methyl

You mean methyl-cytosine; methyl-cysteine would be a derivative of the amino acid cysteine.

The authors find that methyl-adenine seems to play a role opposite to methyl-cytosine, surprisingly.

The addition of methyl groups to DNA bases is a long appreciated way to control gene activity - see ref.

Birds are not affected by capsaicin, but are absolutely repulsed by methyl anthranilate (artificial grape flavoring).

While the existence of methyl-adenine was known in bacteria, its function in gene expression in more complicated organisms was not known.

The only way to maintain the 'methylation marks' is to add a methyl group back on to the newly synthesized cytosine.

Having read the paper, methyl-adenine is the same - interesting new aspect of epigenetics and gene regulation, but not a new base.

How can you possibly determine loss of IQ points due to methyl mercury exposure on this set while controlling for other factors?

For example, since the end of alcohol prohibition, how many cases are there of people being blinded by methyl alcohol from bathtub gin?

Just anecdotally, I have found the methyl- much stronger, like 1 cup coffee vs 1 can soda.

The DNA base cytosine is often modified to yield methyl-cytosine, a methylated base with a well established role in the regulation of gene expression; methyl-cytosine tends to mark genome areas with lower probabilities of gene expression.

If you can, avoid the generic cyanoco- type, get the methyl cobalamin.

So your answer is either "because you're not reading the right books" or possibly "because methyl-cytosine is clearly a variety of cytosine, which is one of the four 'classic' bases".

It's natural in the sense that the healthy human body produces s-ame on it own, and if your body ever stopped producing s-ame, you'd probably die since (quoting Wikipedia) "more than 40 metabolic reactions involve the transfer of a methyl group from SAM to various substrates, such as nucleic acids, proteins, lipids and secondary metabolites."

"In the early 80s, to these four "classic" bases of DNA was added a fifth: the methyl-cytosine (mC) derived from cytosine." How come any biology course or book I've ever seen, whether high-school or top-university level, only teaches about four bases?

Methyl-cytosine is a modified version of cytosine that is a very important part of epigenetics, but since it is almost irrelevant to other aspects of genetics, I would argue that you could not consider it a 'fifth base'.