Question
What is the primary function of the free ribosomes shown in the electron micrograph?
A. Synthesize proteins to be used within the cell
B. Synthesize proteins for use in lysosomes
C. Carry amino acids to mRNA for protein synthesis
D. Synthesize proteins for secretion
▶️Answer/Explanation
Answer: A. Synthesize proteins to be used within the cell
Explanation:
- Free ribosomes are located in the cytoplasm (not attached to the endoplasmic reticulum).
- They synthesize proteins that remain within the cell, such as enzymes in the cytosol or structural proteins.
- In contrast, ribosomes attached to the rough ER synthesize proteins for secretion, for use in lysosomes, or for insertion into membranes.
Why not the others?
B. Proteins for lysosomes are synthesized by ribosomes on the rough ER.
C. This is the role of tRNA, not ribosomes.
D. Proteins for secretion are made by bound ribosomes, not free ones.
Question
Which types of interactions are found in a part of a protein with secondary but not tertiary structure?
I. Hydrogen bonds
II. Disulphide bridges
III. Ionic bonds
A. I only
B. I and II only
C. II and III only
D. I, II and III
▶️Answer/Explanation
Answer: A. I only
Explanation:
- A protein’s secondary structure includes regular folding patterns such as alpha-helices and beta-pleated sheets, which are held together only by hydrogen bonds between the backbone atoms (not the side chains).
- Disulphide bridges (covalent bonds between cysteine side chains) and ionic bonds (between charged side chains) are part of the tertiary structure, which involves interactions among the R-groups (side chains) that give the protein its 3D shape.
- Therefore, only hydrogen bonds are involved in secondary structure, while the others appear later in the folding hierarchy.
Answer Evaluation:
I. Correct – Hydrogen bonds form between backbone atoms in secondary structures like alpha-helices and beta-sheets.
II. Incorrect – Disulphide bridges form in tertiary structure between cysteine side chains.
III. Incorrect – Ionic bonds are interactions between charged side chains and occur in tertiary structure.
Question
This DNA sequence was used to synthesize a polypeptide.
DNA (sense strand): 3′ T A C T G A 5′
DNA (template strand): 5′ A T G A C T 3′
Which are the bases of the tRNA (anticodons)?
A. T A C T G A
B. U A C U G A
C. A U G A C U
D. A T G A C T
▶️ Answer/Explanation
Answer: C. A U G A C U
Explanation:
- DNA sense strand: 3′ T A C T G A 5′
- DNA template strand: 5′ A T G A C T 3′
mRNA is transcribed from the template strand, and tRNA anticodons pair with mRNA codons.
Step 1: Transcribe the mRNA from the template strand
Template DNA: 5′ A T G A C T 3′
mRNA (complementary, with U replacing T): 5′ U A C U G A 3′
Step 2: Determine the tRNA anticodons
tRNA anticodons are complementary to mRNA codons.
mRNA: 5′ U A C U G A 3′
tRNA: 3′ A U G A C U 5′
So, the tRNA anticodons are A U G and A C U
Final Evaluation:
- A. Incorrect – Contains DNA bases, not RNA.
- B. Incorrect – These are mRNA codons, not tRNA anticodons.
- C. Correct – A U G and A C U are the correct tRNA anticodons.
- D. Incorrect – Contains DNA bases (T), not RNA.
Question
A DNA triplet on the strand that is transcribed has the bases TAG. Which anticodon on tRNA is used in translation?
A. AUC
B. UAG
C. TAG
D. ATC
▶️Answer/Explanation
Answer: A. AUC
Explanation:
- The DNA triplet given is TAG on the transcribed strand (also called the sense strand).
- During transcription, the template strand is used to make the mRNA codon complementary to the template. Since the transcribed strand is the same sequence as the mRNA (except T replaced by U), the mRNA codon will be UAG (replacing T with U).
- The tRNA anticodon is complementary and antiparallel to the mRNA codon. The mRNA codon is UAG, so the tRNA anticodon will be AUC.
- The anticodon on tRNA pairs specifically with the mRNA codon during translation.
Answer Evaluation:
A. Correct – AUC is complementary to the mRNA codon UAG and is the correct tRNA anticodon.
B. Incorrect – UAG is the mRNA codon, not the tRNA anticodon.
C. Incorrect – TAG is a DNA triplet, not an anticodon.
D. Incorrect – ATC is DNA bases and does not represent tRNA anticodon.
Question
What is a feature of transcription in the single-celled organism shown in the electron micrograph?
A. mRNA splicing
B. Removal of introns
C. Codon-anticodon binding
D. Synthesis of RNA in a 5′ to 3′ direction
▶️Answer/Explanation
Answer: D. Synthesis of RNA in a 5′ to 3′ direction
Explanation:
The electron micrograph shows a prokaryotic cell likely a bacterium as indicated by:
- Lack of membrane-bound organelles (e.g., no nucleus).
- A single circular DNA region (nucleoid).
- Small cell size (~1.0 μm scale bar).
The question asks about transcription in this prokaryotic organism.
Correct answer: D. Synthesis of RNA in a 5′ to 3′ direction
Why the others are incorrect:
A. mRNA splicing – This occurs in eukaryotes, where introns are removed from pre-mRNA. Prokaryotic mRNA is not spliced.
B. Removal of introns – Again, prokaryotes generally do not have introns in their genes.
C. Codon-anticodon binding – This occurs during translation, not transcription.
Question
The anticodons of three tRNAs and the amino acids they carry are shown in the table.
Which base sequence of an mRNA molecule would code for an arginine–leucine–valine tripeptide?
A. GCA AAU CAG
B. GCA AAT CAG
C. CGT TTA GTC
D. CGU UUA GUC
▶️Answer/Explanation
Answer: D. CGU UUA GUC
Explanation:
Step 1: Understand codon–anticodon pairing
- Codons (on mRNA) are complementary and antiparallel to anticodons (on tRNA).
- If the anticodon is GCA, the codon is CGU (base pairing: G ≡ C, C ≡ G, A ≡ U).
- So we reverse and complement each anticodon:
| tRNA Anticodon | mRNA Codon | Amino Acid |
|---|---|---|
| GCA | CGU | Arginine |
| AAU | UUA | Leucine |
| CAG | GUC | Valine |
Step 2: Find the correct mRNA sequence for the peptide Arginine–Leucine–Valine
- Codon for Arginine: CGU
- Codon for Leucine: UUA
- Codon for Valine: GUC
Correct answer: D. CGU UUA GUC
Why the others are incorrect:
A. GCA AAU CAG → These are anticodons, not codons.
B. GCA AAT CAG → Contains T, which is found in DNA, not RNA.
C. CGT TTA GTC → Also contains T, so it’s DNA, not RNA.