Chemistry lesson: structure of ara C
The following structure represents the natural nucleoside cytosine, or cytosine deoxyriboside. Note the base (blue) and the sugar (reddish). The sugar moiety is deoxyribose, thus this compound is a natural building block of DNA.
The structure below is that of cytosine riboside. This compound is a building block of RNA.
The structure below represents cytosine arabinoside. Note how the sugar is different from both deoxyribose and ribose. This compound is not a building block of either DNA or RNA.
Note that cytosine arabinoside (above) was not the compound isolated from the marine sponges. Instead, it was synthesized in the lab, inspired by spongothymidine, also called thymine arabinoside or ara T - spongothymidine.