Introduction
DNA replication is the process by which a cell copies its DNA before cell division. This ensures that each daughter cell receives an identical set of DNA. There are different proposed modes of DNA replication, but only one is biologically accurate. Let’s explore the various modes of replication and understand which one actually occurs in living cells.
Three Proposed Modes of DNA Replication
1. Conservative Replication
- In this model, the original (parental) DNA molecule remains unchanged.
- A completely new copy is made, so after replication, one DNA is entirely old, and the other is entirely new.
- This model was proposed in the early days of DNA research but later proven incorrect.
2. Semi-Conservative Replication (Actual Mode)
- This is the correct and biologically accurate method of DNA replication.
- In this process, each of the two strands of the original DNA serves as a template.
- New complementary strands are formed on each template, resulting in two DNA molecules.
- Each new DNA has one original (parental) strand and one newly synthesized strand.
- This model was experimentally confirmed by Meselson and Stahl in 1958.
3. Dispersive Replication
- This model suggests that the original DNA breaks into fragments.
- New DNA is formed by mixing old and new segments randomly.
- Each strand of the daughter DNA is a mixture of old and new DNA segments.
- This model was also proven incorrect based on experimental evidence.
Key Experiment: Meselson and Stahl
In 1958, Meselson and Stahl performed a famous experiment using E. coli and nitrogen isotopes (N15 and N14). Their experiment supported the semi-conservative model of DNA replication. They showed that after one round of replication, the new DNA had one old and one new strand.
Conclusion
Among the different proposed modes of DNA replication, only the semi-conservative model is accurate. This mode ensures faithful duplication of genetic material and plays a vital role in the survival and reproduction of all living organisms.