Q.12 Name the technique Mathew Mesalson and Franklin Stahl used to evidence that
DNA replication is ‘semi conservative’
(1)Cesium Chloride (CSCI) density gradient centrifugation
(2)2D Gel Electrophoresis
(3)Mass spectrometry
(4)DNA sequencing
Matthew Meselson and Franklin Stahl used Cesium Chloride (CsCl) density gradient centrifugation to prove semi-conservative DNA replication. In their 1958 experiment with E. coli, they labeled parental DNA with heavy ¹⁵N, switched to light ¹⁴N medium, and observed distinct hybrid (¹⁵N-¹⁴N) and light DNA bands after ultracentrifugation, confirming each daughter DNA retains one parental strand.
Option Analysis
-
(1) Cesium Chloride (CsCl) density gradient centrifugation: Correct—Equilibrium density gradient ultracentrifugation separates DNA by buoyant density. Generation 0: heavy band; Gen 1: single hybrid band; Gen 2: hybrid + light bands, matching semi-conservative prediction exactly.
-
(2) 2D Gel Electrophoresis: Separates DNA by size and topology (e.g., replication forks, plasmids); developed later, not used in 1958 for replication studies.
-
(3) Mass spectrometry: Measures molecular mass/isotopes; post-1958 technique unsuitable for intact genomic DNA separation at that time.
-
(4) DNA sequencing: Sanger method (1977) analyzes nucleotide order, not replication mechanism or strand segregation.
The answer is (1) Cesium Chloride (CsCl) density gradient centrifugation.
Introduction to Meselson-Stahl Experiment
The technique Matthew Meselson and Franklin Stahl used to evidence that DNA replication is semi-conservative was Cesium Chloride (CsCl) density gradient centrifugation. Growing E. coli in ¹⁵N then ¹⁴N revealed precise band shifts proving Watson-Crick model.
How CsCl Centrifugation Works
CsCl forms a density gradient under ultracentrifugation (44,770 rpm, 20-30 hrs); DNA migrates to buoyant density position. ¹⁵N-DNA: heavy band; post-replication: hybrid (exactly midway); rules out conservative/dispersive models.
Why Other Techniques Fail
2D gels map replication intermediates (not strand labeling); mass spec lacks 1958 resolution for genomes; sequencing doesn’t track parental/new strands.
GATE Life Sciences Importance
Iconic experiment tests molecular biology foundations: Distinguish density labeling from modern sequencing/gel methods for replication mechanism questions.
