Genetic code - Lecture 3

Question 1

What is the difference between the ribosome of eukaryotes and prokaryotes?

Answer 1

eukaryote: 80S
prokaryote: 70S

Question 2

How is an aminoacyl-tRNA formed?

Answer 2

• amino acyl tRNA synthetase enzyme attaches tRNA and AA in specific active sites
• ATP becomes AMP can attached to AA
• tRNA replaces AMP forming tRNA to AA high energy covalent bond from the 3’ of tRNA to the 5’ carboxylate terminal of the AA

Question 3

Describe initiation of translation.

Answer 3

• 16S rRNA is a component of the 30S small ribosomal subunit and contains the complement to the shine-dalgarno sequence
• The pairing of the two sequences positions the ribosome near the AUG start codon
• IF-3 binds to the small subunit, preventing the large subunit from binding to the mRNA so the small subunit can attach to the mRNA
• A tRNA charged with N-formylmethionine forms a complex with IF-2 and GTP and binds to the initiation codon while IF-1 joins the subunit
• All IFs dissociate and GTP is hydrolyzed to GDP
  The large subunit joins to create a 70S initiation complex

Question 4

What is the shine-dalgarno sequence?

Answer 4

5’ AGGAGG 3’

Question 5

What are the 3 distinct regions of a ribosome and what are their functions?

Answer 5

Aminoacyl site
-Accepts newly charged aminoacyl tRNAs
Peptidyl site
-Peptide bond forms between the amino acid on the tRNA and the new AA at the A site
Exit site
-tRNA has its AA removed and exit the ribosome

Question 6

How does translation in eukaryotes differ from prokaryotes?

Answer 6

The amino group of the methionine on the initiator sequence is MET not fMET
There is no shine-dalgarno sequence (uses the 7MG cap instead)
Kozak sequence influences the efficiency of which AUG in the vicinity of the 5’ 7MG cap is used to start translation
Poly(A)tail also helps when binding to initiation sequence

Question 7

What is the Kozak sequence?

Answer 7

5’ GCC(A or G)CCAUGG 3’

Question 8

Describe the process of elongation in translation.

Answer 8

• aminoacyl-tRNA binds to the A site of the ribosome
• The amino acid is transferred from the tRNA in the P site to the tRNA in the A site by the formation of a peptide bond
• The ribosome translocates along the mRNA to position the next codon in the A site
  This results in the polypeptide from the A site being translocated to the P site
• The uncharged tRNA is translocated from the P site to the E site and is removed
• A new aminoacyl tRNA binds to the A site and the cycle is repeated

Question 9

Describe termination in translation.

Answer 9

Polypeptide chain termination occurs when a chain-termination codon enters the A site of the ribosome
Stop codons: UAA, UAG, and UGA
When a stop codon is encountered, a release factor (RF) binds to the A site
Release factor 1 (RF-1) recognizes UAG and UAA, RF-2 recognizes UAA and UGA
RF-1 or RF-2 binding alters activity of peptidyl transferase resulting in a water molecule being added to the carboxyl terminus of the new polypeptide releasing the polypeptide and leading to termination
The binding of RF-3 and GTP to the ribosome assists in the dismantling of the entire complex

Question 10

What scientists discovered how ribosomes work?

Answer 10

Ramakrishnan, Steitz, and Yonath