Cesium chloride density gradient centrifugation is commonly used for the separation of DNA
molecules. The buoyant density, ρ, of a double stranded Cs+DNA is given by the equation
ρ = 1.66 + 0.098XG+C
where XG+C denotes
Options:
(A) directly proportional to the square root of pressure drop across the filter medium
(B) inversely proportional to the pressure drop across the filter medium
(C) inversely proportional to the viscosity of the solution
(D) inversely proportional to the square of viscosity of the solution
Official Answer Key:
B
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2016 Q.19)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (1 Mark)
- **Topic**: Biophysics & Molecular Biology (Isopycnic Density Gradient Centrifugation of DNA)
**2. Detailed Step-by-Step Analysis**:
- In cesium chloride ($CsCl$) isopycnic density gradient centrifugation (first pioneered by Meselson and Stahl, and refined by Schildkraut, Marmur, and Doty), DNA molecules migrate under high centrifugal fields until they reach a position where the solution density equals their buoyant density ($\rho$).
- For double-stranded DNA in neutral $CsCl$, the buoyant density ($\rho$ in $\text{g/cm}^3$) increases linearly with the fraction of $G \equiv C$ base pairs due to the higher molecular mass and lower partial specific volume of G-C pairs:
$\rho = 1.660 + 0.098 X_{G+C}$
- In this empirical equation, $X_{G+C}$ represents the **mole fraction of G+C base pairs** in the DNA duplex:
$X_{G+C} = \frac{[G] + [C]}{[A] + [T] + [G] + [C]}$
(ranging from $0.0$ to $1.0$).
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: It is a normalized fractional ratio (dimensionless), not the absolute number of nucleotides.
- **Option (B) [CORRECT]**: $X_{G+C}$ is the mole fraction of $(G+C)$ base pairs in the DNA duplex.
- **Option (C) [INCORRECT]**: It does not refer to tandem repeats.
- **Option (D) [INCORRECT]**: It is the mole fraction, not the ratio $(G+C)/(A+T)$.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **B**
- **Scoring Criteria**: +1.0 Mark (Negative marking: -0.33 marks).
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