Question

DNA molecule having $N^{15}$ isotope in both strands is cultured in $N^{14}$ containing medium. After two generations how many DNA molecules are hybrid DNA?

• A. $2$
• B. $3$
• C. $1$
• D. $4$

Answer 1

Answer: (A)

The double-stranded structure of DNA provides a simple mechanism for DNA replication. Here, the two strands are separated and then each strand's complementary DNA sequence is recreated by an enzyme called DNA polymerase. This enzyme makes the complementary strand by finding the correct base through complementary base pairing, and bonding it onto the original strand. Meselson and Stahl use isotopes in the sources of nitrogen present in the culture medium in their experiment as nitrogen is a major constituent of DNA. Moreover is by far the most abundant isotope of nitrogen, and DNA with the heavier isotope is also functional. E.coli can be grown for several generations in a medium with ease. In this type of replication, one strand of parent is new and one strand is old. It means, one strand of daughter duplex is derived from the parent, while the other strand is formed new. In this way two DNA molecules formed are hybrid DNA.

So the correct option is '2'.