Lecture11_NABPty2012 (1)

o o o p o ch2 o o o p o ch2 o o o opo o o o o oh oh

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Unformatted text preview: . . . . O P O-CH2 O O O P O-CH2 O O O OPO O O O O OH OH CH 2 O RNA O P O-CH2 N O N P O N O OPO O O OH O P OH O O N O H2O H+ OH ... HOCH2 N O OPO O mixture of 2’- and 3’- monophosphate derivatives O OH ... shortened RNA The reaction proceeds through a 2’,3’-cyclic monophosphate intermediate. The Enzymatic hydrolysis of RNA by RNase proceeds through a similar intermediate. Because DNA lacks the 2’-OH group, it is stable under alkaline conditions. Why does DNA contain T rather than U? Cytosine deaminates non-enzymatically to form uracil. If this happens in DNA, it constitutes a mutation. Cells have a proofreading system that recognizes the error and replaces the U by C. 5-methyl cytosine occurs in DNA at a low but significant frequency (1/100-1/1000). When it is deaminated, it forms thymine, a normal constituent of DNA. This is a more serious alteration because there is now a T:G “base-pair”. O NH2 N O C C N CH H2O CH cytosine HN O C C N CH CH uracil Deamination of cytosine is of less consequence in RNA, because RNA is not the permanent repository of genetic information. The phosphate groups of DNA and RNA are negatively charged 5’ HO-CH2 N O O M+ OPO CH 2 O O N O A phosphodiester group has a pKa of about 1, and so will always be ionized and negatively charged under physiological conditions (pH ~7). Nucleic acids require counterions such as Mg2+, polyamines, histones...
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This note was uploaded on 02/17/2012 for the course CHEM 212 taught by Professor Staff during the Fall '10 term at Rutgers.

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