Calculating Proportion of Radioactive DNA After Replication

[Cryptic89]

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?

 

[iansmith]

Your answer is wrong. It is not 2/8.

 

[Cryptic89]

Hmm...what do you think is the answer then?

 

[iansmith]

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.

 

[Cryptic89]

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.

 

[iansmith]

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

 

[Cryptic89]

OMG...you're right! I'm sorry I did some mistake in counting! *embarrassed*
Thank you for all the help!