Past paper Question HELP!

oKAY....So here is the question: enjoy :biggrin:

d) A quantity of DNA was labelled with radioactive nitrogen. It was then allowed to replicate 3 times, using non radioactive nucleotides to synthesise the new DNA strands. What proportion of the final mass of DNA would you expect to be radioactive? Explain your answer.

e) In which stage of the cell cylce does replication of DNA take place?

Thanx :smile:

Reply 1

shinyhappy

14

a) thats to do with the principle of semi-conservative replication.
The first strand is 100% radioactive.
It will then divide, forming two strands, each 50% radioactive, so 50% of total mass.
It will divide a second time, into four strands, the two original halves will now be 2/8, so it will be 25% radioactive.
The third time, there will still only be two radioactive strands, but 16 altogether, so 2/16 will be radioactive. so, express 2/16 as a percentage, and thats your answer !
2. err, mitosis?

Reply 2

jalapeno

7

Milli

A quantity of DNA was labelled with radioactive nitrogen. It was then allowed to replicate 3 times, using non radioactive nucleotides to synthesise the new DNA strands. What proportion of the final mass of DNA would you expect to be radioactive? Explain your answer.

|| = Original DNA molecule (100% radioactive)

1st replication:
|| || = each is 50% radioactive as one strand is from the original, 100% radioactive DNA molecule and the other from non-radioactive (clean) nucleotides.

2nd replication:
|| || = each is 25% radioactive as one strand is from the 50% radioactive DNA molecule and the other from the non-radioactive (clean) nucleotides.

3rd replication:
|| || = each is 12.5% radioactive as one strand is from the 25% radioactive DNA molecule and the other from the non-radioactive (clean) nucleotides.

It wants to know what proportion of the final mass of the DNA is radioactive. We know that the molecules produced after the 3rd replication were each 12.5% radioactive. Therefore 2 x 12.5 = 25. so 25% of the final mass of the DNA was radioactive.

Sorry if that was unclear, I'm not thinking clearly at the moment.

e) In which stage of the cell cylce does replication of DNA take place?

Interphase (S1).

Reply 3

Eau

15

shinyhappy

a) thats to do with the principle of semi-conservative replication.
The first strand is 100% radioactive.
It will then divide, forming two strands, each 50% radioactive, so 50% of total mass.
It will divide a second time, into four strands, the two original halves will now be 2/8, so it will be 25% radioactive.
The third time, there will still only be two radioactive strands, but 16 altogether, so 2/16 will be radioactive. so, express 2/16 as a percentage, and thats your answer !
2. err, mitosis?

I agree with our first answer being 12.5%, but the second answer is wrong.

DNA replication occurs before mitosis, during interphase.

Past paper Question HELP!

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