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  #1  
Old 10-17-2012
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DNA Chargaff Base Pairs

A ssDNA has 20%A, 30%G, 25%C and also 25%T. Now, complement to this ssDNA is synthesized, the T content of entire duplex will have ???
a) 25%
b) 20%
c) 45%
d) 22.5%
e) 90%
f) 5%
g) 27.5%
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  #2  
Old 10-17-2012
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Default B?

Is it B- 20% to complement the A in first ssDNA...
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Old 10-17-2012
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Default my answer :)

c) 45%

on complement DNA - 20% T that are complement to A bases on the template DNA
25% T - on the template DNA
so double helix will have 45%
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Old 10-18-2012
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Correct Answer D is right

the correct answer is D) 22.5%

ssDNA contain 20% A,30%G,25%C ,25%T....when complimentary strand is formed..it contains 25%A(complimentary to T) , 25% G(complimentary to C) , 30% C and 20% T.....

interpretation of the results is the most important in this case

the stem asked-----Q)T content of entire duplex formed------22.5%(average of 25% in one strand and 20% in one strand)[option D]

also A-22.5%, ,G- 27.5% , , C- 27.5%[option G]

{as A=T ;G=C}

the total T content of the duplex-------25%(of one strand) + 20% (of other strand)=45%[optionC]

the total A and T content of the duplex------45% + 45% = 90%[option E]

the total A and T content of the entire duplex-----90%/2==45%

the total G content(or C content) of the duplex-----30% + 25% =55%

the total G(or C) content of the entire duplex------average of 30% and 25%====27.5%


hope it understands....

@casandra and @drsk thanks for answering both of you
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Quote:
Originally Posted by venky2600 View Post
the correct answer is D) 22.5%

ssDNA contain 20% A,30%G,25%C ,25%T....when complimentary strand is formed..it contains 25%A(complimentary to T) , 25% G(complimentary to C) , 30% C and 20% T.....

interpretation of the results is the most important in this case

the stem asked-----Q)T content of entire duplex formed------22.5%(average of 25% in one strand and 20% in one strand)[option D]

also A-22.5%, ,G- 27.5% , , C- 27.5%[option G]
ok, so we don't really calculate the mean here. I think the issue is that what was 25% on 1 strand will be 12,5% on two strands bc the number of bases is 2 times higher.

nice question where is it from btw?
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Originally Posted by Casandra View Post
ok, so we don't really calculate the mean here. I think the issue is that what was 25% on 1 strand will be 12,5% on two strands bc the number of bases is 2 times higher.

nice question where is it from btw?
thanks.....i found it in other forum...though to share with you all... i modified the values and presented here..that's it...

ya i also thought of 45% only as answer(that becomes the easy question right..?)
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Wait.. so the question is asking the T content of entire duplex, meaning T base pair makes up 22.5% out of the entire complex? Is that the correct reasoning behind it?

I never liked Chargaff questions, always gets me.

Thanks for the question venky
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Question

Quote:
Originally Posted by Casandra View Post
ok, so we don't really calculate the mean here. I think the issue is that what was 25% on 1 strand will be 12,5% on two strands bc the number of bases is 2 times higher.

nice question where is it from btw?
sorry i didnt catch your sentence........how could we get 12.5% here( i mean if 25% T present in one strand,they cannot be present in other strand right..?..).....

can you elaborate it..?
thanks
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Old 10-18-2012
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All the numbers were getting to me, so I have decided to post here an answer which will be easier to understand for all of us who have difficulties with numbers.

Take one strand (the original ssDNA) with:
20 Adenine
30 Guanine
25 Cytosine
25 Thymi

Notice how they sum up to a total of 100 bases. Now in order to consider a duplex, you will require another strand of the same length. That complementary strand will have:

20 Thymine
30 Cytosine
25 Guanine
20 Adenine

The total is once again 100.

To calculate the total Thymine content in duplex, add the thymines (25+20). This comes out to be 45. But the question is asking you for the percentage of Thymine in duplex, so you divide 45 by 200 (total number of bases) and multiply that by 100. This gives you 22.5%
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ssDNA contain 20% A,30%G,25%C ,25%T....when complimentary strand is formed..it contains 25%A(complimentary to T) , 25% G(complimentary to C) , 30% C and 20% T..... HOW?


Oh boy! why this stuff is looking like a russian code book to me.
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Quote:
Originally Posted by koolkiller88 View Post
ssDNA contain 20% A,30%G,25%C ,25%T....when complimentary strand is formed..it contains 25%A(complimentary to T) , 25% G(complimentary to C) , 30% C and 20% T..... HOW?


Oh boy! why this stuff is looking like a russian code book to me.
Oh saw above explanation and now can RIP
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Quote:
Originally Posted by venky2600 View Post
sorry i didnt catch your sentence........how could we get 12.5% here( i mean if 25% T present in one strand,they cannot be present in other strand right..?..).....

can you elaborate it..?
thanks
what I mean was that if you have 1 strand with 100 bases and you have 45 Ts, then the % of T will be 45%.
but if you have 2 strands and still the number of Ts is 45, then the % is 22,5% bc instead of 100 now we have 200 bases
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Quote:
Originally Posted by Casandra View Post
what I mean was that if you have 1 strand with 100 bases and you have 45 Ts, then the % of T will be 45%.
but if you have 2 strands and still the number of Ts is 45, then the % is 22,5% bc instead of 100 now we have 200 bases
k...got it
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