How large is the smallest piece of DNA compared to a hydrogen atom? What chemical elements go into the average strand of DNA? If the human body is mostly H2O, then why are we considered "carbon-based" lifeforms?
1. DNA is not "one size" - it is an enormously long chain made of small building blocks: adenine, thymine, guanine, cytosine. Even small strands of DNA are millions of time larger than a single atom.
2. See the URL above: Carbon, hydrogen, oxygen, nitrogen
3. All of what we are depends on carbon chemistry, water is the medium for the reactions. Sometimes water is bound up into the chemicals created or is released as a product of the reaction.