Question

Both strands of a DNA molecule are labeled with radioactive thymidine and are allowed to duplicate in an environment containing non-radioactive thymidine. The number of DNA molecules that will contain radioactive thymidine after three duplications is .............

Hint:

In DNA replication, the number of radioactive molecules increases with each round of duplication, but the amount of radioactivity decreases as non-radioactive thymidine is incorporated.

Answer 1

Correct Answer: 2

Understanding DNA replication:

DNA replication is semi-conservative: each new DNA molecule contains one original strand and one newly synthesized strand.

Tracking through replications:

Initial state:

• 1 DNA molecule with both strands radioactive (let's call them R-R)

After 1st replication:

• Original strands separate
• Each pairs with a new non-radioactive strand (N)
• Result: 2 DNA molecules, both hybrid (R-N)

After 2nd replication:

• The 2 hybrid molecules (R-N) replicate
• Each R-N produces:
  • 1 hybrid molecule (R-N)
  • 1 non-radioactive molecule (N-N)
• Result: 4 DNA molecules total
  • 2 with radioactive strand (R-N)
  • 2 without radioactive strand (N-N)

After 3rd replication:

• The 2 hybrid (R-N) and 2 non-radioactive (N-N) molecules replicate
• From 2 hybrid (R-N):
  • Each produces 1 hybrid (R-N) and 1 non-radioactive (N-N)
  • Total: 2 hybrid (R-N) + 2 non-radioactive (N-N)
• From 2 non-radioactive (N-N):
  • Each produces 2 non-radioactive (N-N)
  • Total: 4 non-radioactive (N-N)
• Result: 8 DNA molecules total
  • 2 with radioactive thymidine (R-N)
  • 6 without radioactive thymidine (N-N)

Answer: 2