DNA Computers:

After a longish time lag, have just started understanding the beauty of organisation of DNA. HAve no idea yet about how DNA computers work. The following article offers tantalising glimples of what it could be....

DNA is also a wonderful way to store information. One gram of genetic material, which would occupy about one cubic centimeter, can hold as much information as 1 trillion CDs, according to Adleman. It's also incredibly cheap: Commercial labs sell a molecule of DNA for about one-thousand-trillionth of a cent. The cost is about $30 for a DNA sequence big enough to compute on. Intel sells its latest P4 chip for more than $500. "DNA has been storing the blueprint of life for several billion years," says Adleman. "Its powers are an untapped legacy for the 21st century."

billions of years of evolution have pushed cells to the brink of what thermodynamics says is possible. Take ligase, a molecule whose job it is to stick strands of DNA together. With just one joule of energy -- the amount of energy a human expends to lift one kilogram one meter, ligase molecules can perform 20x10 to the 18th operations, Adleman says. That's a million times a million times a million times 20 operations. Such efficiency could push computing to new levels since electronics are limited by the amount of power -- and the heat it gives off -- needed to run increasingly sophisticated operations.

In the lab, Adleman made a strand of DNA to represent each city and the path between each city. He then encoded the sequences so that a strand representing a road would connect to any two strands representing a city according to the rules of DNA binding. Then he mixed trillions of copies of each strand in a test tube. Within seconds, the strands wove themselves together in a myriad of possible combinations.

Over a period of time, Adleman performed a series of biochemical reactions to eliminate the wrong answers -- strands encoding routes that either started or ended in the wrong city, those that visited a city more than once, and so on. When all the wrong answers had been destroyed, Adleman was able to look under the microscope and find only strands that carried the right answer.

Adleman's experiment used just seven cities, a problem that isn't hard to solve on modern computers. But Adleman's biological computation showed that DNA has the potential to solve far more complex problems than even the most advanced electronic computers can. The fastest supercomputer wouldn't be able to solve a problem with more than about 50 cities, Adleman says. He believes that a test tube full of DNA could solve the problem with as many as 200 cities.

. Take the Traveling Salesman problem. The DNA in the test tube produced 100 trillion answers in less than one second. Most of those answers were repeats -- and incorrect. Adleman had to discard the erroneous answers using lab procedures that took about a week.