Ribosomes & translation Flashcards
Describe a ribosomes structure
- Large RNP
2 distinct subunits: - Where the subunits join together = the interface
- Small subunit (SSU)
- Large subunit (LSU)
Describe the function of the SSU of a ribosome
codon/anticodon (on tRNA) binding occurs
Describe the function of the LSU of a ribosome
peptide bond formation
Where on ribosome do tRNAs join?
At the subunit interface
How many tRNA binding sites are there at the SSU of a ribosome?
- 3 diff tRNA binding sites (A, P and E)
- But, normally only 2 sites are occupied at the same time
Describe tRNAs moving to each binding site on a ribosome
tRNAs move from A, to P to E sites:
1. Aminoacyl (A) site → (charged) tRNAs that have single amino acid
2. Peptidyl (P) site → tRNAs that are attached to polypeptide chain
3. Exit (E) site → Where non-charged tRNA bind before they leave the ribosome
What does it mean when a tRNA is ‘charged’
It has a protein attached to it
Describe ribosome synthesis in eukaryotes
Requires a lot of energy and involves hundreds of diff proteins and RNAs
Occurs in the nucleoli → later processing occurs in the nucleoplasm + cytoplasm
Where does ribosome synthesis occur?
the nucleous
Why can translation only occur in the cytoplasm?
You only generate ribosomes that can carry out translation once they’ve been exported to the cytoplasm = translation can only occur in cytoplasm
In translation, which end are amino acids added to?
Amino acids added to carboxyl end of the chain
In translation, where does the peptide bond form between the amino acids?
Peptide bond formed between the amino group of the amino acid and the carboxyl group of the chain
In translation, describe the linkage that is broken at the same time as peptide bonds are formed.
Ester linkage that connects the polypeptide chain to the tRNA in the P site is broken
In each turn of the translation elongation cycle, how many amino acids are added to the chain?
1 amino acid
Describe the A & P sites in the translation elongation cycle
A & P sites → pre-translocation state (before ribosome moved)
Describe the P & E sites in the translation elongation cycle
P & E sites → post-translocation state (after ribosome moved)
Describe the tRNA pre-translocation.
- tRNA bound at P site
- Using codon-anticodon binding a tRNA is selected to bind to the A site
- Peptide bond forms = amino acid is transferred to tRNA in A site
Describe the tRNA post-translocation.
- tRNA that was bound in the A site → P site
- tRNA that was bound in the P site → E site
= New charged tRNA can now bind to A site
= Non-charged rRNA in the E site is removed
What is the function of a GTPase?
drives hydrolysis of GTP to GDP
→ Key to translation
What is the function of elongation factor (EF1A)?
Required to bring charged tRNA to A site
How does GTPase ensure the correct tRNA is brought to the A site?
GTP hydrolyses GTP, this is used to ensure correct tRNA molecule is brought to A site
What is the function of elongation factor (EF2)?
Translocation of tRNA requires EF2, it allows for the movement of the ribosome along the mRNA
How is GTPase used with EF2?
EF2 action is coupled with GTP hydrolysis
In translation, how many GTP mols are hydrolysed per incorporated amino acid?
2 GTP
How does a tRNA charged with Methionine bind differently to normal? and why?
- The GTPase elF2 is associated with initiator tRNA, makes it bind to the P site instead of A site
- (AUG) Methionine = start codon and initiates translation
Describe the Shine-Dalgarno sequence
Translation initiation in prokaryotes
- interaction between ribosome and mRNA must occur in a particular way so that the initiation codon is positioned in the P site
- Positioning is dictated by base pairing
How is the m7g key to translation initiation in eukaryotes?
Cap binding complex (CBC) binds to the 5’ end of mRNAs (m7G cap), this initiates translation
Describe the process of translation initiation in eukaryotes
1) Formation of a complex between SSU and the initiator tRNA + eIF2
2) tRNA interacts with the SSU and is targeted to the 5’ end of the mRNA (due to cap)
3) Scans the sequence of the mRNA in a 3’-5’ direction, until it finds the initiation codon (AUG) in the appropriate context (Kozak sequence)
4) Large subunit recruited = translation
Define kozak sequence
the sequence around the initiation codon
Describe the process of translation termination
1) Ribosome translocates in its final cycle = stop codon in the A site
2) Enables the binding of a termination factor to the A site (molecular mimicry = it mimics the tRNA molecule)
3) 1st recognition factor recognises the nucleotide termination codon
4) Triggers hydrolysis of polypeptide carboxyl-ester linkage to the tRNA in the P site = polypeptide is released from ribosome
5) Second termination factor (Rf3 in prokaryotes, eRF3 in eukaryotes) is a GTPase and hydrolyses GTP to GDP in order to release the first termination factor from the ribosome
