Question

If a hybrid DNA molecule labelled with ${}^{15}N$is allowed to replicate twice in a normal culture medium, the percentage hybrid DNA after second replication will be
A. \[50\% \]
B. $12.5\% $
C. $25\% $
D. $75\% $

Answer 1

Hint: A DNA molecule holds all the genetic information of an organism. DNA replication is an important molecular process that involves the use of a DNA molecule to produce two identical DNA strands. The process doubles the amount of genetic material. For cell division to occur DNA replication is a necessary step. A family of enzymes called the DNA polymerases are responsible for carrying out this process.

Complete answer:
In the laboratory, it takes about $15 - 20$minutes for bacteria to reproduce. Meselson and Stahl collected samples from culture media containing only the heavy isotope of nitrogen. Then they collected samples from both generations after the culture media was changed.
Centrifugation separated cell DNA based on its density. Here a single band was observed for cells grown in the heavy isotope culture media. This is considered as the zero generation. The cells collected after the change in culture media are called generation one and generation two respectively. Generation one showed only a single band. The band was formed in an intermediate position i.e. DNA isolated had a density that was between DNA labelled with heavy and light isotope. This implies that the DNA isolated from this generation of bacteria contained both heavy and light isotope i.e. a hybrid.
The DNA isolated from the second generation proved that DNA replication has a semiconservative nature. In this case, two bands were formed one was in the intermediate position as in the case of the first generation. The second band was in a position higher than that of the intermediate band. The intermediate band was formed by the hybrid DNA while the other band was formed by the DNA with lighter isotope.
In the question above, both strands are labelled with ${}^{15}N$. After the first round of replication in a normal media i.e. a media containing no radioactive isotope; a hybrid DNA molecule is formed. The replicated DNA molecules have one ${}^{15}N$strand and one normal strand.
The second generation produced in the same normal media will have hybrid DNA molecules and normal DNA as well. This is because the hybrid DNA from the first generation will unwind and then a DNA polymerase will act on both strands to produce new strands. The labelled strand will become a template for a complementary normal strand (the media is normal so the cell has only the unlabeled nitrogen for use). The normal strand (original from generation one) will form a complementary normal strand. The percentage of hybrid DNA after the second replication process is \[50\% \]as the DNA contains both hybrid DNA molecules and normal DNA molecules in equal proportion.

Hence, the correct answer is option (A).

Note: Meselson and Stahl's experiments eliminated two theories regarding the nature of DNA replication. The biologists had proposed that the DNA replication followed a conservative pattern or a dispersive pattern of replication. Conservative replication theory proposed that after replication there is a parental DNA and a new DNA; no hybrids are formed. The dispersive theory proposed that hybrid DNA is formed, however; the DNA strands had a patchwork of the original and the new DNA.