07_Protein Synthesis (cont'd)

Question 1

During which phase of the cell cycle does the nucleolus disappear? Why?

Answer 1

mitosis

DNA is packed, so no RNA can be made

Question 2

In (eukaryotes/prokaryotes), transcription and translation occur simultaneously.

Answer 2

Prokaryotes

Question 3

Match
70S and 80S ribosomes
eukaryotes, prokaryotes

Answer 3

70S: P
80S: E

Question 4

What are the small and large subunits of prokaryotic ribosomes, and what are each comprised of?

Answer 4

small subunit: 30S (16S rRNA + 21 proteins)

large subunit: 50S (5S rRNA + 23S rRNA + 34 proteins)

Question 5

What are the small and large subunits of eukaryotic ribosomes, and what are each comprised of?

Answer 5

small subunit: 40S (18S rRNA + 33 proteins)

large subunit: 60S (28S rRNA + 5.8S rRNA + 5S rRNA + 49 proteins)

Question 6

Operons are found in (prokaryotes/eukaryotes).

Answer 6

Prokaryotes

Question 7

When is the m7 guanine cap added to the 5’ end of the mRNA transcript?

Answer 7

at initiation of transcription

Question 8

What type of modification is the addition of the poly-A-tail? What enzyme catalyzes this modification?

Answer 8

post-transcriptional; poly (A) polymerase

Question 9

What prevents translation in the 5’ UTR?

Answer 9

secondary structure elemnents, like hairpins and loops

Question 10

What does PABP bind to?

Answer 10

mRNA poly A tail

Question 11

What protein on mRNA allows it to be exported from the nucleus through nuclear pores?

Answer 11

nuclear export receptor

Question 12

The eIF4F complex is composed of 3 proteins. What are they, and what does each bind to?

Answer 12

eIF4E: cap-binding proteins, binds to 5’ cap.
eIF4A: is an RNA helicas; unwinds loops and hairpin structures in the 5’UTR
eIF4G: bridge between eIF4E and PABP

Question 13

What is the ternary complex composed of?

Answer 13

initiatory methionyl-tRNA, eIF2 and GTP

Question 14

What does the ternary complex bind to? What forms as a result?

Answer 14

Binds to the 40S ribosomal subunit to form a 43S pre-initiation complex

Question 15

As the initiator tRNA scans the mRNA for AUG, what is used to provide energy for helicase activity by eIF4A?

Answer 15

Hydrolysis of ATP

Question 16

What happens when the initiator tRNA anneals to a codon? (eukaryotes)

Answer 16

GTP is hydrolized, and eIF2 and the other initiation factors dissociate. Then, the large ribosomal subunit (60S) binds.

Question 17

When the large subunit first binds, which site is methionyl tRNA loaded in?

Answer 17

the P site

Question 18

What kind of bond is formed between the two amino acids in the P and A sites?

Answer 18

phosphoester bond

Question 19

Which end of the peptide emerges from the ribosome first? (i.e. which direction are peptides synthesized in?)

Answer 19

The amino end

amino –> carboxyl

Question 20

What catalyses binding of an aa-tRNA to the A-site?

Answer 20

eEF1α in eukaryotes

EF-TU in prokaryotes

Question 21

What does eEF2 catalyze?

Answer 21

Movement of the amino-acid peptide from the A-site to the P-site and movement of the ribosome to the next codon.

Question 22

What are the equivalents of eEF1 and eEF2 in prokaryotes?

Answer 22

eEF1 –> EF-TU

eEF2 –> EF-G

Question 23

Which eukaryotic elongation factor is a G-protein with proofreading activity?

Answer 23

eEF1

Question 24

What are the three stop codons?

Answer 24

UAA, UAG, UGA

Question 25

What binds to the A-site when a stop codon is encountered?

Answer 25

Release factors

Question 26

What are polysomes?

Answer 26

Numerous ribosomes translating the same mRNA strand

Question 27

What does presence of the release factor in the P site trigger?

Answer 27

Disassembly of of the 80S ribosome-mRNA complex

Question 28

How does tetracycline act as an antibiotic?

Answer 28

blocks binding of aminoacyl-tRNA to A-site of ribosomes

Question 29

How does chloramphenicol act as an antibiotic?

Answer 29

blocks peptidyl transferase reaction on ribosomes

peptidyl transferase = ribozyme; forms bonds b/w adjacent amino acids

Question 30

What is an example of a translation inhibitor that only acts on eukaryotes? What does it target?

Answer 30

Cycloheximide; blocks translocation reaction on ribosomes