Question

A DNA molecule of Escherichia coli is heavy (full labelled with nitrogen-15) and is allowed to be duplicated in a medium containing nitrogen-14. After one generation, the two daughter strands molecules have:

• A. Same density as parent DNA, but do not resemble their parent DNA
• B. Different densities from parents but their sequence resembles with parent DNA
• C. Different densities from parent DNA and also do not resemble the parent DNA
• D. Same density as parent DNA but do not resemble their parent DNA

Answer 1

The correct option is B Different densities from parents but their sequence resembles with parent DNA

When a heavy DNA (with nitrogen-15) is replicated in nitrogen-14 medium, after one generation the DNA formed has one strand with heavy nitrogen-15 and other strand with light nitrogen-14. This is because of semi-conservative nature of DNA replication. Thus, the density of the hybrid DNA will be different from parental DNA, but its sequence is same as that of parental DNA.