Q.57 A cDNA was synthesized from the mRNA of a eukaryotic gene. After cloning and
sequence analysis, the double–stranded cDNA of 614 bp revealed 125 bp 5′–UTR and 120
bp 3′–UTR. The number of amino acids present in the polypeptide encoded by this gene
is _____________. (answer in integer)
📚 Introduction
In many CSIR NET and IIT-JAM Life Sciences questions, students are asked to determine the number of amino acids encoded by a gene from the total cDNA length and the sizes of the 5′ and 3′ untranslated regions (UTRs). Such problems test understanding of eukaryotic mRNA structure, coding sequence (CDS), and the triplet nature of the genetic code.
⚙️ Step-by-Step Solution
1. Total cDNA Length (Double-Stranded)
Given: 614 bp cDNA. This corresponds to 614 nucleotides in the single-stranded mRNA sequence (cDNA is a DNA copy of the mature mRNA, excluding introns).
2. Subtract 5′ and 3′ UTRs to Get Coding Region (CDS)
5′-UTR length = 125 bp
3′-UTR length = 120 bp
Coding region length = Total length − (5′-UTR + 3′-UTR)
$$ \text{CDS} = 614 – (125 + 120) = 614 – 245 = 369\ \text{bp} $$
3. Convert Coding Region Length into Codons
Protein-coding sequence is read in triplets (codons), each codon = 3 nucleotides and codes for one amino acid, except the stop codon, which does not add an amino acid.
4. Account for the Stop Codon?
In this type of exam problem, when the CDS length is exactly divisible by 3 and no extra information is given, the entire coding stretch of 369 nucleotides is treated as the open reading frame from start codon to just before the stop codon, so the number of codons directly equals the number of amino acids.
Therefore, the polypeptide encoded contains 123 amino acids.
💡 Concept Recap: UTRs and Coding Region
- 5′-UTR and 3′-UTR are untranslated regions; they are part of mature mRNA but do not encode amino acids.
- Only the coding sequence (CDS) between start and stop codons determines the amino acid sequence.
- To find the number of amino acids from such data:
- Subtract 5′- and 3′-UTR lengths from total cDNA length to get CDS.
- Divide CDS length (in nucleotides) by 3.
🎯 Final Answer (Integer)
