreads to ensure stop codon was correctly recongised and stimulates release of RF1 and RF2
these release factors mimic the tRNA.
does the polypeptide grow from the c terminus to the n terminus
no its the opposite
what are the three termination codons ?
UGA, UAG or UAA
how is the polypeptide hydrolysed in prokaryotes ?
when the release factors recognise a termination codon, they cause hydrolysis of the peptide-tRNA. Release of the completed polypeptide is probably accompanied by GTP hydrolysis, providing energy for dissociation of the release factors from the ribosome.
what is the name of the elongation factor that docks an aminoacyl-tRNA in the A site
EF-Tu-GTP. Hydrolysis, release of factor, aminoacyl-tRNA remains in the A site where it is positioned for peptide bond formation. elongation factors maintain the processtivity .
what are the two subunits of prokaryotic ribosomes ?
50s + 30s to give 70s
what are the two subunits of eukaryotic ribosomes ?
60s and 40s to give 80s non-linear scale based on centrifugation
what catalyses peptide bond formation
peptide transferase, this enzymatic activity is localised in the large ribosomal subunit
what does peptide bond formation involve ?
physically transferring the polypeptide attached to the p site tRNA onto the amino terminus of the amonoacyl-tRNA bound in the ribosome’s A site
what are the three steps in the chain elongation microcycle ?
- elongation factors dock an aminoacyl-tRNA in the A site
- peptide transferase catalyses peptide bond formation
- translocation moves the ribosome by one codon
describe the ribosome
nucleic acids buried in the inside of the ribosome where catalytic activity is localised. alpha helices on the outside surface
describe the function of the initiation factors 1A and 3 in eukaryotes
1A blocks the A site
1 and 3 block the E site stoping large subunit from binding
P site is left vacant - only site that can be occupied
describe mRNA in eukaryotes
5’ end is capped. 3’ end is polyadenylated. secondary structure of mRNA is regulatory of translation.
function of eIF2-GTP.
eIF2 is a GTPase. binds to Met-tRNA to form a ternary complex. the complex then binds to the vacant p site. The binding isn’t finalised. remember the other two sites are blocked. - a 43s preintiation complex is formed.
components of eIF4 complex
subunits 4E, 4G, 4A and 4B
role of subunit 4e of eIF4 complex
recognises the 5’ end methyl cap
role of subunit 4b, 4a of eIF4 complex
4a is a helicase. 4b has hydrolytic capacity for ATP to ADP. can also stimulate and activate the helicase.
