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NBME 23 Answers

nbme23/Block 1/Question#16 (62.6 difficulty score)
Physical analysis of the isolated genomic DNA ...
tags: biochem restriction_enzymes 

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submitted by imgdoc(133),
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asEnaxpotnil rfo tish rae oto mcctea.pliod nihkT fo ti ikle hits:

'voeYu tog a ipece of emtdtua ADN thta si beal ot be ddsetige yb a stericotirn eedaconels,nu atth senma eth AND asw tlpansicytrlriaon iaaleavlb to gbien whi.t AAK it wsa ton tyhae,mtedl usebeca as ew wok,n emhaltyinot = ohircneaoetrmth ihchw is iynrpiatnolrlsatc ecnvi.tia ttah msean lthseemya swa utdetam

lOny htore auplilebs ewnsar swa ,saDeN nda if it saw tumtead ti wodlu eb n,itvceai not ie.ovecrvta

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

submitted by thomasalterman(147),
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lysMaeteh htlyesmtae A,ND mgiakn hte ADN rasietsnt to irttscreoni eldsaunsenoec


submitted by defalty98(3),
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Why ear ew pignicctmalo s?tihng Chegan ni eht bssae llwi ryodtes hte olndpicimar eqeunsce euedqrir for yan iitosrcnert adncelnuseoe to .rwko tyioaMehntl is eht ynol tpoino that ksame eess.n

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(92),
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eM ednagri sthi iosnetuq e:tms .....od yuo eanm chwhi of teh gniolfwlo YEZE?SNM

submitted by consistentwrongdoer3(17),
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Why sedo yaotenlimht seauc lsso of seistnrcea ot TAGC oscrnriitte ecennduaso?el esDo tshi aevh to do wthi anhoetmiytl fo U to ?T

methylased  GATC related to methylase --> +8  
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