FHMP 007 translation

Question 1

What is translation?

Answer 1

the decoding of an mRNA message into a protein

Question 2

Describe the process of translation

Answer 2

1. mRNA attaches to the ribosome and translation begins with the start codon.
2. Beginning with the start codon, a complementary (anti-codon) tRNA molecule, carrying an amino-acid is brought to the ribosome and attached to that codon. This releases the amino-acid, to be used in constructing a new polypeptide (protein). Then this tRNA molecule is released.
3. The ribosome advances through the mRNA molecule’s codons, one by one, repeating the process above and attaching the new amino-acid to the chain of the previous ones.
4. Once it reaches the stop codon the process is ended, and the completed amino-acid chain, which is the new polypeptide, or protein, is released to the cell.

Question 3

What is tRNA?

Answer 3

• tRNA is transfer RNA. It carries amino acids around during translation.
• 1 strand of RNA looped
• amino acid attaches to the 3’ end on the acceptor arm
• 1 tRNA can bind to more than 1 codon
• the anticodon loop pairs with the mRNA codon

Question 4

How does an amino acid attach to tRNA?

Answer 4

• amino acid tRNA synthase is specific to each tRNA amino acid combination and attaches the amino acid to the 3’ of tRNA
• the codon reads 5’ to 3’ and the anticodon reads 3’ to 5’
• uses ATP

Question 5

How many amino acids are there and how many codons are there?

Answer 5

• 20 amino acids and 61 codons and 3 stop codons

- this is because many amino acids have multiple codons that can code for them

Question 6

how many combinations can you make of codons?

Answer 6

64 as 4 bases and 3 bases in a codon, so 4^3 = 64

Question 7

what is the wobble mechanism?

Answer 7

• explains how 1 anticodon can bind to more than 1 codon ( 61 codons but only 20 amino acids)
• the first or wobble position of the anticodon doesn’t always follow Watson-Crick base pairing rules
• the first anticodon base = 3rd codon base
• Inosine (I) = naturally occurring rRNA purine base
• in the 3rd base (mRNA/codon) Us and Cs can be read by a G in the anticodon, and As and Gs can be read by the U anticodon
• if the tRNA contains I in the anticodon at the wobble position, it can read As, Cs and Us on the codon
• but still gives the same amino acid as the tRNA is the same (only 20 tRNAs anticodons = 20 amino acids)

Question 8

describe the differences between bacteria and eukaryote ribosomes

Answer 8

bacteria = 70s (smaller)
eukaryote = 80s (bigger)

Question 9

what are the leader and trailer sections?

Answer 9

leader section = upstream of the start codon

trailer section = downstream of the stop codon

Question 10

How is translation initiated in bacteria?

Answer 10

• the ribosome binding site ( the Shine-Dalgarno sequence) has direct base pairing with the mRNA just upstream of the start codon
• incorporation of IF1,2 (GTPase), 3, and GTP to the small ribosomal subunit
• GTPase hydrolysis GTP to GDP + Pi
• the initiation factors are then released by the energy from the hydrolysis substrate and the ribosome is ready for translation

Question 11

How is translation initiated in eukaryotes?

Answer 11

• ribosome scans until the Kozak sequence on the mRNA is found
• initiation factors 1,2(GTPase),3 and GTP bind to small ribosome subunits
• the cap structure at the 5’ end of the mRNA is recognised by IF4, which binds and activates a subunit containing initiator tRNA to the 5’ end of mRNA
• the mRNA bound subunit travels along the 5’ untranslated end of the mRNA until it reaches the first start codon
• this process needs ATP and IFs
• when the activated small subunit has reached the first start codon, the large subunit binds to the small subunit which requires IF5 and hydrolysis IF2-bound GTP, thus releasing IFs

Question 12

what is translation elongation?

Answer 12

Elongation is the addition of amino acids by the formation of peptide bonds as the ribosome is moving 5’ - 3’ along mRNA

Question 13

describe the process of translation elongation

Answer 13

• aminoacyl-tRNA is carried to the ribosome where mRNA codons are matched through complementary base pairing to specific tRNA anticodons
• Ribosome has 3 tRNA binding sites (E-P-A 5’-3’ )
• A = aminoacyl site = binds to tRNA charged with the mRNA codon
• P = peptidyl site = holds the tRNA with the growing peptide chain
• E = exit site = holds uncharged tRNA
• tRNA in the E site leaves and another amino-acyl-tRNA enters site A
• aminoacyl-tRNAs are picked up by EF1 in the presence of GTP
• proofreading occurs where the tRNA is checked to see if anti-codon matches codon, if not it is removed and another is added
• when the correct amino-acyl-tRNA enters A site, the growing polypeptide in the P-site is almost immediately linked via a peptide bond
• the ribosome catalyses the peptide bond formation
• tRNA now moves from the P site to the E site without the amino acid and process repeats with a new tRNA
• translocation is facilitated by EF2 and GTP
• repeats until termination/stop codon

Question 14

How is translation terminated?

Answer 14

• stop codon
• When a protein called a release factor (RF1) recognises the stop code and then enters and binds to the A site it terminated the chain by adding a water molecule to the end of the chain
• this hydrolyses the polypeptide from tRNA

Question 15

give 3 examples of how antibiotics interfere with translation

Answer 15

• tetracycline = inhibits aminoacyl-tRNA binding to site A
• erythromycin = binds to peptidyl transferase site and blocks polypeptide exit and stops translation
• chloramphenicol = blocks correct positioning of the A-site aminoacyl-tRNA for peptidyl transferase reaction