Calculating Proportion of Radioactive DNA After Replication

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Discussion Overview

The discussion revolves around calculating the proportion of radioactive DNA after multiple rounds of replication. Participants explore the implications of radioactive and non-radioactive strands during the replication process, focusing on the mathematical reasoning behind the expected proportions after three replications.

Discussion Character

  • Mathematical reasoning, Debate/contested

Main Points Raised

  • One participant initially proposes that the proportion of radioactive DNA after three replications is 2/8 (1/4).
  • Another participant challenges this claim, suggesting that the calculation is incorrect.
  • A participant explains that the total amount of DNA doubles with each division while the amount of radioactive DNA remains constant, leading to a different expected outcome.
  • One participant calculates the final number of DNA strands after three replications, concluding that there would be 16 strands, of which only 2 are radioactive, leading to a proportion of 2/16 (1/8).
  • A later reply acknowledges a mistake in the initial counting and expresses gratitude for the clarification.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the initial claim regarding the proportion of radioactive DNA, as there are competing calculations presented. The discussion reflects differing views on the correct mathematical approach to the problem.

Contextual Notes

The discussion highlights the importance of accurately tracking the number of strands through each replication cycle and the implications of radioactive versus non-radioactive strands. There are unresolved mathematical steps in the initial claims that lead to different conclusions.

Cryptic89
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Hi,

I was doing this question & I just want to make sure I've got it correct:

A quantity of DNA was labelled with radioactive nitrogen. It was then allowed to replicate three times using non radioactive to synthesise the new DNA strands. What proportion of the final mass of DNA would you expect to be radioactive?

OK so is the answer 2/8th?
 
Last edited:
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Your answer is wrong. It is not 2/8.
 
Hmm...what do you think is the answer then?
 
For each division your total amount of DNA doubles and your radioactive DNA stay the same. Keep in mind that both radioactive and non-radioactive strands are both replicated.

You start with 2 strands and both strands are radioactive. After the first division you get 4 strands, 2 non-radioactive and 2 radioactive.

If you do the math, you will get the answer.
 
I did the math! At then end of the third replication sequence, you get 8 DNA molecules...of which two strands are radioactive. So naturally you'd expect 2/8th (1/4th) of the final mass of the DNA to be radioactive.
 
You get 16 strands and two are radioactive. 2/16 or 1/8 is the final mass.

Start: 2 strands
After 1 rep: 4 strands
After 2 rep: 8 strands
After 3 rep: 16 strands
 
OMG...you're right! I'm sorry I did some mistake in counting! *embarrassed*
Thank you for all the help!
 

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