Over the last few years it’s been shown theoretically and with some prototype devices that a biological computer is possible. That is, a digital computer where the components are built not of silicon or metal but with organic material. The question has become not can a biological computer be developed, but how – or more to the point, how best? Therein lie the ongoing lines of research. It’s tempting to call it a competition, although all approaches at this point are so new that it’s difficult to even compare them, much less evaluate them.

One of those approaches, in development by a team at the Imperial College of London (England) led by Baojan Wang and Richard Kitney, uses DNA cultivated from the common stomach bacteria Escherichia coli (E. coli). As published in Nature Communications [13 October 2011, paywalled,Engineering modular and orthogonal genetic logic gates for robust digital-like synthetic biology], their approach uses gene expression – the ability of a gene to produce proteins – as the core mechanism for a logic gate. Logic gates with names like AND, OR, NOT, NAND are key pieces of digital computers that perform many of the processing functions. Building these gates from DNA uses the combinatorial capability of genes in much the same way as “on” or “off” works for electronic circuits.

In this research, a DNA logic gate starts with a custom grown snippet of the E. coli DNA. It consists of two genes (hrpR and hrpS) that are controlled (activated) by two separate inputs (promoters). Only when the two genes are expressed, they trigger an output (hrpL promoter). In basic computer logic this is equivalent to AND (both inputs are TRUE). The output promoter can, in turn, be linked to another gate, which the researchers also tested; showing that this approach to DNA logic gates can be hooked together into more complex circuits.

This is a baby step in terms of computer processing, but behind the development of a DNA gate are the techniques for developing components from common bacteria and the ability to isolate, manipulate and link the components into more complex devices. It is, by demonstration of concept, the first step toward a biological computer. Whether it goes further down the road in a few years depends on whether the approach is inexpensive, reliable, scalable (can be done in large devices and many times), efficient and competitive with other approaches.

In that respect, this research hopes that using DNA expression will be easier to implement than some other more complex approaches. For example, a team at the California Institute of Technology (USA) built the first DNA logic gate in 2006, which uses individual molecules that attach to specific points on a DNA strand and thereby release an output molecule. The output molecule can then be counted to derive the logic (AND, OR etc.) Obviously, this is very different than using gene expression and to a certain extent is operating at a different (and more complicated) level of chemistry. There are some details of this in a previous SciTechStory post:
DNA computing: Advances in organic circuits.

These and other efforts at biological computing point to years, perhaps a decade or more, before computing devices that are truly useful become available. But at this point, it seems it will only be a matter of time. Keep in mind, the goal is not to compete with other forms of computing, but to provide a form of computing device that is compatible with living things – biologically compatible. For example, a biological computing device could be used within organs, tissues or even cells within the human body to monitor biochemistry or control various medical procedures. The applications are endless, if also typically open to both good and not so good uses.