Prokaryotic Translation

Question 1

What does it mean that the codon code is degenerate?

Answer 1

There are 61 triplets coding for only 22 AAs. Therefore, often a coded AA will have multiple sequences coding for it.

Question 2

Can codons have different AA meaning in different organisms?

Answer 2

Yes

Question 3

What are the 3 types of point mutations and what do they involve?

Answer 3

• Missense mutation – substitution of one nucleotide so that an alternative AA is incorporated into the polypeptide
• Nonsense mutation – substitution of one nucleotide so that a stop codon is prematurely introduced and as a result transcription of a polypeptide is terminated too early
• Frameshift mutation – mutation as a result of ribosome either reading 4 nucleotides as 3, or backing up one nucleotide and reading from a different reading frame

Question 4

What are the roles of different types of RNA in tranlation ?

Answer 4

• mRNA – carries the genetic information for protein structure from transcribed DNA
• tRNA – acts as an adaptor between codons and AAs (keys to deciphering the codons
• rRNA – associates with a set of proteins to form ribosomes, which are the machinery for translation

Question 5

What are ribosomes? What are the S values of both prokaryotic ribosomal subunits and the ribosome as a whole?
In what direction does The ribosome progress?
Can 1 protein be synthesised by multiple ribosomes?

Answer 5

Ribosome is a protein synthesis factory.
Large - 50S, Small - 30S, Ribosome - 70S.
5’-3’ direction.
Yes

Question 6

What are the 4 important structural features of tRNA?

Answer 6

• AA accepting arm
• T loop
• D loop
• Anticodon

Question 7

What is the function of aminoacyl tRNA synthetases and how many are there per AA?

Answer 7

Enzymes that attach tRNA to the appropriate AA. Each synthetase is specific for a single AA

Question 8

What are the 2 steps in which the codon is decoded in translation?

Answer 8

• linkage of AA to tRNA by aminoacyl tRNA synthetase

- tRNA binds to the mRNA codon

Question 9

What is the wobble position and what does it mean for translation?

Answer 9

Wobble position is the 1st base at the tRNA and the 3rd at mRNA. There is a certain degree of flexibility of base recognition at that position, which allows for binding of different tRNA sequences to a single mRNA sequence.

Question 10

In translation initiation, where does the Ribosome assemble? What does this determine?

Answer 10

It assembles at Shine-Dalgarno sequence, which determines the reading frame

Question 11

Which initiation factor keeps the ribosomal subunits separate by occupying which site of which subunit?

Answer 11

IF3 occupies the E site of the small ribosomal subunit.

Question 12

What events cause the release of IF3 and ribosomal assembly at Shine-Dalgarno?

Answer 12

1) IF1, IF2 and GTP bind to the A site of small ribosomal subunit
2) Methionine-tRNA binds to the P site
3) IF3 is released and Ribosome asssembles

Question 13

What happens after the ribosomal assembly?

Answer 13

1) IF2 is dephosphorylated by the large ribosomal subunit
2) IF1, IF2, GDP and Pi are released
3) Ribosome is ready to start translation

Question 14

What is happening at each ribosomal site during translation?

Answer 14

• New aminoacyl-tRNAs carrying AAs bind to A site
• Peptide bond is formed between AAs in P and A sites
• Used aminoacyl-tRNAs not carrying AAs are discarded from E site

Question 15

How is translation terminated? What is the function of RF3?

Answer 15

RF1/RF2 binds to the stop codon, the synthesised polypeptide is terminated with a COOH group and released from the ribosome, ribosome dissociates.
RF3-GTP proofreads to ensure the stop codon was recognised corectly and if so stimulates the release of RF1 &RF2