DNA replication is a fundamental biological process that has been extensively studied to understand how genetic information is passed on during cell division. Initially, three hypotheses were proposed to explain the mechanism of DNA replication: the Conservative Replication Hypothesis, the Dispersive Replication Hypothesis, and the Semi-Conservative Replication Hypothesis.
The Conservative Replication Hypothesis suggests that the original DNA double helix remains intact while a completely new double helix is synthesized. In this model, the old strands serve as templates, but they do not mix with the new strands. After replication, one double helix consists entirely of old strands, while the other is made entirely of new strands.
In contrast, the Dispersive Replication Hypothesis posits that the original DNA strands break apart and are interspersed with newly synthesized DNA. This means that after replication, both resulting double helices contain segments of both old and new DNA, creating a mosaic of genetic material.
The Semi-Conservative Replication Hypothesis is the model that was ultimately confirmed through experimentation. According to this hypothesis, each of the two new double helices consists of one old strand and one newly synthesized strand. This means that after replication, each double helix retains half of the original DNA, hence the term "semi-conservative."
To determine which model accurately describes DNA replication, scientists Meselson and Stahl conducted a pivotal experiment using E. coli bacteria. They grew the bacteria in a medium containing heavy nitrogen (N15) for several generations, ensuring that all the DNA incorporated this heavy isotope. After this initial growth phase, the bacteria were transferred to a medium with normal nitrogen (N14) and allowed to replicate their DNA.
After one round of replication, the resulting DNA showed a single band of mixed density, indicating that each double helix contained one strand of heavy DNA and one strand of light DNA. This result was consistent with the Semi-Conservative model. When the bacteria underwent a second round of replication, two distinct bands were observed: one of mixed density and another of light density. This further supported the Semi-Conservative model, as it demonstrated that the new strands were being synthesized from the light nitrogen while retaining the old strands from the previous generation.
In summary, the experiment by Meselson and Stahl provided compelling evidence that DNA replication occurs in a semi-conservative manner, where each new double helix consists of one old strand and one new strand. This understanding is crucial for comprehending how genetic information is accurately transmitted during cell division, ensuring the continuity of life.
