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NBME 20 Answers

nbme20/Block 2/Question#3 (54.5 difficulty score)
An investigator compares the DNA sequences of ...
5′ CCGG🔍,📺
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 +3 
submitted by hayayah(1101),
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tsMo ictitrornes syeenmz ibnd ilmnader.osp

oS ohbt G'5CGC ro C'GGC3 oluwd heav been eaclcpbeat ni hsti ronasiec.

meningitis  Yes, correct. The 5'GGCC option could cause some confusion. +1  
guillo12  I really don't understand the question nor the answer. Can someone explain it for dummies like me? +11  
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 +56  
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 +6  



 +1 
submitted by topgunber(44),

Restriction enzymes cut at palindromic sequences... i knew this would bite me in the ass someday.+ We are testing two pieces of DNA- one with a mutation, one without.+ We need to use a restriction enzyme to cut exactly where the mutation is in order to see which piece of DNA has the sequence of interest (the restriction enzyme site). I chose 5' ACCG, which would cut the mutated strand and not the wild type. Why is this wrong? because when you write the complementary strand you get TGGC, which is not a mirror image of ACCG. + Correct answer 5' CCGG, the complementary strand: GGCC. This is a palindromic sequence (1), would cut the mutated strand of DNA and not the wild type (2), which when using gel electrophoresis the mutated strand would show 2 bands small bands, while the non mutated strand would show one large band (3)

topgunber  sorry bout the format on this +  
utap2001  The definition of palindrome is "mirror sequence". The inverse of complementary strand is the same with main strand. https://socratic.org/questions/why-do-most-restriction-enzyme-cuts-at-palindromic-sequence +  
drdoom  palindromemordnilap +