Introduction

DNA replication is a fundamental process in all living organisms, ensuring the faithful transmission of genetic information from one generation to the next. One of the most elegant experiments demonstrating the mechanism of DNA replication was performed using isotopic labeling of nitrogen atoms in bacterial DNA and density gradient centrifugation. This experiment, involving heavy (^15N) and light (^14N) nitrogen isotopes, provided crucial evidence supporting the semi-conservative model of DNA replication.

The Experimental Setup

• Bacteria were initially grown in medium containing the heavy isotope ^15N. As a result, all newly synthesized DNA incorporated ^15N, making it denser.

• These bacteria were then transferred to medium containing only the lighter isotope ^14N and allowed to replicate for one generation.

• DNA was extracted from these cells and subjected to cesium chloride (CsCl) density gradient centrifugation, which separates DNA molecules based on their buoyant density.

Observations and Interpretation

• After one generation in ^14N medium, the DNA formed a hybrid band positioned exactly between the heavy (^15N) and light (^14N) DNA bands.

• This hybrid band indicates that each DNA molecule contains one strand with ^15N (heavy) and one strand with ^14N (light).

Conclusion: Semi-Conservative Replication

The presence of hybrid DNA molecules supports the semi-conservative model of DNA replication, where:

• Each of the two parental DNA strands serves as a template.

• The newly synthesized DNA molecules consist of one old (heavy) strand and one new (light) strand.

This model contrasts with:

• Conservative replication: Parental DNA remains intact, and an entirely new DNA molecule is synthesized.

• Dispersive replication: Parental and new DNA are interspersed in both strands.

• Rolling circle replication: A mechanism primarily observed in certain viruses and plasmids, not typical bacterial chromosomal DNA replication.

Summary Table

Correct Answer

(2) Replication of DNA is semi-conservative

Biological Significance

This experiment, famously performed by Meselson and Stahl in 1958, is a cornerstone in molecular biology. It conclusively demonstrated that DNA replication conserves one parental strand in each daughter molecule, ensuring genetic stability and fidelity.

Keywords

DNA replication, semi-conservative replication, Meselson-Stahl experiment, 15N labeling, 14N labeling, cesium chloride density gradient centrifugation, hybrid DNA, bacterial DNA replication, conservative replication, dispersive replication, rolling circle replication, molecular biology.

Conclusion

The observation of a hybrid DNA band in the 15N/14N density gradient experiment provides compelling evidence that DNA replication in bacteria is semi-conservative. Each daughter DNA molecule contains one parental strand and one newly synthesized strand, a mechanism fundamental to the accurate transmission of genetic information.