Chargaff Base Pairs - USMLE Forums
 USMLE Forums         Your Reliable USMLE Online Community     Members     Posts
 Home USMLE Articles USMLE News USMLE Polls USMLE Books USMLE Apps
 USMLE Forums Chargaff Base Pairs
 Register FAQs Members List Search Today's Posts Mark Forums Read

 USMLE Step 1 Forum USMLE Step 1 Discussion Forum: Let's talk about anything related to USMLE Step 1 exam

#1
10-17-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,186 Threads: 119 Thanked 821 Times in 434 Posts Reputation: 831
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%
 The above post was thanked by: alice (10-18-2012), Easy (10-18-2012), vnagubandi (10-17-2012)

#2
10-17-2012
 USMLE Forums Addict Steps History: 1 + CS Posts: 106 Threads: 3 Thanked 40 Times in 32 Posts Reputation: 50
B?

Is it B- 20% to complement the A in first ssDNA...
#3
10-17-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 570 Times in 323 Posts Reputation: 580

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%

#4
10-17-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,186 Threads: 119 Thanked 821 Times in 434 Posts Reputation: 831
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
#5
10-17-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 570 Times in 323 Posts Reputation: 580

Quote:
 Originally Posted by venky2600 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?
#6
10-17-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,186 Threads: 119 Thanked 821 Times in 434 Posts Reputation: 831

Quote:
 Originally Posted by Casandra 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..?)
#7
10-17-2012
 USMLE Forums Addict Steps History: 1 + CS Posts: 106 Threads: 3 Thanked 40 Times in 32 Posts Reputation: 50

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
#8
10-18-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,186 Threads: 119 Thanked 821 Times in 434 Posts Reputation: 831

Quote:
 Originally Posted by Casandra 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
#9
10-18-2012
 USMLE Forums Scout Steps History: Not yet Posts: 75 Threads: 3 Thanked 35 Times in 23 Posts Reputation: 45

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:
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

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%
 The above post was thanked by: DrSK (10-18-2012), koolkiller88 (10-18-2012), venky2600 (10-18-2012)
#10
10-18-2012
 USMLE Forums Master Steps History: Step 1 Only Posts: 864 Threads: 62 Thanked 374 Times in 225 Posts Reputation: 384

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.
__________________
One day i will infect these microbes and they will search for "Anti-MEbiotic"....
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
#11
10-18-2012
 USMLE Forums Master Steps History: Step 1 Only Posts: 864 Threads: 62 Thanked 374 Times in 225 Posts Reputation: 384

Quote:
 Originally Posted by koolkiller88 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
__________________
One day i will infect these microbes and they will search for "Anti-MEbiotic"....
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
#12
10-18-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 570 Times in 323 Posts Reputation: 580

Quote:
 Originally Posted by venky2600 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
#13
10-18-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,186 Threads: 119 Thanked 821 Times in 434 Posts Reputation: 831

Quote:
 Originally Posted by Casandra 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

 Tags Molecular-Biology-, Step-1-Questions

Message:
Options

## Register Now

In order to be able to post messages on the USMLE Forums forums, you must first register.
User Name:
Medical School
Choose "---" if you don't want to tell. AMG for US & Canadian medical schools. IMG for all other medical schools.
 AMG IMG ---
USMLE Steps History
What steps finished! Example: 1+CK+CS+3 = Passed Step 1, Step 2 CK, Step 2 CS, and Step 3.

Choose "---" if you don't want to tell.

 Not yet Step 1 Only CK Only CS Only 1 + CK 1 + CS 1+CK+CS CK+CS 1+CK+CS+3 ---
Favorite USMLE Books
 What USMLE books you really think are useful. Leave blank if you don't want to tell.
Location
 Where you live. Leave blank if you don't want to tell.

## Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.