How did the results prove the semiconservative model of DNA
replication? Explain.
The Correct Answer and Explanation is :
The semiconservative model of DNA replication was proven through the famous Meselson-Stahl experiment in 1958. This experiment provided strong evidence that, during DNA replication, each new DNA molecule consists of one original (parent) strand and one newly synthesized strand.
Key Details of the Meselson-Stahl Experiment:
- Isotopic Labeling: The researchers grew E. coli bacteria in a medium containing a heavy isotope of nitrogen, N-15 (15N), which was incorporated into the bacterial DNA. After several generations, the bacteria’s DNA was fully labeled with N-15. Then, they transferred the bacteria to a medium containing the lighter nitrogen isotope, N-14 (14N), and allowed DNA replication to occur.
- Density Gradient Centrifugation: After a number of replication cycles, the DNA was extracted and analyzed using a density gradient centrifugation technique. This method separates DNA molecules based on their density, which is affected by their nitrogen content. The DNA with heavier nitrogen (15N) would form a distinct band lower in the gradient, while DNA with lighter nitrogen (14N) would form a band higher up.
Results:
- After one round of replication in the N-14 medium, the DNA showed a band in the middle of the gradient, indicating a hybrid of heavy and light DNA strands (one old N-15 strand and one new N-14 strand).
- After two rounds of replication, there were two distinct bands: one in the middle (hybrid DNA) and one near the top (light DNA), showing that some DNA strands were made entirely of N-14.
Explanation:
The results supported the semiconservative model because:
- After one replication cycle, the DNA did not consist of entirely heavy or light strands but rather hybrids, which indicates that each new DNA molecule contained one parent strand and one newly synthesized strand.
- After two cycles, there was a mix of hybrid and completely light DNA, showing that the first generation of hybrid DNA was used as a template in the next round, leading to some completely light DNA strands.
Thus, the experiment demonstrated that DNA replication is semiconservative, where each daughter DNA molecule contains one strand from the parent DNA and one newly synthesized strand.