15.
You are given an oligonucleotide whose molecular weight is 10000 Daltons. Assuming
you want to make 100 μl of 100 μM stock how much oligonucleotide would you need?
a. 10 μg
b. 1 μg
c. 100 μg
d. 1000 μg
Oligonucleotide Stock Calculation: 100 μl 100 μM from 10000 Daltons MW
The oligonucleotide stock calculation for 100 μl of 100 μM solution from a 10,000 Daltons molecular weight oligo is a core skill in molecular biology labs and CSIR NET Life Sciences exams. This oligonucleotide molecular weight problem tests molarity-mass conversions essential for PCR primers, probes, and gene synthesis. Precise oligonucleotide amount ensures experiment reproducibility—here, exactly 100 μg yields the target concentration.
Calculation Breakdown
100 μg of oligonucleotide is required to prepare 100 μl of 100 μM stock from a 10,000 Da oligo. This calculation ensures precise molar concentration for molecular biology experiments like PCR or hybridization.
The mass needed equals concentration × volume × molecular weight.
For 100 μM (10⁻⁴ mol/L) in 100 μl (10⁻⁴ L), moles required are (10⁻⁴ mol/L) × (10⁻⁴ L) = 10⁻⁸ mol.
With MW = 10,000 g/mol, mass = 10⁻⁸ mol × 10,000 g/mol = 10⁻⁴ g = 100 μg.
Option Analysis
- a. 10 μg: Yields 10 μg / 10,000 g/mol = 1 nmol total, or 10 μM in 100 μl—too dilute by 10-fold.
- b. 1 μg: Gives 0.1 nmol total, or 1 μM in 100 μl—100-fold under-concentrated.
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c. 100 μg: Matches exactly: 100 μg / 10,000 g/mol = 10 nmol total, or 100 μM in 100 μl (correct).
- d. 1000 μg: Provides 100 nmol total, or 1 mM in 100 μl—10-fold too concentrated.
Step-by-Step Oligonucleotide Amount Formula
Convert units: Concentration (100 μM = 10⁻⁴ M), volume (100 μl = 10⁻⁴ L), MW (10,000 g/mol). Moles = C × V = 10⁻⁸ mol; mass (g) = moles × MW = 10⁻⁴ g = 100 μg.
Resuspend in nuclease-free water or TE buffer, vortex gently, and verify via NanoDrop (A260).
Common Errors in Oligo Stock Prep
- Forgetting unit conversions (μM to mol/L, μl to L) leads to off-by-10³ mistakes.
- Option pitfalls: 10 μg suits 10 μl stocks; 1 μg for pmol scales; 1000 μg risks precipitation.
- Always use MW from oligo certificate—~330 Da/base × ~30 bases ≈ 10 kDa here.
CSIR NET Exam Tips
This MCQ reinforces biochem stoichiometry. Practice: mass (μg) = [conc (μM) × vol (μl) × MW (Da)] / 10⁶.
Similar questions cover siRNA dilution or probe quantification.
