Question

A completely radioactive, double-stranded DNA molecule undergoes two rounds of replication in a non-radioactive medium. What will be the radioactive status of the four daughter molecule?
A. All four still contain radioactively
B. Radioactivity is lost from all four
C. Out of four, three contain radioactivity
D. Half of the number contain no radioactivity

Answer 1

Hint: The mechanism of DNA replication in all known cells is represented by Semiconservative Replication. DNA replication occurs along the DNA template strand at different roots of replication. As the DNA double helix is unwound by helicase, replication in antiparallel directions occurs separately on each template strand. This method is known as semi-conservative replication because it produces two copies of the initial DNA molecule.

Complete answer:
The Meselson-Stahl experiment in 1958 validated the hypothesis that DNA replication was semi-conservative. After the discovery of the structure of DNA, there were three basic models for DNA replication suggested by the scientific community. A semi-conservative model was one of them.
Semi-conservative replication. The two strands of DNA unwind from each other in this model, and each serves as a synthesis template for a new, complementary strand. This results in two DNA molecules with one original strand and one new strand.
DNA replication is semi-conservative since one strand from the helix from which it was copied is used in each helix that is formed. Two daughter helices, each of which incorporates one of the original parental helical strands, result in the replication of one helix. So, four daughter molecules are produced after two rounds of DNA replication, out of which two daughters contain non-radioactive DNA, while the other two each contain one radioactive strand. So, the correct answer is option D i.e. Half numbers contain no radioactivity.
So, the correct answer is option D.

Note: The other two models were Conservative and Dispersive models. Conservative DNA replication in this model results in a molecule composed of the same original DNA strands as the original DNA molecule, as well as a second molecule consisting of two new strands containing exactly the same sequences as the original molecule. Replication Dispersive DNA replication in the dispersive model results in two DNA molecules of parental and daughter DNA that are mixtures, or "hybrids." Each individual strand in this model is a patchwork of original and new DNA.