Q: A tRNA anticodon contains Inosine (I) at its wobble position (e.g., $$3'\text{-}UAI\text{-}5'). $$Which mRNA codons can this tRNA recognize, and what amino acid does it deliver?

It recognizes $$5'\text{-}AUA, AUU, AUC\text{-}3'$$ and delivers isoleucine

Question 1

Which experimental finding led to the shift from the One Gene–One Enzyme hypothesis to the One Gene–One Polypeptide theory?

Accepted Answer

Vernon Ingram’s identification of a single amino acid substitution in sickle-cell hemoglobin

Question 2

Why is a doublet code insufficient to specify all 20 amino acids, and how does a triplet code solve this problem? Use the formula for possible codon combinations.

Accepted Answer

A doublet code yields $$4^2 = 16$ combinations, which is less than 20; a triplet code yields 4^3$ = 64$$ combinations, which is sufficient

Question 3

Crick and Brenner’s proflavin-induced indel experiments in phage T4 demonstrated that the genetic code is read in non-overlapping triplets. Which result best supports this conclusion?

Accepted Answer

Mutations that added or removed a single base caused frameshifts, but adding/removing three bases restored the original reading frame

Question 4

Which statement correctly distinguishes between 'degenerate' and 'unambiguous' as applied to the genetic code?

Accepted Answer

'Degenerate' means multiple codons can specify the same amino acid; 'unambiguous' means each codon specifies only one amino acid

Question 5

A tRNA anticodon contains Inosine (I) at its wobble position (e.g., $$3'	ext{-}UAI	ext{-}5'). $$Which mRNA codons can this tRNA recognize, and what amino acid does it deliver?

Accepted Answer

It recognizes $$5'	ext{-}AUA, AUU, AUC	ext{-}3'$$ and delivers isoleucine

Question 6

Which two structural features of tRNA are essential for aminoacyl-tRNA synthetase recognition, and how does the enzyme ensure high fidelity?

Accepted Answer

The anticodon loop and the acceptor stem; proofreading removes incorrectly attached amino acids

Question 7

During eukaryotic translation initiation, which sequence is recognized by the preinitiation complex after scanning the mRNA, and what is its significance?

Accepted Answer

The Kozak consensus sequence; it ensures correct start codon selection in eukaryotes

Question 8

Which ribosomal RNA acts as a ribozyme to catalyze peptide bond formation during polypeptide elongation?

Accepted Answer

$$23S$$ rRNA in prokaryotes and $$28S$$ rRNA in eukaryotes

Question 9

Protein release factors recognize stop codons in the A site of the ribosome. What mechanism allows them to mimic tRNA and facilitate polypeptide release?

Accepted Answer

Molecular mimicry: release factors structurally resemble tRNA, allowing them to enter the A site and trigger hydrolysis of the polypeptide from tRNA

Question 10

How do Hsp70 and Hsp60 (GroEL/GroES) chaperones differ in their mechanisms for assisting protein folding, and what role does ATP play?

Accepted Answer

Hsp70 binds nascent chains and prevents aggregation; Hsp60 provides an isolated chamber for folding. Both use ATP hydrolysis to drive conformational changes and correct misfolding caused by hydrophobic interactions.

Question 11

Which mutation type results in a codon change that produces a premature stop codon, and how can a suppressor tRNA overcome this effect?

Accepted Answer

Nonsense mutation; a suppressor tRNA with a mutated anticodon can recognize the stop codon and insert an amino acid, allowing translation to continue

Question 12

Which scenario best illustrates codon usage bias in a real-world context?

Accepted Answer

Certain organisms preferentially use specific synonymous codons, affecting translation efficiency and protein expression levels

Question 13

A patient’s mRNA contains a premature stop codon upstream of an Exon Junction Complex (EJC). Which quality control mechanism is triggered, and why?

Accepted Answer

Nonsense-Mediated Decay (NMD) is triggered because the EJC downstream of the stop codon signals incomplete translation

Question 14

Which step in eukaryotic translation initiation involves the recruitment of the preinitiation complex to the mRNA cap, and which factors mediate this process?

Accepted Answer

Recruitment to the 5' cap is mediated by eIF4E and eIF4G

Question 15

During polypeptide elongation, which factor is responsible for delivering aminoacyl-tRNA to the A site, and what energy molecule is required?

Accepted Answer

EF-Tu with GTP

Question 16

Which mutation type results in a codon change that does NOT alter the amino acid sequence, and what is its effect on protein function?

Accepted Answer

Silent mutation; typically has no effect on protein function

Question 17

What is the main difference between Nonsense-Mediated Decay (NMD) and Nonstop Decay in eukaryotic cells?

Accepted Answer

NMD targets mRNAs with premature stop codons and downstream EJCs; Nonstop Decay targets mRNAs lacking stop codons

Question 18

A mutation in the tRNA anticodon loop prevents proper recognition by aminoacyl-tRNA synthetase. What is the likely consequence for protein synthesis?

Accepted Answer

The tRNA will not be charged with its amino acid, leading to translation errors or stalling

Cell Biology Study Guide: Genetic Code, Translation, and Protein Folding

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