Question

E. Coli fully labelled with ¹⁵N is allowed to grow in ¹⁴N medium. The two strands of DNA molecule of the first generation bacteria have

• A. Same density and resemble with their parent DNA
• B. Same density but do not resemble with their parent DNA
• C. Different density but do not resemble with their parent DNA
• D. Different density but resemble with their parent DNA

Answer 1

The Correct Option is D

This scenario describes the setup of the classic Meselson and Stahl experiment, which demonstrated the semi-conservative nature of DNA replication. 

1. Initial State (Parent DNA): The *E. coli* are initially grown in a medium containing the heavy isotope of nitrogen, ¹⁵N. This means that both strands of the DNA double helix in the parent bacteria are composed of nucleotides containing ¹⁵N. Let's represent this parent DNA as ¹⁵N/¹⁵N. This DNA has a high density.
2. Transfer and First Generation: The bacteria are then transferred to a medium containing only the normal, lighter isotope of nitrogen, ¹⁴N. They are allowed to replicate once (first generation).
3. Semi-conservative Replication: According to the semi-conservative model:
   • The parent DNA molecule (¹⁵N/¹⁵N) unwinds.
   • Each of the original ¹⁵N strands serves as a template for the synthesis of a new complementary strand.
   • Since the available nucleotides are now ¹⁴N-labeled, the newly synthesized strands will contain ¹⁴N.
   • Therefore, each new DNA molecule formed after one generation will consist of one original "heavy" strand (¹⁵N) and one newly synthesized "light" strand (¹⁴N). These are hybrid DNA molecules, represented as ¹⁵N/¹⁴N.
4. Analyzing the Strands of First Generation DNA: The question asks about the two strands within a single DNA molecule of the first generation bacteria.
   • One strand contains ¹⁵N (the original parent strand).
   • The other strand contains ¹⁴N (the newly synthesized strand).
   • Since ¹⁵N and ¹⁴N have different masses, the two strands within the molecule have different densities.
   • Does this molecule "resemble" the parent DNA? Yes, because one of its strands (the ¹⁵N strand) is directly inherited from the parent molecule. It retains half of the original parental material.

Therefore, the two strands of a DNA molecule in the first generation bacteria have different densities but resemble their parent DNA (as one strand is parental).

Answer 2

In the Meselson-Stahl experiment, E. coli bacteria were initially cultured in a medium containing the heavy nitrogen isotope \((^{15}N)\), fully labeling their DNA. The bacteria were then transferred to a medium with the lighter nitrogen isotope \((^{14}N)\) and allowed to replicate.

First Generation (After One Replication):

• Parental DNA: Consisted entirely of \(^{15}N\)-labeled DNA, which is denser.​
• Newly Synthesized DNA: Incorporated \(^{14}N\), making it lighter.​

Upon density gradient centrifugation, the DNA separated based on density, resulting in two distinct bands:​

• Hybrid DNA: DNA molecules comprising one strand of \(^{15}N\) DNA and one strand of \(^{14}N\) DNA, appearing at an intermediate density.​
• Light DNA: DNA molecules composed entirely of ^14N DNA, appearing at a lighter density.​

The two strands of DNA molecules in the first-generation bacteria will have different densities but will resemble their parent DNA in terms of base sequence, as one strand is original (^15N) and the other is newly synthesized \((^{14}N)\).

So, the correct Option is (D): Different density but resemble with their parent DNA​ Sources The correct answer is (D) : Different density but resemble with their parent DNA.