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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 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  




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.





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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.