Translation

Question 1

What is translation and where is it carried out?

Answer 1

The process of converting the information present in mRNA to proteins.
Translation is carried out by the ribosomes in the cytoplasm.

Question 2

How many possible codes are there of a codon?

Answer 2

64 (4^3)

Question 3

How many sense codons and stop codons are there?

Answer 3

61 sense and 3 stop/nonsense.

Question 4

What is the main exception to the universal genetic code?

Answer 4

Mitochondria

Question 5

What is the open reading frame?

Answer 5

The protein coding sequences in a gene.

Question 6

Explain why the genetic code is not ambiguous?

Answer 6

Each codon codes for only one amino acid.

Question 7

What amino acid does the start codon code for?

Answer 7

Methionine (AUG)

Question 8

What does the stop codon code for?

Answer 8

No amino acid

Question 9

What 3 RNAs are involved with translation?

Answer 9

mRNA - gene code
rRNA - components of ribosomes
tRNA - anticodon + amino acid

Question 10

What is the adaptor between the codon and amino acids?

Answer 10

tRNA.

Question 11

What is the role of the anticodon?

Answer 11

Recognises one or more codons in mRNA by base pairing, triggering tRNA to bring the correct amino acid.

Question 12

Three functions of tRNA?

Answer 12

-Binds to an amino acids and is then charged
-It associates with mRNA molecule through anticodon loop
-Interacts with ribosomes.

Question 13

What is the amino acid attachment site and how does it attach?

Answer 13

3’ OH group of tRNA molecule.
Covalent linkage of tRNA 3’ end to cognate amino acid.
Catalysed by aminoacyl-tRNA synthetase.
Linkage of amino acid is via an ester bond to OH group.

Question 14

3 features of tRNA structure?

Answer 14

TC loop
D loop
Anticodon

Question 15

How is accuracy of tRNA charging maintained?

Answer 15

Both amino acid selection and tRNA selection are subject to proofreading.
Proofreading either disfavours the forward reaction (kinetic proofreading) or reverses the catalytic reaction (chemical proofreading) if the wrong component has been selected.
Avoids the need for absolute accuracy and slowness.

Question 16

Why does proofreading in tRNA charging matter?

Answer 16

Mutation affects the accuracy of the enzyme.

Question 17

How do the 30 tRNA molecules recognise the 61 sense codons?

Answer 17

Wobble and modified bases allow one tRNA to decode more than one codon.
Wobble hypothesis - base pair rules between anticodon position 1 and codon position 3 are relaxed due to flexibility and modifications of the anticodon loop.
Single tRNA can recognise multiple codons due to inosine wobble as can base pair with uracil cytosine and adenine.

Question 18

Wobble codon base and possible anticodon bases?

Answer 18

U - A,G OR I
C - G OR I
A - U OR I
G - C OR U

Question 19

The ribosome - 50S crystal structure?

Answer 19

RNA forms (grey) the bulk of structure.
Protein (gold) fill in gaps between rRNA.

Question 20

Critical role of rRNA in ribosome?

Answer 20

rRNA is highly structured.
Not just a scaffolding for proteins - most of proteins are non-essential.
Peptide bond formation is catalysed by 23S rRNA.
Subunit association and tRNA binding directly involve rRNA - specific changes in rRNA conformation are associated with these events.

Question 21

Three sites for tRNA binding?

Answer 21

A (aminoacyl tRNA or entry) site - bind with the anticodon of charged tRNA.
P (peptidyl tRNA or donor) site - where tRNA adds its amino acid to the growing peptide chain.
E (exit) site - where tRNA sits before being released from the ribosome.

Question 22

Translation steps?

Answer 22

Initiation
Elongation
Termination

Question 23

Initiation steps?

Answer 23

Nucleotide sequences in mRNA signal where to start protein synthesis.
Start codon AUG methionine.
Recognition of correct start codon requires 3’ end of 16S of the small subunit.
Initiator tRNA in the P-site in bacteria and A - site in all other aminoacyl tRNAs.
Initiation factors facilitate assembly of the initiation complex.

Question 24

3 steps in elongation?

Answer 24

Codon recognition
Peptide bond formation.
Translocation.

Question 25

Events of codon recognition elongation?

Answer 25

Aminoacyl-tRNA entry into A-site. Ribosomes have fidelity function small subunits rRNA validates H-bonds and if they have not formed between all three base pairs, the tRNA must be incorrect match, and its is rejected.

Question 26

Events of peptide bond formation elongation?

Answer 26

Met is linked to pro in A site. After peptidyl transfer there is a new peptidyl-tRNA in the A site and a deacylated tRNA in the P site. Carried out by 23S rRNA. Occurs when A and P sites are appropriately occupied. Generates the new peptide bond. Polypeptide is now in the A site.

Question 27

Events of translocation in elongation?

Answer 27

Simultaneously, the free tRNA is moved to the E-site, the peptidyl-tRNA is moved to the P-site. The deacylated tRNA is ejected via E-site the peptidyl-tRNA is moved to the P-site together with its codon and the next codon is exposed in the A-site.

Question 28

The steps in termination?

Answer 28

Stop codons mark the end of translation. Stop codons are not recognised by any aa~tRNA but rather by protein release factors. Released factors promote hydrolysis of completed peptide from the tRNA and expulsion of the tRNA. RRF promotes dissociation of the ribosomal subunits, tRNA and mRNA.

Question 29

How are proteins made on polyribosomes (polysomes)

Answer 29

Several ribosomes can work together to translate the same mRNA producing multiple copies of the polypeptide. A strand of mRNA with associated ribosomes is called polysome.

Question 30

Cooperation between transcription and translation in bacteria?

Answer 30

During transcription of protein-coding genes, bacterial RNA polymerase is closely followed by a ribosome that is engaged in translation of the newly synthesized transcript. As a result of translation-transcription coupling the overall elongation rate of transcription is tightly controlled by translation.

Question 31

How to do some proteins fold?

Answer 31

While still being synthesized. Growing peptide chain folded N-terminal domain. Folding C-terminal domain. Folding protein completed after release from ribosome.