Question

Caesium chloride density gradient centrifugation is commonly used for the separation of DNA molecules. The buoyant density, \(\rho\), of a double stranded Cs+DNA is given by the equation \(\rho = 66 + 0.098 X_{G+C}\), where \(X_{G+C}\) denotes

• A. total number of G and C
• B. mole fraction of G+C
• C. number of GC repeats
• D. ratio of G+C to A+T content

Hint:

Because of their three hydrogen bonds, G-C base pairs pack tighter and have a higher density than A-T base pairs, meaning G-C rich DNA bands deeper in a CsCl density gradient.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Analytical ultracentrifugation using a CsCl density gradient separates DNA based on its buoyant density (\(\rho\)).
The density of DNA is influenced by its base composition because G-C base pairs are held together by three hydrogen bonds, packing more tightly and having a higher density than A-T base pairs, which are held by two hydrogen bonds.

Step 2: Detailed Explanation:
The empirical equation relating the buoyant density of double-stranded DNA to its base composition in a cesium chloride gradient is: \[ \rho = 66 + 0.098 X_{G+C} \]
In this linear equation:
- \(\rho\) represents the buoyant density of the DNA in \(\text{g/cm}^3\).
- \(66\) is the buoyant density of a theoretical DNA containing only A-T base pairs.
- \(X_{G+C}\) represents the mole fraction of G+C base pairs in the DNA molecule, which ranges from \(0\) to \(1\).
As the proportion of G+C base pairs in the DNA increases, its buoyant density increases linearly.
Measuring the buoyant density allows researchers to determine the overall G-C content of a genomic DNA sample.

Step 3: Final Answer:
In the buoyant density equation, \(X_{G+C}\) denotes the mole fraction of G+C content.