Translation

Question 1

What are the 3 stages for protein synthesis? Summarize each briefly

Answer 1

1. Initiation - ribosome is assembled at initiation codon (AUG) on mRNA from a large and small subunit
2. Elongation - repeated cycles of amino acids are added to the amino acid chain, causing transpepidation and translation
3. Termination - the protein chain is released

Question 2

Describe the size of subunits making up the ribosome in Prokaryotes

Answer 2

Prokaryotic ribosome = 70s
Small subunit = 30s
Large subunit = 50s

Question 3

Where does the amino acid attach to tRNA?

Answer 3

The amino acid attaches to the 3’ end of tRNA

Question 4

What is the initiaition codon? After translation what happens to it?

Answer 4

AUG codes for Met - the point on mRNA where a ribosome is synthesized. After translation, methionine will split off from the final polypeptide

Question 5

List binding sites on the assembled ribosome

Answer 5

Assembled ribosomes have 3 tRNA binding sites:

1. Aminoacyl “A site”
2. Peptidyl “P site”
3. Exit site “E site”

Question 6

What binds to the A site on a ribosome?

Answer 6

Incoming AA-tRNA binds to A site

Question 7

What binds to the P site on a ribosome?

Answer 7

Initially, tRNA coding for Met binds to P site. The polypeptide chain then grows from the P site. AA-tRNA’s from the A site are moved into the P site during translocation

Question 8

Describe elongation in translation

Answer 8

1. tRNA carrying Met/initiaition codon binds to the P site on the ribosome
2. aa-tRNA then arrives in the A-site and binds to complementry codon on mRNA. This hydrolyses GTP to GDP
3. Transpepidation - a peptide bond is formed between adjacent a.a. via a condensation reaction where water is released
4. Translocation - polypeptide is transferred from the a-site to the p-site as the ribosome moves 3 nucleotides along. A new aa-tRNA can bind to the A site

Question 9

Describe transpepidation in detail and what happens to GTP

Answer 9

A peptide bond is formed, removing water. This only happens if GTP has been hydrolysed to GDP, allowing the transcription factor to leave.. This forms a delay - allows the tRNA to be proof read before peptide bond is formed

Question 10

How is GTP regenerated?

Answer 10

Elongation factors EF-TU and EF-TS bind to regenerate GTP.

Question 11

What is needed for eukaryotic transcription?

Answer 11

Met-tRNA (unformulated methionine initiation codon)
3 initiation factors - EF-Ts, EF-Tu, EF-G
GTP

Question 12

What is needed for prokaryotic transcription?

Answer 12

FMet-tRNA (formulated methionine initiaition codon)
3 initiation factors
GTP

Question 13

What is the role of the elongation factors?

Answer 13

Bind to GTP or GDP

Question 14

How does tRNA recognize the correct codon?

Answer 14

Wobble pairing - allows tRNA to recognise multiple codons for the amino acid it is carrying

Question 15

Describe termination in transcription

Answer 15

Occurs when ribosome reaches the stop codon, releasing the new polypeptide.
2GTP is hydrolysed to 2GDP upon termination

Question 16

What are stop codons?

Answer 16

Triplets of mRNA not recognized by any tRNAs. Stop codons are recognized by releasing factor proteins (RF1 and RF2)

Question 17

List the stop codons and proteins that recognize them

Answer 17

UAA, UAG, UGA
RF1 - recognises UAA and UAG
RF2 - recognises UAA and UGA

Question 18

Give an example of an antibody

Answer 18

Antibody = immunoglobin proteins

Question 19

What are the roles of chaperone proteins?

Answer 19

Chaperone proteins - release the polypeptide upon termination, preventing it from folding