Q.78 The length of a coding region in an mRNA is 897 bases. How many amino acids will be there in the
polypeptide synthesized using this mRNA?
(A) 297 (B) 298 (C) 299 (D) 897
Genetic Code Fundamentals
The coding region (CDS) of mRNA contains multiples of 3 nucleotides called codons. Each codon specifies one amino acid during translation at the ribosome.
- Start codon (AUG = Met) begins translation
- Stop codons (UAA/UGA/UAG) end translation without coding amino acids
- “Coding region” length excludes UTRs and stop codons
✅ Correct Answer: (C) 299
897 bases ÷ 3 bases/codon = 299 amino acids
Number of amino acids = 897 / 3 = 299
This assumes perfect triplet reading frame with no stops in the coding sequence definition.
❌ Why Not the Other Options?
| Option | Calculation | Why Wrong |
|---|---|---|
| (C) 299 | 897 ÷ 3 = 299 | ✅ Correct |
| (A) 297 | 891 ÷ 3 | ❌ Subtracts 6 bases (2 stop codons) prematurely |
| (B) 298 | 894 ÷ 3 | ❌ Subtracts 3 bases (1 stop codon) from CDS |
| (D) 897 | 897 ÷ 1 | ❌ Ignores triplet genetic code (3 bases = 1 amino acid) |
📊 Translation Diagram
mRNA: 5′-AUG-XXX-XXX-…-XXX-UAA-3′
Coding: ||||||||||| <– 897 bases = 299 × 3
Protein: Met-AAA-AAA-…-AAA <– 299 amino acidsKey: CDS excludes start counting? NO! AUG codes Methionine (counts as aa #1)
Stop codon NOT counted in CDS length
Coding: ||||||||||| <– 897 bases = 299 × 3
Protein: Met-AAA-AAA-…-AAA <– 299 amino acidsKey: CDS excludes start counting? NO! AUG codes Methionine (counts as aa #1)
Stop codon NOT counted in CDS length
🎯 Key Exam Concepts
✅ Perfect Score Checklist:
- 897 divisible by 3 = valid CDS (no remainder)
- No remainder = perfect reading frame
- Stop codon NOT counted in CDS length
- AUG counts as amino acid #1 (Methionine)
🧠 Exam Mnemonic
“897 Triplets = 299 Proteins”
8+9+7=24 → 2+4=6 → 6×50=300 → 300-1=299
8+9+7=24 → 2+4=6 → 6×50=300 → 300-1=299
Or simply: Divide by 3, pick the answer!
