cytosine

I would take adenosine, guanine, taurine, and cytosine and pair them into double helices.

Seems like replacing cytosine could have some negative side effects if it occurs in the patient's cells as well as the virus's.

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

'When DNA is replicated, methyl-cytosine pairs with guanine just like cytosine does.

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".

I agree that it could be useful to introduce students to the concept of epigenetics, of which base pair modifications such as methyl-cytosine are just one aspect.

It may be that our DNA uses only the four specific bases guanine, cytosine, adenine, and thymine because wobble pairings provided additional mutation resistance, or offered additional structural options for transcribed proteins.

> Because methylcysteine and methyladenine aren't really fundamentally different from their unmethylated counterparts...You mean methyl-cytosine; methyl-cysteine would be a derivative of the amino acid cysteine.

All these mechanisms interact to ensure genes are turned on and off at the right time and place, and this field is known as 'epigenetics'.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'.