Translation - L9

Question 1

Describe the basic ribosome structure.

Answer 1

Ribosomes are a large and abundant RNP particles with 2 unequal subunits.

Each subunit is mostly of RNA. ~ 20 in small subunit and ~ 50 in the large subunit. Each subunit contains one larger rRNA.

Question 2

How does peptide bond formation occur?

Answer 2

Peptide bond formation (peptidyltransfer reaction) involves nucleophilic attack of the alpha-amino group of aminoacyl-tRNA on the carboxyl group of the peptidyl-tRNA.

Question 3

What is the translation elongation cycle?

What tRNA recognises internal AUG codons?

Answer 3

During the elongation stage, the ribosome continues to translate each codon in turn. Each corresponding amino acid is added to the growing chain and linked via a bond called a peptide bond. Elongation continues until all of the codons are read.

Elongator methionyl-tRNAs are used to recognise internal AUG codons. Elongator tRNAs are bound by EF-Tu.

Question 4

In the translation elongation cycle, how many ribosomal tRNA binding sites are there and what are they?

Answer 4

There are 3:

• the A (aminoacyl) site
• the P (peptidyl) site
• the E (exit) site

Two tRNAs are bound at any one time (A+P sites or P+E sites).

Question 5

What are the GTPases in translation elongation?

Answer 5

EF-Tu and EFG are GTPases.

Two molecules of GTP are hydrolysed for each round of translation.

Question 6

How does translation initiation occur?

Answer 6

A specific initiator tRNA binds to the start codon. initiator methionyl-tRNA recognises the AUG start codon.

Initiator methionyl-tRNA is bound by the initiation factor IF2 (eIF2 in eukaryotes). IF2, like EF-Tu, is a GTPase.

Question 7

What is the Shine-Dalgarno (SD) sequence?

Answer 7

It is a ribosomal binding site in bacterial and archaeal messenger RNA. - prokaryotes

16S/mRNA base-pairing selects the initiation codon in prokaryotic cells. The Shine-Dalgarno (SD) sequence within the mRNA is recognised by base-pairing with nucleotides at the 3’ end of the 16S rRNA.
The SD/anti-SD interaction positions the initiation codon in the ribosomal P-site.

Question 8

Describe translation initiation in eukaryotes.

Answer 8

• methionyl-tRNA is bound by the initiation factor eIF2 in eukaryotes.
• The Met-tRNA/eIF2 complex binds to the small subunit and is recruited to the 5’ end of the mRNA (Interaction of the Met-tRNA/eIF2/SSU complex with the mRNA is dependent upon the cytoplasmic cap binding complex) (CBC).
• The complex scans along the mRNA, using the helicase activity of CBC, until it finds an AUG codon within an appropriate context: the Kozak sequence.
• GTP hydrolysis by eIF2 leads to its release, recruitment of the LSU (large subunit) and formation of the translation initiation complex.

Question 9

In translation termination, how are stop codons recognised?

Answer 9

by protein termination (or release) factors, not tRNAs.

Binding of a release factor (RF1 or RF2) (eRF1 in eukaryotes) triggers peptide hydrolysis.