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Really Hard DNA Exam Question

13

Hi all - i am really stuck on this question, i have spent ages online and using my textbooks but i can't find any information on light and heavy DNA. So i have attached the past paper sheet which has the question on it in the hope that one of you can answer it :biggrin:

The first attachment is information on the question and the other one is the questions

Thanks in advance,

Regards, Asad

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Reply 1

Cirsium

19

Ok so basically this question is testing your knowledge of semi conservative replication of DNA. (i.e. two new strands are made from one old strand plus a load of free nucleotides, so that each new DNA molecule contains one old and one new strand)

If the B DNA was grown in light nitrogen for one generation then each of its DNA molecules will compromise one strand made with the heavy nitrogen nucleotides and one with the light. So therefore it will be slightly lighter than the A DNA which will all have 2 strands made of heavy nitrogen.

With me so far?

Now the C DNA after one generation will be exactly like the B DNA. So we have 2 molecules of C DNA. Strand I in molecule A is heavy and strand II is light. Strand III in molecule B is heavy and Strand IV is light. Then the DNA replicates again and makes 4 DNA molecules. Molecule C contains Strand I and some new light nucleotides. Molecule D Contains Strand II (which was light) and some more new light nucleotides. Molecule E will be identical to C, and F to D. So therefore you will have 2 molecules that are 'medium' - one heavy and one light - and 2 molecules that are 'light'.

So therefore when you centrifuge the solution the DNA will split into two layers.

That make sense?

Reply 2

Stranded

11

Asad, u're doing edexcel right?

Reply 3

asadtamimi

OP

13

hira89

Asad, u're doing edexcel right?

yea this was a question from an edexcel past paper

Reply 4

asadtamimi

OP

13

Bekaboo

Ok so basically this question is testing your knowledge of semi conservative replication of DNA. (i.e. two new strands are made from one old strand plus a load of free nucleotides, so that each new DNA molecule contains one old and one new strand)

If the B DNA was grown in light nitrogen for one generation then each of its DNA molecules will compromise one strand made with the heavy nitrogen nucleotides and one with the light. So therefore it will be slightly lighter than the A DNA which will all have 2 strands made of heavy nitrogen.

With me so far?

Now the C DNA after one generation will be exactly like the B DNA. So we have 2 molecules of C DNA. Strand I in molecule A is heavy and strand II is light. Strand III in molecule B is heavy and Strand IV is light. Then the DNA replicates again and makes 4 DNA molecules. Molecule C contains Strand I and some new light nucleotides. Molecule D Contains Strand II (which was light) and some more new light nucleotides. Molecule E will be identical to C, and F to D. So therefore you will have 2 molecules that are 'medium' - one heavy and one light - and 2 molecules that are 'light'.

So therefore when you centrifuge the solution the DNA will split into two layers.

That make sense?

I understand the first bit but stuck on the explanation on the results of batch C.

Also when asked to draw the results for batch E would they all be light DNA?

Reply 5

Cirsium

19

asadtamimi

I understand the first bit but stuck on the explanation on the results of batch C.

Also when asked to draw the results for batch E would they all be light DNA?

Hope this makes things a bit clearer:

The DNA masses work on an exponential system. So in Group B it's 100% mixed DNA, in Group C it's 50% mixed, in Group D it's 25% mixed and in Group E it's 12.5% mixed and the rest is all light. (This happens because you only have the 2 original heavy strands from the original chromosome). So you'll have a very thick 'light' band and a very thin 'heavy' band.

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Reply 6

asadtamimi

OP

13

Bekaboo

Hope this makes things a bit clearer:

The DNA masses work on an exponential system. So in Group B it's 100% mixed DNA, in Group C it's 50% mixed, in Group D it's 25% mixed and in Group E it's 12.5% mixed and the rest is all light. (This happens because you only have the 2 original heavy strands from the original chromosome). So you'll have a very thick 'light' band and a very thin 'heavy' band.

Alright , cool - thanks

Really Hard DNA Exam Question

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