Protein Synthesis

Question 1

What is the central dogma?

Answer 1

DNA -> RNA -> Protein

Question 2

Where does protein synthesis occur?

Answer 2

• In the ribosomes (either free floating in the cytosol or on the RER)

Question 3

What is a UTR?

Answer 3

Untranslated region of the the mRNA

Question 4

What does the 5’ UTR do?

Answer 4

Determines the rate of protein synthesis

Question 5

What does the 3’ UTR do?

Answer 5

Affects the stability of the mRNA

Question 6

What are codons?

Answer 6

A triplet code of aa

Question 7

How many codons are there?

Answer 7

64 (4 x 4 x 4)

Question 8

Where on the ribosome does protein synthesis occur?

Answer 8

The cleft between the large ribosomal subunit and small ribosomal subunit

Question 9

What are the differences between the large and small ribosomal subunit?

Answer 9

• Large is 60s and small is 40s (together forms 80s ribosome)
• Large has three RNA types ( 5s, 5.8s, 28s) but small has one (18s)
• Large has 4700 nucleotides but small has 1900
• Large has 49 proteins but small has 30

Question 10

How many active sites does the cleft have? What are the names of the sites and what binds to them?

Answer 10

• 3 active sites
• Aminoacyl tRNA binding site binds to aminoacyl tRNAs (charged tRNAs = met-tRNA)
• Peptidyl binding site binds to met-tRNAi with the growing polypeptide chain
• Exit site where the tRNA is release back into the cytoplasm

Question 11

Describe initiation

Answer 11

• Initiation factors , EIF4 and EIF5, bind with the cap and the poly A binding protein associates with the poly A tail.
• Initiation factors assemble on the small subunit and have GTP bound to it.
• A met-tRNA binds to the peptidyl binding site on the small ribosomal subunit.
• The small subunit binds to the 5’ cap of the mRNA and the ribosome scans along until it finds the AUG start codon which is next to a kozak sequence.

Question 12

What happens when a start codon is found?

Answer 12

• Initiation is paused, 60s subunit binds to 40s to form 80s ribosome

–EIF4 and EIF5 dissociate

Question 13

What is the name of the enzyme that binds the aa on the P site to the aa on the A site?

Answer 13

Peptidyl transferase

Question 14

Describe elongation

Answer 14

• tRNA with the next amino acid comes to the A site which involves elongation factor 1 (EF1)
• When the covalent bond forms between methionine and the next amino acid, peptidyl transferase moves the amino acid in the P site to the A site
• The tRNA on the P site is now empty
• The tRNA in the A site has both amino acids attached to it.
• P site tRNA now moves to the E site so it can go and get recharged so a new tRNA can enter the A site

Question 15

What is translocation?

Answer 15

• tRNAs from the A & P sites moving to the P & E sites
• Requires energy from GTP, EF2 and enzyme translocase

Question 16

What is a polysome?

Answer 16

Multiple ribosomes attached to a mRNA strand

Question 17

Describe termination

Answer 17

• Ribosome encounters stop codon
• Causes protein release factor eRF1 to be acquired
• Triggers the hydrolysis of GTP and terminal peptide tRNA to release polypeptide chain
• Dissociation of ribosome

Question 18

What are some differences between bacterial ribosomes and eukaryotic ribosomes?

Answer 18

• BR are 70s whereas ER are 80s

- BR made of 50s+30s subunits but ER made of 60s+40s

Question 19

What does streptomycin do?

Answer 19

• Targets small subunit
• Inhibits initiation
• Cause misreading of genetic code

Question 20

What does erythromycin do?

Answer 20

Inhibition of translocation