CB9 Translation

Question 1

Explain the nature of the genetic code & outline the concept of reading frame.

Answer 1

The genetic code = 3 nucleotide letters specify one amino
“Degenerate” as several combinations code for the same amino acid
~ 20 amino acids

Question 2

What are the start & stop codons?

Answer 2

Start codon: methionine - AUG

Stop codon: UAG, UAA, UGA

Question 3

Explain the role of aminoacyl tRNA synthetases in attachment of the appropriate amino acid to tRNA

Answer 3

Aminoacyl tRNA synthetases link amino acids to their corresponding tRNA

Two active site - amino acid, anticodon

Question 4

Outline the structure & composition of the ribosomes

Answer 4

• Large complex of 80 proteins & 4 rRNAs
• large subunit binds to tRNA (3 binding sites)
• small subunit binds to mRNA (1 binding site)

Question 5

Describe initiation in protein synthesis

Answer 5

Initiation: prokaryotes
Small ribosomal subunit recognises shine-dalgarno sequence
mRNA attaches to subunit
tRNAmet bonds to start codon (AUG)
Ribosome pulls in large ribosomal subunit

Eukaryotes
Generally occurs at 5’ end of mRNA
Requires imitation factors & 40s ribosome subunit

Question 6

Explain the roles of the A & P sites on the ribosome

Answer 6

A = amino acid loaded, matched to correct codon, peptide bond formed

P = tRNA carries elongated peptide chain

Question 7

Explain the special role of tRNAmet in the initiation of translation

Answer 7

Recruites the large ribosomal subunit

Question 8

Describe the differences between free & Er ribosome

Answer 8

Rough ER ribosomes make
- proteins secreted from cell
- integral membrane proteins
- soluble proteins within ER, Golgi, lysosomes, endosomes

Free ribosome (cytosolic) make
- enzymes
- peripheral proteins
- nuclear proteins

Question 9

Explain how proteins destined for secretion are made

Answer 9

• All protein synthesis initiated in the cytosol
• short signal sequence recognised by SRP
• SRP binds to SRP receptor on ER
• ribosome binds to translocon and enters lumen of ER

Question 10

Describe elongation in protein synthesis

Answer 10

Ribosome moves along the mRNA
tRNAs occupy successlively three sites as the ribosome moves along
A, P, E

Question 11

Describe termination in protein synthesis

Answer 11

• Ribosome reaches a stop codon (UAG, UAA, UGA)
• release factor enters the A site
• protein released, ribosome dissociates into large subunit & small subunit
• protein may then be processed: folding, addition of sugars & lipids, cleavage, secretion)