%ASSEMBLY DIRECTIONS %judyfig1a.eps and judyfig1b.eps go in the section titled %``DNA NDTM: description and operation''. % %editfig2.eps and orthopict.eps go in the %``RNA editing'' section toward the end, e.g. %between its end and the start of %the ``RNA life'' section. DNA computers in vitro and vivo Warren D. Smith NECI Abstract We show how DNA molecules and standard lab techniques may be used to create a nondeterministic Turing machine. This is the first scheme that shows how to make a universal computer with DNA. We claim that both our scheme and previous ones will work, but they probably cannot be scaled up to be of practical computational importance. In vivo, many of the limitations on our and previous computers are much less severe or do not apply. Hence, lifeforms ought, at least in principle, to be capable of large Turing universal computations. The second part of our paper is a loose collection of biological phenomena that look computational and mathematical models of computation that look biological. We observe that cells face some daunting computational problems, e.g. gene regulation, assembly of complex structures, and antibody synthesis. We then make simplified mathematical models of certain biochemical processes and investigate the computational power of these models. The view of ``biology as a computer programming problem'' that we espouse, may be useful for biologists. Thus our particular Turing machine construction bears a remarkable resemblance to recently discovered ``RNA editing'' processes. In fact it may be that the RNA editing machines in {\it T.Brucei} and other lifeforms are clonable, extractible and runnable in vitro, in which case one might get a far better performing Turing machine than all constructions so far, including our own. The fact that RNA editing {\it is} a Turing machine may in turn have a lot to do with the origins of life. We also have a computer science explanation for ``junk DNA.'' Keywords DNA based computing, nondeterministic Turing machine, RNA editing, transposons, replicon killers, cellular automata, multicellular development, gene regulation, hypotrichous ciliates, junk DNA, biologically based models of computation, NP, PSPACE. %RNA world, %immune system