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Computer Chip of the Future May Be Built on DNA


The recipe for a computer chip of the future may read something like this: Take some wires. Add DNA. Stir.

In an advance that may provide a practical method for making molecular-size circuits -- the smallest possible -- scientists in Israel used strands of DNA, the computer code of life, to create tiny transistors that could literally build themselves.

“What we’ve done is to bring biology to self-assemble an electronic device in a test tube,” said Dr. Erez Braun, a professor of physics at the Technion-Israel Institute of Technology in Haifa, Israel, and a senior author of a paper describing the research in Friday’s issue of the journal Science.

Scientists have in the past few years accomplished feats of the incredibly small, constructing devices not much larger than individual molecules, but they also realize that their current painstaking techniques are too slow and inefficient. “In order to construct a circuit, you need to invent ways to tell molecules where to go and how to connect to each other,” Braun said.

To that end, many scientists have turned to the biologically inspired notion of self-assembly: using molecules like DNA and proteins that can automatically link together in the correct configuration.

“It’s all of the dynamics on that scale rather than just making small stuff,” said Dr. Horst Stormer, a professor of physics at Columbia University. Stormer, who was not involved in the new research, described the work as a “good first step” toward self-assembling electronic devices.

The Technion-Israel scientists constructed transistors from carbon nanotubes, cylindrical molecules that are about one 10-millionth of an inch in diameter and resemble rolled-up chicken wire. Other researchers have made similar transistors, which offer a promising potential to replace silicon when current technology reaches fundamental limits in about a decade, but a challenge remains how to guide nanotubes to specific locations. In the earliest work, the nanotubes were placed randomly; by chance, some made the correct electrical connections.

Since then, researchers have looked for a more practical way of wiring together the billions of transistors that would be needed for a computer chip. Scientists at Duke University reported in August that they coated DNA with silver to produce ultrathin wires. The Israeli group is the first to use DNA to build a working electronic device.

“It’s a very interesting demonstration of a completely new concept of assembling devices,” said Dr. Cees Dekker, a professor of physics at the Delft University of Technology in the Netherlands, whose research group made the first nanotube transistor in 1998.

The new technique takes advantage of a biological process known as recombination, where a segment of DNA is swapped out for an almost identical piece. The cell uses recombination to repair damaged DNA and to swap genes. A special protein helps connect the replacement DNA to the desired location.

By attaching a nanotube to the protein, the nanotube moves to an exact location along the DNA strand. “The DNA serves as a scaffold, a template that will determine where the carbon nanotubes will sit,” Braun said. “That’s the beauty of using biology.”

The scientists then coated the DNA with gold, producing a simple electronic device consisting of the nanotube connected to gold wires at each end. Current through the nanotube could be switched on or off by applying an electric field -- the definition of a transistor.

In earlier work, the same researchers showed that they could stretch DNA across a surface to provide a template to hook the transistors together into a circuit. The next step would be to actually build the circuit, Braun said.

Other groups are looking at alternative ways of building molecular circuits. Dekker’s group now lays catalyst that will grow nanotubes at desired places. Dekker is also exploring use of DNA, although using a different approach: DNA molecules attached at the end of nanotubes that would act like “smart glue.” Each strand would be able to attach to only one other one. “It’s programmable Velcro,” Dekker said.

In Sniper Trial, Ex-Wife Testifies to Rage, Threats


There was a time, Mildred Muhammad recalled on Wednesday, when life with her former husband John seemed normal, even happy. They had three young children, a thriving auto repair business in Tacoma, Wash., and a reputation for caring for children from broken homes.

But in 1999, their marriage fell apart, and so did John Allen Muhammad, his former wife testified. By the end of that year, the couple had separated, their business had closed and Muhammad had threatened her life.

“You have become my enemy,” he told her in early 2000, she said, testifying for the prosecution before a jury that is weighing a death sentence for her former husband. “And as my enemy, I will kill you.”

In the monthlong trial of Muhammad, who was convicted on Monday of directing the sniper attacks in the Washington area last fall, there has been little testimony to answer the inescapable question: Why?

The jury got perhaps its clearest indication on Wednesday, as Mildred Muhammad described her former husband’s long odyssey from devoted parent and hard-working business owner to the central figure in 10 cold-blooded killings.

The jury of seven women and five men must now decide whether to sentence Muhammad to death or life in prison without parole for the killing of Dean H. Meyers last October. Deliberations are expected to begin on Thursday.

The prosecution has argued that the principal motive for the killing spree was money, with Muhammad and Lee Boyd Malvo, accused of being his co-conspirator, demanding $10 million from the government to stop the shootings. Malvo, 18, is on trial in adjacent Chesapeake on capital murder charges in a separate killing. The prosecution is expected to rest its case on Thursday.