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NBME 20 Answers

nbme20/Block 2/Question#3 (49.3 difficulty score)
An investigator compares the DNA sequences of ...
5β€² CCGGπŸ”

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submitted by hayayah(1056),
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tosM itetscirrno zmenyes ibdn adonismpr.el

oS tboh GCG'C5 or G'C3GC lwuod eahv eben btapcceeal ni isht i.socerna

meningitis  Yes, correct. The 5'GGCC option could cause some confusion. +  
guillo12  I really don't understand the question nor the answer. Can someone explain it for dummies like me? +8  
whossayin  yes please.. I'm with guillo12 on this +  
sugaplum  @guillo12 @whossayin questions says you've created a new cut site, 1. look at the region on the sick vs healthy. The C to G is the change 2. Write out the sick "CCGG" from 5'3'- you could write out the whole thing, but the answer only has 4 letters, so being lazy here 3. write under it, its complement, the dna base pair. So "GGCC" 4. remember both strands are going in opposite directions when you write them out on top of each other. 5. So the bottom strand actually reads 5' CCGG 3' so that is the answer I hope that clears it up +47  
shirafune  To add to the palindrome part, many restriction endonucleases actually function as dimers. Each individual subunit usually has a nickase, so to create a double-stranded break in DNA, they must bind a palindrome so that each enzymatic domain creates a single-stranded break (thus a double-stranded break). +1  
alimd  Why do we start from CCGG? Why not CGGG or TACC? +2  
alimd  Why do we start from CCGG? Why not CGGG or TACC? +1  
ssbhatti  I think its due to the palindrome requirement? +  
bbr  Maybe I'm missing a part here, but the substrate that the enzyme will bind to will be the DNA. I went with the line that was from the questions stem, as it is the mtuated DNA will be recognized by the restriction enzyme. I didnt see the need to convert it into base pairing. Let me know what you guys think. +1  
uloveboobs  @bbr I agree. I'm definitely not an expert in these lab tests, but the question asks "substrate specificity." I was thinking that it would recognize the abnormal DNA; nothing to do with RNA. I didn't know about the palindromic preference of restriction enzymes, but I don't think there's any need to figure out base-pairing and whatnot here. (At least for this question it didn't work out that way!) +  
spaceboy98  sugaplum, I'd give you an award if this was Reddit +5