IB DP Biology- D1.2 Protein synthesis - IB Style Questions For SL Paper 1A -FA 2025
▶️ Answer/Explanation
✅ Answer: (B)
▶️ Answer/Explanation
Answer: A. I only
Explanation:
A protein’s secondary structure includes regular folding patterns such as alpha-helices and beta-pleated sheets. These are stabilized only by hydrogen bonds formed between the backbone atoms (carbonyl and amide groups) of the polypeptide chain, not by side-chain interactions.
- Disulphide bridges (covalent bonds between cysteine side chains) and ionic bonds (between charged side chains) occur in the tertiary structure — these involve the R-groups and determine the overall 3D conformation of the protein.
- Thus, only hydrogen bonds are characteristic of the secondary structure, while disulphide and ionic interactions are features of the tertiary level of folding.
Answer Evaluation:
I. Correct – Hydrogen bonds occur between backbone atoms in alpha-helices and beta-sheets.
II. Incorrect – Disulphide bridges form between cysteine side chains during tertiary folding.
III. Incorrect – Ionic bonds are interactions between charged side chains and are also part of tertiary structure.
▶️ 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: Assemble the mRNA for Arginine–Leucine–Valine
- Arginine codon: CGU
- Leucine codon: UUA
- Valine codon: GUC
Therefore: CGU UUA GUC → Option D.
Why the others are incorrect:
A. GCA AAU CAG — these are anticodons, not mRNA codons.
B. GCA AAT CAG — contains the letter T, which occurs in DNA, not RNA.
C. CGT TTA GTC — also contains T, so this is a DNA sequence, not mRNA.