








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

Thread Tools  Search this Thread  Display Modes 
#1




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:  
#2




B?
Is it B 20% to complement the A in first ssDNA...

#3




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% 
#4




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 askedQ)T content of entire duplex formed22.5%(average of 25% in one strand and 20% in one strand)[option D] also A22.5%, ,G 27.5% , , C 27.5%[option G] {as A=T ;G=C} the total T content of the duplex25%(of one strand) + 20% (of other strand)=45%[optionC] the total A and T content of the duplex45% + 45% = 90%[option E] the total A and T content of the entire duplex90%/2==45% the total G content(or C content) of the duplex30% + 25% =55% the total G(or C) content of the entire duplexaverage of 30% and 25%====27.5% hope it understands.... @casandra and @drsk thanks for answering both of you 
#5




Quote:
nice question where is it from btw? 
#6




Quote:
ya i also thought of 45% only as answer(that becomes the easy question right..?) 
#7




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




Quote:
can you elaborate it..? thanks 
#9




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% 
The above post was thanked by:  
#10




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 "AntiMEbiotic".... To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. 
#11




Oh saw above explanation and now can RIP
__________________
One day i will infect these microbes and they will search for "AntiMEbiotic".... To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts. 
#12




Quote:
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




k...got it

Tags 
MolecularBiology, Step1Questions 
Thread Tools  Search this Thread 
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
The cause of Acid Base Disturbance?  mosallam  USMLE Step 2 CK Forum  15  09112014 09:54 PM 
Acid Base Physiology!  usmleroad  USMLE Step 1 Forum  5  02272012 05:16 AM 
summary of acid base  usmleman2020  USMLE Step 1 Forum  0  07312011 10:05 PM 
DNA base percentages  saintdizzy  USMLE Step 1 Forum  11  07122011 03:17 PM 
acid base status  INCOGNITO  USMLE Step 1 Forum  7  07092011 07:56 AM 
