By Patricia Reaney

LONDON (Reuters) - Following Mother Nature's lead, Israeli scientists have built a DNA computer so tiny that a trillion of them could fit in a test tube and perform a billion operations per second with 99.8 percent accuracy.

Instead of using figures and formulas to solve a problem, the microscopic computer's input, output and software are made up of DNA molecules -- which store and process encoded information in living organisms.

Scientists see such DNA computers as future competitors to for their more conventional cousins because miniaturization is reaching its limits and DNA has the potential to be much faster than conventional computers.

"We have built a nanoscale computer made of biomolecules that is so small you cannot run them one at a time. When a trillion computers run together they are capable of performing a billion operations," Professor Ehud Shapiro of the Weizmann Institute in Israel told Reuters on Wednesday.

It is the first programmable autonomous computing machine in which the input, output, software and hardware are all made of biomolecules.

Although too simple to have any immediate applications it could form the basis of a DNA computer in the future that could potentially operate within human cells and act as a monitoring device to detect potentially disease-causing changes and synthesize drugs to fix them.

The model could also form the basis of computers that could be used to screen DNA libraries in parallel without sequencing each molecule, which could speed up the acquisition of knowledge about DNA.

ENORMOUS POTENTIAL

DNA can hold more information in a cubic centimeter than a trillion CDs. The double helix molecule that contains human genes stores data on four chemical bases -- known by the letters A, T, C and G -- giving it massive memory capability that scientists are only just beginning to tap into.

"The living cell contains incredible molecular machines that manipulate information-encoding molecules such as DNA and RNA (its chemical cousin) in ways that are fundamentally very similar to computation," said Shapiro, the head of the research team that developed the DNA computer.

"Since we don't know how to effectively modify these machines or create new ones just yet, the trick is to find naturally existing machines that, when combined, can be steered to actually compute," he added.

Writing in the science journal Nature, Shapiro and his team describe their DNA computer, which is a molecular model of one of the simplest computing machines -- the automaton which can answer certain yes or no questions.

Data is represented by pairs of molecules on a strand of DNA and two naturally occurring enzymes act as the hardware to read, copy and manipulate the code.

When it is all mixed together in the test tube, the software and hardware operate on the input molecule to create the output.

The DNA computer also has a very low energy consumption, so if it is put inside the cell it would not require much energy to work.

DNA computing is a very young branch of science that started less than a decade ago, when Leonard Adleman of the University of Southern California pioneered the field by using DNA in a test tube to solve a mathematical problem.

Scientists around the globe are now trying to marry computer technology and biology by using nature's own design to process information.