Translation (Part 2)

Which statement BEST explains why the ribosome is considered a ribozyme?
A. It binds tRNA molecules
B. It contains proteins that catalyze reactions
C. rRNA catalyzes peptide bond formation
D. It hydrolyzes GTP during elongation

C

The peptidyl transferase center is located in the:
A. Small subunit decoding center
B. Large subunit rRNA
C. tRNA anticodon loop
D. mRNA exit channel

B

During elongation, which factor directly delivers aminoacyl-tRNA to the A site?
A. EF-G
B. EF-Ts
C. EF-Tu
D. RF1

C

What is the primary role of EF-G?
A. Deliver tRNA to A site
B. Hydrolyze peptide bonds
C. Recognize stop codons
D. Drive ribosome translocation

D

Why does peptide bond formation not require direct ATP hydrolysis?
A. Energy comes from the high-energy acyl bond
B. GTP hydrolysis substitutes for ATP
C. Ribosome provides energy
D. No energy is required

A

Which of the following BEST describes the A, P, and E sites?
A. They coordinate sequential tRNA movement
B. They exist only in prokaryotes
C. All bind aminoacyl-tRNAs
D. They are located only in the small subunit

A

The initiator tRNA in prokaryotes differs because it:
A. Enters the A site first
B. Is charged with N-formyl methionine
C. Does not recognize AUG
D. Binds only to the large subunit

B

What triggers release of initiation factors during initiation?
A. ATP hydrolysis
B. Ribosome reaching stop codon
C. EF-Tu activity
D. GTP hydrolysis by IF2

D

Which mechanism ensures correct codon-anticodon pairing?
A. Peptidyl transferase
B. EF-G activity
C. Poly-A tail
D. 16S rRNA interactions and EF-Tu proofreading

D

Class I release factors resemble tRNA because they:
A. Mimic codon recognition and occupy A site
B. Have anticodon loops
C. Bind amino acids
D. Catalyze peptide elongation

A

What happens if an mRNA lacks a stop codon in prokaryotes?
A. Translation stops normally
B. Ribosome stalls and is rescued by tmRNA
C. Ribosome falls off
D. Protein synthesis speeds up

B

In eukaryotes, nonsense-mediated decay is triggered when:
A. Premature stop codon is detected before exon junction complexes are removed
B. Ribosome stalls at poly-A tail
C. mRNA lacks a start codon
D. tRNA is mischarged

A

The ribosome consists of a __________ and __________ subunit.

large and small

The decoding center is located in the __________ subunit.

small

The peptidyl transferase reaction is catalyzed by __________.

rRNA (23S rRNA / peptidyl transferase center)

The A site binds __________ tRNA.

aminoacyl (charged)

The P site holds the __________ tRNA.

peptidyl

The E site is responsible for __________.

exit of uncharged tRNA

EF-Ts functions as a __________ factor.

GTP exchange

Translation requires ___ ATP and ___ GTP per elongation cycle.

1 ATP and 2 GTP

Stop codons are recognized by __________ instead of tRNAs.

release factors

tmRNA has properties of both __________ and __________.

tRNA and mRNA

The ribosome is primarily made of protein. (True/False)

False

Peptide bond formation is catalyzed by rRNA. (True/False)

True

Initiator tRNA enters the A site first. (True/False)

False

GTP hydrolysis is required for accuracy in translation. (True/False)

True

EF-Tu binds tRNA in its GDP-bound form. (True/False)

False

Translocation moves the ribosome by one nucleotide. (True/False)

False

The polypeptide grows from C-terminus to N-terminus. (True/False)

False

Release factors hydrolyze the polypeptide from tRNA. (True/False)

True

All stop codons are recognized by a single factor in prokaryotes. (True/False)

False

mRNA decay mechanisms require translation to occur first. (True/False)

True

Match the factor to its function:

Column A	Column B
1. EF-Tu	A. Translocation
2. EF-G	B. GTP exchange
3. EF-Ts	C. Delivers aminoacyl-tRNA
4. RF1/RF2	D. Stop codon recognition
5. RF3	E. Release of RFs

1-C, 2-A, 3-B, 4-D, 5-E

Match structure to function:

Column A	Column B
1. A site	A. Exit of tRNA
2. P site	B. Peptide bond formation
3. E site	C. Holds growing peptide
4. Large subunit	D. Aminoacyl-tRNA entry
5. Small subunit	E. Decoding center

1-D, 2-C, 3-A, 4-B, 5-E

Explain why the ribosome is considered both a structural and catalytic machine.

Ribosome is structural (holds mRNA/tRNA) and catalytic (rRNA catalyzes peptide bonds).

Describe the sequence of events during one elongation cycle.

• EF-Tu delivers aminoacyl-tRNA to A site (GTP-dependent)

• Codon-anticodon pairing checked

• Peptide bond formed (peptidyl transferase)

• EF-G mediates translocation

• Ribosome moves one codon

Explain how EF-Tu and EF-Ts work together during translation.

EF-Tu delivers tRNA in GTP-bound form → hydrolyzes GTP → EF-Ts reloads EF-Tu with GTP.

Why is GTP hydrolysis important for translation accuracy rather than peptide bond formation?

GTP ensures correct tRNA selection and proper timing, not bond formation.

Compare initiation in prokaryotes vs eukaryotes at the molecular level.

• Prokaryotes: Shine-Dalgarno alignment, no scanning

• Eukaryotes: 5’ cap binding + scanning to AUG

Explain how release factors mimic tRNA and why this is important.

RFs structurally mimic tRNA → fit into A site → trigger hydrolysis of peptide.

Describe the mechanism and importance of translocation.

Ribosome shifts 3 nucleotides; tRNAs move A→P→E; requires EF-G + GTP.

Explain how nonsense-mediated decay distinguishes normal vs defective mRNA.

Premature stop codons leave exon junction complexes intact → trigger decay.

Why must defective mRNAs be translated before being degraded?

Translation exposes errors (e.g., premature stop or no stop), enabling detection.

Predict the effect of a mutation in EF-G that prevents GTP hydrolysis.

No translocation → ribosome stalls → translation halts.