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NBME 20 Answers

nbme20/Block 1/Question#34 (46.2 difficulty score)
A strain of Escherichia coli produces a ...
MissenseπŸ”
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 +24 
submitted by hayayah(1056),
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sMnsseie ttnmiosau neivolv a ecieoulndt outstusbniti ugrtlsnie ni gaendch naimo csiad. eomtsSeim eht fsftece fo esmnssie unomtaits yam eb noyl pnaprtea urnde enirtac lrnoevienntma ioc;nidtnos hucs esmsesin tsmutaion rea ldaecl intioaldnoc ati.mtnsou Myna eessismn uamnsotit lsteru ni oripnste ttha rea tisll lnac,fuiton ta altse to eoms gee.dre

lsAo, lla het etrho swseran doulw abpobylr veeal uyo hiwt reieth a ygeartl edtaler ro lfoo-ucnatinnn ptiroen.

thefoggymist  I chose nonsense because I thought it'll make the enzyme shorter (since less bonds = more heat liable = can't work at 42 degrees) but yea, probably won't work even at 30 if it's an early nonsense. +6  
joanmadd  A good clinically related example is HbC disease on electrophoresis. HbC disease is due to a missense mutation (glutamic acid (-) for valine (neutral)). The mutated hemoglobin is still fairly functional but when placed on gel electrophoresis the HbC migrates the least far from the negative electrode due to less negative propulsion (lack of wild type glutamic acid). +1  
fatboyslim  @Joanmadd Just to clarify, glutamic acid substitution for valine occurs in sickle cell disease not HbC. HbC is glutamic acid substitution for lysine :) +6  
rockodude  lyCCCCine for HbC +2  



 +0 
submitted by andremosq(1),

Missense - Nucleotide substitution results in CHANGED aminoacid..like what happens on Sickle cell disease, substitution of glutamic acid with valine..

the question says requires exogenous tryptophan for growth at 42 but no at 30.. so its still partially functional,so it cant be NONSENSE.