Lecture 15 - Protein Synthesis

Question 1

What are the major steps of protein biosynthesis?

Answer 1

1) Activation of amino acids (attachment to tRNA)
2) Initiation
3) Elongation
4) Termination/release
5) Post-translational modifications

Question 2

What is required for activation of translation?

Answer 2

Matching up a 3 base sequence (codon) on the mRNA with a 3 base anticodon on the tRNA carrying a specific amino acid

Question 3

What is the most important enzyme in bringing about the process of translation and what does it do?

Answer 3

• Aminoacyl tRNA synthetase

- Brings together the correct amino acid with the correct tRNA

Question 4

How many amino acids can one tRNA accept?

Answer 4

Only one

Question 5

What must happen to the amino acid for it to be able to bind to the tRNA?

Answer 5

Must be activated at the carboxyl end

Question 6

__ ATP are required for activation of translation

Answer 6

2

Question 7

What is the difference between class 1 and class 2 aminoacyl tRNA synthetases?

Answer 7

• Class 1 enzymes attach the amino acid to the 2’ OH and then move it to the 3’ OH
• Class 2 enzymes attach the amino acid directly to the 3’ OH

Question 8

The 3’ nucleotide of most tRNA’s is _____

Answer 8

Adenosine

Question 9

What are ribosomes made of?

Answer 9

RNA and protein

Question 10

What are the 2 subunits of a ribosome in E. coli?

Answer 10

The 70S ribosome is composed of the 50S subunit and the 30S subunit

Question 11

Which amino acid starts synthesis in bacteria and which amino acid in eukaryotes?

Answer 11

• Bacteria - formyl methionine

- Eukaryotes - methionine

Question 12

What is always the start codon and what does it correspond to?

Answer 12

AUG corresponds to the methionine tRNA

Question 13

What enzyme adds methionine to tRNA?

Answer 13

Methionyl tRNA synthetase

Question 14

What factors are needed for initiation of translation and what do the factors do?

Answer 14

Initiation factors - IF1 and IF3 bind to the 30S, which then binds the mRNA

Question 15

____ helps the 30S subunit bind to the AUG codon

Answer 15

16S rRNA

Question 16

What are the steps of elongation?

Answer 16

1) Insertion of amino acyl tRNA
2) Peptide bond formation
3) Movement of the ribosome down the mRNA

Question 17

What is required for insertion of amino acyl tRNA during elongation?

Answer 17

ETFu and EFTs to activate the aminoacyl tRNA for insertion into A site

Question 18

What are the 3 possible stop codons?

Answer 18

UAA, UAG, or UGA because there are no complementary tRNA’s

Question 19

What are the functions of release factors?

Answer 19

1) Hydrolyze the terminal peptidyl-tRNA bond
2) Release the free polypeptide and last tRNA from the P site
3) Dissociate the 70S ribosome to 50S and 30S subunits

Question 20

What are the energy requirements for translation?

Answer 20

• Activation of fMet-tRNA = 2 ATP
• Insertion of fMet-tRNA = 1 ATP
• Activation of aa-tRNA = 2 ATP/amino acid
• Insertion of aa-tRNA = 1 ATP/amino acid
• Translocation = 1 ATP/amino acid
• Termination = 1 ATP
• Total = 4 ATP + 4 * # of amino acids after fMet

Question 21

Is ATP used for translation or ATP equivalents?

Answer 21

ATP equivalents (ATP is cleaved to AMP in aminoacyl-tRNA and GTP is used with factors)

Question 22

How many codons are there and how many tRNA molecules do most organisms produce?

Answer 22

• Codons = 64, with 3 being termination codons

- About 40 tRNA molecules are produced

Question 23

What is the wobble hypothesis?

Answer 23

The 5’ base of the anticodon is not spatially constrained (it can wobble) allowing for an alternate form of base pairing

Question 24

What is alternate base pairing?

Answer 24

When U and G pair - U uses a different =O group and G doesn’t use one of the NH2 groups

Question 25

What does alternate base pairing allow for?

Answer 25

One tRNA’s anticodon can recognize more than one codon (the third U can be paired with an A or G; the third G can be paired with a C or U)