Translation (l)

Question 1

which types of RNA is most abundant in the cell?

Answer 1

Ribosomal and tRNAs are the most abundant RNAs in the cell

Question 2

Structure of ribosomal RNA

Answer 2

• Ribosomal ribonucleic acid is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes
• rRNA makes up the ribosome together with protein
• rRNA do not encode protein
• formation of secondary shape due to base pairing of complementary base pairs in one piece of RNA

Question 3

The molecular organisation of the ribosome

Answer 3

• eukaryotes ribosome is twice as big as the prokaryote’s ribosome
• Every ribosome consists of two subunits of unequal size, and each subunit is itself a complex mixture of rRNA and protein molecules.
• Ribosomes provide frame work for protein synthesis
• Located in cytoplasm

Question 4

What is the role of rRNAs?

Answer 4

•The 16S/18S rRNA of the small ribosomal subunit binds the mRNA
- binds mRNA through a ribosome binding site (RBS; prokaryotes)
- binds mRNA at the 5’ cap (eukaryotes)
- recognition of correct codon-anticodon matches
•rRNA interacts with specific regions of the tRNAs
- Binding is by complementary base pairing
•Large subunit rRNA catalyses peptide bond formation
- Ribozyme activity

Question 5

Where does the mRNA bind on the ribosome?

Answer 5

Small ribosomal subunit

16S/18S rRNA

Question 6

Polymerisation center in ribosome?

Answer 6

Large ribosomal subunit

peptidyl transferase activity

Question 7

In which direction does translocation occur?

Answer 7

5’–>3’

Question 8

Steps in Protein Synthesis

Answer 8

1. Activation of amino acids
• loading of amino acids or charging tRNA✓
2. Initiation
3. Elongation
4. Termination

Question 9

Activation

Answer 9

The activation reaction is catalyzed by specific aminoacyl-tRNA synthetases, which are also called activating enzymes
The enzyme binds ATP to the amino acid to form an amino acid–AMP complex linked by a high energy bond (PP released)
The amino acid is then coupled to tRNA and the AMP is released – the tRNA molecule is now “charged” and ready for use

Question 10

Initiation in Prokaryotes

Answer 10

• Translation initiation is the process in which mRNA initiator tRNA and small and large ribosomal subunits associate with each other to form a complex initiation of translation
• This process if facilitated by shine Dalgarno sequence in the mRNA which is complimentary to a component of the small ribosomal subunit called 16S ribosomal RNA.
• Initiation Factor 3 (IF3 ) also facilitates the binding of the mRNA to small ribosomal subunit. Once the 30S subunit, IF3 and mRNA complex is formed the Initiation Factor 2 (IF2) binds to the complex and promotes the binding of tRNA, N-Formylmethionine (Fmet) into the complex
• IF2 and IF3 hydrolyse GTP as an energy source to promote the association of tRNA and mRNA and the ribosomal subunit. Translation initiation is completed when the large ribosomal subunit binds and IF2 and IF3 are released.

Question 11

what is the aim of initiation?

Answer 11

Aim:

(1) dock mRNA onto ribosome
(2) set correct reading frame

Question 12

The reading frame

Answer 12

In principle, any given mRNA has three possible reading frames
• Each reading frame gives a different sequence of codons
• Only one reading frame encodes the correct protein
• Other reading frames unlikely to encode proteins

Question 13

Initiation in reading frame

Answer 13

•eIF2 has the same role as IF2 – forms a complex with the initiator tRNAi Met
the complex binds to the P site of the small rRNA subunit
•A complex of eIFs binds both ends of the mRNA:
eIF4E binds the 5’ cap
eIF4G binds the poly(A) tail
Ensures the integrity of the mRNA molecule
•Recruitment of the mRNA to the small rRNA subunit and scanning for the first AUG
•hydrolysis of GTP allows release of eIF2 and recruitment of the large rRNA subunit

Question 14

Which of the following steps during initiation of translation is
similar between prokaryotes and eukaryotes?
A. eIF2 has the same role as IF2 in binding to charged initiator tRNA
B. Initiator tRNA has to be first charged with methionine before binding
to the start codon AUG in the E site of the small ribosomal subunit
C. eIF2 and IF2 both bind to the A site of the large ribosomal subunit
D. Release of initiation factor(s) from the small ribosomal subunit is
followed by assembly of the complete ribosome

Answer 14

A, D

Question 15

Elongation

Answer 15

1- Binding of a new aminoacyl-tRNA to the A site
2- Formation of the peptide bond between the amino acid on the tRNA in P site and the tRNA in A site
3-Translocation of the ribosome, the tRNA that was located in P site moves to E site and the one in A site moves to P site
4- Release of deacylated tRNA

Question 16

Termination

Answer 16

• signalled by codons UAA, UAG and UGA
• recognised by protein release factors (RFs)
• causes hydrolysis of last peptidyl-tRNA bond
• releases polypeptide and tRNA
• causes ribosomal subunits to dissociate

Question 17

Total number of high-energy phosphate bonds used:

Answer 17

Initiation 1 high-energy bond (GTP → GDP) per protein
Activation of aa 2 high-energy bonds (ATP → AMP tRNA) per peptide bond
Elongation 2 high-energy bonds (GTP → GDP) per peptide bond
Release 1 high-energy bond (GTP → GDP)per protein