A method of determining whether a cytosine present at a predefined position within a single strand of a double-stranded DNA sequence of known sequence is non-methylated comprising: a) contacting the double-stranded DNA with a mutant CpG methyltransferase and an S-adenosylmethionine analog having the structure: wherein R is a chemical group capable of (a) being transferred from the S-adenosylmethionine analog by the mutant CpG methyltransferase to the 5 carbon of any non-methylated cytosine within the double-stranded DNA and (b) forming a covalent bond between the chemical group and the 5 carbon of any such non-methylated cytosine within the double-stranded DNA, thereby producing a derivative of the double stranded DNA and R has any one of the following structures: wherein each of R 1, R 2 and R 3 is independently H, alkyl, aryl, C NH 2, C R', CN, NO 2, C R', or S 2 NHR'; wherein X is 0 or NR'; wherein R' is H, alkyl or aryl; and wherein n is an integer from 1 to 8; and b) photocatalytically converting any cytosine to which the chemical group R is covalently bound to a uracil analog by photoirradiating the derivative of the double-stranded DNA at a wavelength greater than 330 nm up to 700 nm, wherein the presence of the uracil analog at the predefined position within the single strand of the double-stranded DNA indicates that the cytosine at that position is non-methylated, wherein the mutant CpG methyltransferase comprises relative to non-mutant M. Sss I methyltransferase from E. coli K12 strain ER1821 (i) substitution of serine for the amino acid corresponding to Q142 and (ii) substitution of serine for the amino acid corresponding to N370. a) contacting the double-stranded DNA with a mutant CpG methyltransferase and an S-adenosylmethionine analog having the structure: wherein R is a chemical group capable of (a) being transferred from the S-adenosylmethionine analog by the mutant CpG methyltransferase to the 5 carbon of any non-methylated cytosine within the double-stranded DNA and (b) forming a covalent bond between the chemical group and the 5 carbon of any such non-methylated cytosine within the double-stranded DNA, thereby producing a derivative of the double stranded DNA and R has any one of the following structures: wherein each of R 1, R 2 and R 3 is independently H, alkyl, aryl, C NH 2, C R', CN, NO 2, C R', or S 2 NHR'; wherein X is 0 or NR'; wherein R' is H, alkyl or aryl; and wherein n is an integer from 1 to 8; and wherein each of R 1, R 2 and R 3 is independently H, alkyl, aryl, C NH 2, C R', CN, NO 2, C R', or S 2 NHR'; wherein X is 0 or NR'; wherein R' is H, alkyl or aryl; and wherein n is an integer from 1 to 8; and b) photocatalytically converting any cytosine to which the chemical group R is covalently bound to a uracil analog by photoirradiating the derivative of the double-stranded DNA at a wavelength greater than 330 nm up to 700 nm, wherein the presence of the uracil analog at the predefined position within the single strand of the double-stranded DNA indicates that the cytosine at that position is non-methylated, wherein the mutant CpG methyltransferase comprises relative to non-mutant M. Sss I methyltransferase from E. coli K12 strain ER1821 (i) substitution of serine for the amino acid corresponding to Q142 and (ii) substitution of serine for the amino acid corresponding to N370.