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NBME 23 Answers

nbme23/Block 1/Question#16

Physical analysis of the isolated genomic DNA from ...

Methylase

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Methylase methylates DNA, making the DNA resistant to restriction endonucleases

*https://d2jmvrsizmvf4x.cloudfront.net/7Pf4zZ3TuGwLTRKvUGYL_methylatedrestriction.jpg





Me reading this question stem: .....do you mean which of the following ENZYMES?





Why are we complicating things? Change in the bases will destroy the palindromic sequence required for any restriction endonuclease to work. Methylation is the only option that makes sense.





Why does methylation cause loss of resistance to GATC restriction endonuclease? Does this have to do with methylation of U to T?

methylased  GATC related to methylase --> https://en.wikipedia.org/wiki/Dam_methylase +2  
sympathetikey  Dam methylase, alright +2  




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submitted by imgdoc(33),

Explanations for this are too complicated. Think of it like this:

You've got a piece of mutated DNA that is able to be digested by a restriction endonuclease, that means the DNA was transcriptionally available to begin with. AKA it was not methylated, because as we know, methylation = heterochromatin which is transcriptionally inactive. that means methylase was mutated

Only other plausible answer was DNase, and if it was mutated it would be inactive, not overactive.