Steps In protein Synthesis Flashcards
What direction is mRNA translated
-5’ -3’
What is translation
-assembly of amino acids from tRNA into a peptide chain sequence directed by mRNA template and done by ribosomes
List the 3 steps of translation
1 initiation
2 Elongation
3 termination
Which organism has translation and transcription linked, how and why
- prokaryotes
- translation starts before transcription is done
- genetic material not separated from ribosomes via nucleus
What does initiation need before starting peptide formation and list them
- assembly of components of translation
- 2 ribosomal subunits
- 1st aminoacyl-tRNA
- mRNA bound to ribosomes
- GTP
- initiation factors
List the initiation factors in eukaryotes and prokaryotes and what they do
1 prokaryotes
- IF 1
- IF 2 GTP : beings charged met-tRNA to P site
- IF 3 : prevents premature association of subunits
2 eukaryotes
- eIF 3 : prevents immature association of subunits
- eIF 2 GTP : beings charged met-tRNA to P site
- eIF 4 : locates AUG along with small subunit
Mechanism by which ribosomes recognizes nucleotide sequence to initiate translation ( AUG )
1 Shine-Dalgarno sequence - E. coli has purine rich region 6-10 bases upstream of initiation codon
-16S rRNA component of 30S subunit has sequence complimentary to SD and binds to it
2 5’ cap - no SD sequence In eukaryotes. Small subunit ( 40S with eIF-4 proteins ) binds close to cap at 5’ and scans for AUG
Use of eIF-4 proteins near cap and poly A tail cap
-mediate circularization of the mRNA and prevent use of incompletely processed mRNA
How is met-tRNA bound to initiator codon and energy source and factors responsible
- AUG recognized by special tRNA and IF-2-GTP ( pro ) and eIF-2-GTP ( eu ) factors binds it to AUG
- GTP hydrolyzed to GDP
Special features of initiation tRNA
- only one to go directly to P site
- only one recognized by eIF-2
What type of initiate tRNA and its aas is carried in prokaryotes
-N-formulated methionine
When and how is methionine N-formulated
- after it has been added to tRNA
- uses transformylase ( enzyme ) and N10-formyl-THF as carbon donor
-N10-formyl-THF is cleaved by enzyme and N-formly added to N link ( COH ) of methionine forming fMet-tRNA
What usually happens to methionine
-removed before translation is complete
What happens after net-tRNA has been bound to mRNA
/large subunits then joins complex and functional Ribosome formed with charged initiator tRNA
Function of guanine nucleotide exchange factor
-reactivates eIF-2-GTP and IF-2-GTP by replacing GDP with GTP
How does elongation occur in translation
-amino acids are added to carboxyl of present chain via amide ( peptide bond ) of incoming aas
What is decoding and mediator
-bringing of aminos up-tRNA next on template mRNA to A site
- facilitated by EF-Tu-GTP and EF-Ts ( prokaryotes )
- EF-1alpha-GTP and EF-l beta gamma ( eukaryotes )
What catalyses peptide bond formation and where does it occur
- peptidytransferase
- intrinsic to 23S rRNA in large subunit
What is a ribozyme
-rRNA which catalyses a reaction
23S of large subunit in prokaryotes catalyses peptide formation
Describe what happens after aas added and translocation and the protein mediators
- after peptide bond formed P site tRNA now empty and A site tRNA has peptide chain
- ribosomes moves 3 nucleotides towards 3’ end to next codon
- EF-G-GTP ( prokaryotes)
- EF-2-GTP ( eukaryotes)
How is empty P site tRNA moved to E site and what else moves
- through GTP dependent translocation
- A site tRNA with peptide chain now moves into P site
How does termination occur
-when termination codon moves into A site
What factors recognizes termination codon at A site and what do they do
- RF-1/2 in prokaryotes
- eRF in eukaryotes
-they bind to termination sequence and result in hydrolysis of bond linkage ester of tRNA and peptide leasing nascent chain
What does RT-3-GTP and eRF-3 do
-hydrolytically remove RF-1/2 and eRF from termination signal
