Excerpt on Majority Gate:
All three inputs get a vote, majority rules.
Excerpt on OR Gate:
Anchor (fix) one input at 1. Then if either of two free inputs goes to 1, get 1 as output.
Excerpt on AND Gate:
Anchor (fix) one input at 0. Then both of the free inputs must go to 1 in order to get 1 as output.
QCA's combined to form a binary adder (building block that allows computers to do math)
Aldeman's seminal paper on DNA Computing (cached copy with added notes at end)
Alderman's: "Computing with DNA" review in Scientific American (cached copy)