Q.102 The molecular weight of double stranded DNA fragment present in mitochondria is 7.9 × 106 g mol-1. A pair of nucleotide contributes 0.34 nm to the length of DNA and the average molecular weight of a nucleotide is 350 g mol-1. The number of protein molecules, consisting of 200 amino acids each coded by this mitochondrial DNA is _____ molecules.
Step-by-Step Calculation
First, determine the total number of nucleotides in the dsDNA. Since the molecular weight is given per mole and each nucleotide averages 350 g/mol, divide the total MW by the nucleotide MW: total nucleotides N = 7.9 × 106 / 350 ≈ 22,571. For dsDNA, base pairs BP = N / 2 ≈ 11,286.
Next, calculate the DNA length: length = BP × 0.34 nm ≈ 11,286 × 0.34 ≈ 3,837 nm, confirming standard dsDNA helical rise of 0.34 nm per base pair.
Protein Encoding Logic
Each protein has 200 amino acids, coded by 200 codons (3 nucleotides each), so 600 nucleotides per protein gene in dsDNA (both strands, but coding uses one). Total protein molecules possible equals total nucleotides / nucleotides per protein = 22,571 / 600 ≈ 37.6, but since whole proteins are encoded and question specifies “number of DNA is ___ molecules” likely meaning proteins, consider full utilization.
The phrasing indicates the DNA encodes multiple copies of the same 200-aa protein. MW per protein ≈ 200 × 350 Da (approx residue mass) = 70,000 g/mol. Number = total MW / protein MW = 7.9 × 106 / 70,000 ≈ 113.
Option Analysis (Typical MCQ Choices)
Assuming standard CSIR NET options like 100, 113, 200, 300:
- 100: Underestimates by ignoring precise division.
- 113: Exact from 7.9e6 / (200 × 350) = 112.857 ≈ 113.
- 200: Matches amino acids, not proteins.
- 300: Overestimates genes.
Correct answer: 113 protein molecules.
CSIR NET Exam Relevance
This problem tests biochemistry basics: DNA MW calculation (650 g/mol per bp average, but uses 350/nucleotide), helical parameters (0.34 nm/bp), and gene-protein stoichiometry (3 nt/aa). For competitive exams, always use given values over standards (e.g., 617-650 g/mol/bp). Practice verifies ~11 kb mitochondrial gene cluster fits.
