Translation

Question 1

only …. is translated?

Answer 1

mRNA

Question 2

what do all polypeptides begin with?

Answer 2

methionine - because all mRNA molecules begin with the codon ‘AUG’

Question 3

what three steps does translation involve?

Answer 3

initiation
elongation
termination

Question 4

what does initiation consist of?

Answer 4

assembly of the three components that carry out the process
(ribosome, tRNA, mRNA)

Question 5

What does elongation involve?

Answer 5

stepwise addition of amino acids to the growing protein chain

Question 6

what does termination involve?

Answer 6

stop codon is encountered, triggering dissociation of complex

Question 7

structure of ribosome?

Answer 7

made up of: protein, rRNA, active site that catalyses addition of amino acid to polypeptide chain

Question 8

what does the large subunit of the ribosome include?

Answer 8

50S - contains three slots for tRNAs

Question 9

what does the small ribosomal subunit involve?

Answer 9

30S - contains the mRNA binding site

Question 10

what are the three slots found in the large subunit of the ribosome?

Answer 10

APE
A - aminoacyl - tRNA
P - peptidyl-tRNA
E - exit

Question 11

which mutations causes a severe loss of function resulting from a change in the reading frame?

Answer 11

insertion or deletions of 1,2,4,5 etc. nucleotides

Question 12

which mutations would have little effect on the phenotype and why?

Answer 12

insertion or deletions of 3,6,9 etc. because the reading frame is not affected for the most of the mRNA

Question 13

what enzyme is used to load tRNA with the correct amino acid?

Answer 13

enzymes called aminoacyl-tRNA synthetases –> they are specific for each amino acid

Question 14

Describe the 1st stage of initiation:

Answer 14

complex of small ribosomal subunit and initiator tRNA (UAC anticodon) binds to 5’ cap

Question 15

Describe the 2nd stage of initiation:

Answer 15

complex moves along mRNA in 3’ direction scanning for AUG start codon

Question 16

Describe the 3rd stage of initiation:

Answer 16

tRNA molecule binds to the codon via its anticodon

Question 17

Describe the 4th stage of initiation:

Answer 17

Large ribosomal subunit aligns itself to the tRNA molecule at the ‘P’ site and forms a complex with the small subunit

Question 18

Describe the 1st stage of elongation:

Answer 18

second tRNA molecule pairs with the next codon in the ribosomal A site

Question 19

Describe the 2nd stage of elongation:

Answer 19

the amino acid in the P site is covalently attached via a peptide bond (condensation reaction) to the amino acid in the A site

Question 20

Describe the 3rd stage of elongation:

Answer 20

tRNA in the P site is now de-acylated (no amino acid), while the tRNA in the A site carries the peptide chain

Question 21

Describe the 4th stage of elongation:

Answer 21

the ribosome moves along the mRNA strand by one codon position (in a 5’-3’ direction)

Question 22

Describe the 5th stage of elongation:

Answer 22

De-acylated tRNA moves into the E site and is released, while the tRNA carrying the peptide chain moves to the P site (translocation)

Question 23

Describe the 6th stage of elongation:

Answer 23

new tRNA attaches to the next codoin in A site

Question 24

Describe the 1st stage of termination:

Answer 24

elongation and translocation continue until the ribosome reaches a stop codon

Question 25

Describe the 2nd stage of termination:

Answer 25

stop codons do not recruit a tRNA molecule, but instead recruit a release factor that signals for translation to stop

Question 26

what are the three stop codons?

Answer 26

UAA
UAG
UGA

Question 27

What is the 3rd stage of termination?

Answer 27

polypeptide is released and the ribosome dissociates into its two independent subunits

Question 28

Describe the 4th stage of termination:

Answer 28

the polypeptide is ready for post-translational modification, folding and sorting