Ribosomes & translation

Question 1

Describe a ribosomes structure

Answer 1

• Large RNP
  2 distinct subunits:
• Where the subunits join together = the interface
• Small subunit (SSU)
• Large subunit (LSU)

Question 2

Describe the function of the SSU of a ribosome

Answer 2

codon/anticodon (on tRNA) binding occurs

Question 3

Describe the function of the LSU of a ribosome

Answer 3

peptide bond formation

Question 4

Where on ribosome do tRNAs join?

Answer 4

At the subunit interface

Question 5

How many tRNA binding sites are there at the SSU of a ribosome?

Answer 5

• 3 diff tRNA binding sites (A, P and E)
• But, normally only 2 sites are occupied at the same time

Question 6

Describe tRNAs moving to each binding site on a ribosome

Answer 6

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

Question 7

What does it mean when a tRNA is ‘charged’

Answer 7

It has a protein attached to it

Question 8

Describe ribosome synthesis in eukaryotes

Answer 8

Requires a lot of energy and involves hundreds of diff proteins and RNAs

Occurs in the nucleoli → later processing occurs in the nucleoplasm + cytoplasm

Question 9

Where does ribosome synthesis occur?

Answer 9

the nucleous

Question 10

Why can translation only occur in the cytoplasm?

Answer 10

You only generate ribosomes that can carry out translation once they’ve been exported to the cytoplasm = translation can only occur in cytoplasm

Question 11

In translation, which end are amino acids added to?

Answer 11

Amino acids added to carboxyl end of the chain

Question 12

In translation, where does the peptide bond form between the amino acids?

Answer 12

Peptide bond formed between the amino group of the amino acid and the carboxyl group of the chain

Question 13

In translation, describe the linkage that is broken at the same time as peptide bonds are formed.

Answer 13

Ester linkage that connects the polypeptide chain to the tRNA in the P site is broken

Question 14

In each turn of the translation elongation cycle, how many amino acids are added to the chain?

Answer 14

1 amino acid

Question 15

Describe the A & P sites in the translation elongation cycle

Answer 15

A & P sites → pre-translocation state (before ribosome moved)

Question 16

Describe the P & E sites in the translation elongation cycle

Answer 16

P & E sites → post-translocation state (after ribosome moved)

Question 17

Describe the tRNA pre-translocation.

Answer 17

• 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

Question 18

Describe the tRNA post-translocation.

Answer 18

• 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

Question 19

What is the function of a GTPase?

Answer 19

drives hydrolysis of GTP to GDP
→ Key to translation

Question 20

What is the function of elongation factor (EF1A)?

Answer 20

Required to bring charged tRNA to A site

Question 21

How does GTPase ensure the correct tRNA is brought to the A site?

Answer 21

GTP hydrolyses GTP, this is used to ensure correct tRNA molecule is brought to A site

Question 22

What is the function of elongation factor (EF2)?

Answer 22

Translocation of tRNA requires EF2, it allows for the movement of the ribosome along the mRNA

Question 23

How is GTPase used with EF2?

Answer 23

EF2 action is coupled with GTP hydrolysis

Question 24

In translation, how many GTP mols are hydrolysed per incorporated amino acid?

Answer 24

2 GTP

Question 25

How does a tRNA charged with Methionine bind differently to normal? and why?

Answer 25

• 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

Question 26

Describe the Shine-Dalgarno sequence

Answer 26

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

Question 27

How is the m7g key to translation initiation in eukaryotes?

Answer 27

Cap binding complex (CBC) binds to the 5’ end of mRNAs (m7G cap), this initiates translation

Question 28

Describe the process of translation initiation in eukaryotes

Answer 28

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

Question 29

Define kozak sequence

Answer 29

the sequence around the initiation codon

Question 30

Describe the process of translation termination

Answer 30

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