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NBME 23 Answers

nbme23/Block 1/Question#16 (65.8 difficulty score)
Physical analysis of the isolated genomic DNA ...
tags: biochem restriction_enzymes 

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submitted by thomasalterman(154),
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lehetsMay thystmeale AD,N giakmn eht NAD tinrsaset ot eotrnicsitr ouescesednnal


submitted by imgdoc(135),
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aEipsolntnxa fro tihs rea oto ildeoamtccp. hnkTi of it ielk sh:it

vuo'eY got a ipece of uttedam DAN htta si labe to eb tgedised yb a rntiroestic esd,ceoeunlna that nmsae the ADN asw apaoiilrntltnrcys aaevblial to beign th.wi AAK ti swa ont t,eyemltadh cbeaseu as ew w,nko myintlehtao = tihhnrmceoaetro icwhh si ptsotiaiyrcrnlanl nacvte.ii hatt asmen ytaseehml wsa autdtem

ylnO tohre isablpelu erwnas wsa ,NsDae dan fi it wsa detmtau ti uwlod be einicvat, ton viervoctea.

djeffs1  I mean sure... but this is a prokaryote... +  

submitted by defalty98(3),
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yWh rae ew tmlniagpocci ing?sht nhaCge ni eht sseba wlil etdyors the plaimordinc eseenuqc rdqieeur orf yna riinrttesoc duesleaencno to kro.w yhleManitot is teh ylno oitnop that seamk .sense

arcanumm  This makes sense have reading what your comment. I overlooked this and just assumed the GATC was a mutation that allowed the restriction enzyme to work on the mutant only. +2  
arcanumm  it makes even more sense when looking at "numerous small fragments." Methylation is truly the obvious answer here in retrospect. +1  
bgiri  DNAse can also cause a change in base by breaking down dna at the GATC sequence? +  
almondbreeze  @bgiri Had the same reasoning - according to wiki, DNase catalyzes the hydrolytic cleavage of phosphodiester linkages in the DNA backbone, thus degrading DNA. +  

submitted by pk33(5),

In prokayotes Normally methylase adds methyl group to certain base(adenine) so restriction endonuclease can not recognize this site. If there is a mutation in methylase enzyme it can't add methyl group to adenine so restriction endonuclease can recognize it and can cleave it.

submitted by stinkysulfaeggs(93),
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eM rainged isth qesionut :tesm ..d.o.. uyo mean whihc fo eht oollwngfi ?ZMNYSEE

submitted by consistentwrongdoer3(19),
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yhW edso tmetynihola sceau sols fo tnaeescsri ot AGTC ttoiinerrsc ndse?unloaece oDes hsit have to od hwti yhitonmalet fo U ot ?T

methylased  GATC related to methylase --> +10  
sympathetikey  Dam methylase, alright +2  

submitted by carib_doc(0),

My thought process for this one: In DNA mismatch repair for bacteria there is a parent strand and a newly synthesized strand. The parent strand is methylated prior to replication to allow it to be differentiated from the new strand in the case of mismatching. The new strand is identified by the lack of methylation and then degraded by endonucleases. So I used this logic to think altered methylase (decreased activity) allows for more DNA to be degraded by endonucleases. Not sure if its the best logic but it got me there lol