Q.19 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 + 0.098X 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
Cesium Chloride Density Gradient Centrifugation: Understanding XG+C in DNA Buoyant Density Formula
Cesium chloride (CsCl) density gradient centrifugation separates DNA molecules based on buoyant density, with the formula ρ = 1.66 + 0.098 XG+C determining density for double-stranded Cs+ DNA. XG+C represents the mole fraction of G+C bases. The correct answer is (B) mole fraction of G+C.
Technique Overview
CsCl density gradient centrifugation creates a stable gradient under ultracentrifugation, allowing DNA to band at its buoyant density. G+C-rich DNA has higher density due to stronger base pairing and Cs+ binding, enabling separation of molecules like bacterial genomes or replicons.
Option Analysis
-
(A) Total number of G and C: Incorrect, as density depends on proportion, not absolute count; a longer AT-rich DNA would otherwise appear denser, which contradicts observations.
-
(B) Mole fraction of G+C: Correct; XG+C is the fractional proportion (G+C)/(A+T+G+C), typically expressed as mole percent (0-1 or 0-100%), directly correlating with density via the linear equation.
-
(C) Number of GC repeats: Incorrect; this ignores dispersed G+C and focuses only on dinucleotide repeats (G-C), not overall composition.
-
(D) Ratio of G+C to A+T content: Incorrect; while related, this is (G+C)/(A+T), which equals mole fraction only if total bases equal A+T+G+C, but the formula specifies mole fraction as the standard notation.
