26. If a completely radioactive double stranded DNA molecule undergoes two rounds of replication in a nonradioactive medium, what will be the radioactivity status of the four resulting molecules?
1. Half the number of molecules contains no radioactivity
2. All four molecules contain radioactivity
3. Three out of four molecules contain radioactivity
4. Radioactivity is lost from all four molecules

What Happens to Radioactivity After Two Rounds of DNA Replication?

To understand the radioactivity status of DNA after replication, it’s essential to grasp the principle of semi-conservative DNA replication. In this model, each strand of the DNA serves as a template to produce a complementary strand, resulting in daughter DNA molecules that contain one old (template) strand and one newly synthesized strand.

Let’s break it down step-by-step.

Initial Condition

• We start with 1 double-stranded DNA molecule.

• Both strands are radioactive, as it’s described as a “completely radioactive double-stranded DNA.”

After First Round of Replication

• Each of the two new double-stranded DNA molecules has:

  • 1 radioactive (parental) strand

  • 1 non-radioactive (new) strand

• So, both resulting molecules are half-radioactive.

After Second Round of Replication

Each of the two hybrid DNA molecules (from the first round) now serves as a template again:

• The radioactive strands serve as templates to form new non-radioactive strands → producing 2 hybrid (half-radioactive) molecules.

• The non-radioactive strands serve as templates to form new non-radioactive strands → producing 2 completely non-radioactive molecules.

So, after two rounds, we have:

• 2 molecules with radioactivity (hybrids: one strand radioactive, one non-radioactive)

• 2 molecules without radioactivity (both strands non-radioactive)

Summary of Results:

✅ Correct Answer:

(1) Half the number of molecules contains no radioactivity

Why This Matters

This outcome is a classic example from the Meselson-Stahl experiment that confirmed the semi-conservative model of DNA replication. It’s a cornerstone concept in molecular biology that helps explain how genetic information is accurately passed on during cell division.

Understanding how labeled DNA segregates through replication is also important in experimental biology and techniques like isotope labeling and DNA tracking in molecular genetics.