The First Mathematical

Computation Using DNA

Leonard Adleman published an article in Science, 1994, in which he outlined how DNA was used to solve a well known mathematical problem called the Hamilton Path (or “travelling salesman”) problem.

The task is to find the shortest route between a number of cities, going through each city only once. As you add more cities to the problem, the problem quickly becomes very complicated. In this example seven cities were chosen – in reality the answer could probably be worked out by hand quicker than Adleman achieved in the DNA computer. However it is the principle which is important. Every experiment such as this, nudges the fledgling technology of DNA computing further out of the world of science fiction and into the realm of the possible.

The steps taken by Adleman in his “DNA test-tube computer”:-

   Strands of DNA represent the seven cities. All of the genetic information in DNA is coded for by the letters A, T, C and G. See What is DNA? Some sequence of these four letters would therefore represent each city and possible path between them.

   These molecules are then mixed in a test tube, resulting in some of these DNA strands sticking together. So a chain of these strands represents a possible answer to the problem, i.e. the sequence of cities, which results in the shortest path.

   The reaction which occurs to joins the DNA strands together, occurs very rapidly, and so within a few seconds, all of the possible combinations – representing every possible answer, right and wrong – will be formed in the test-tube.

Most of the answers generated are incorrect, but one or a few may be correct, the task is to check each of them and winnow out the incorrect ones. The DNA computer does that by subjecting all of the strands simultaneously to a series of chemical reactions that mimic the mathematical computations an electronic computer would perform on each possible answer.

For example, some mathematical operations are performed by enzymes whose function depends on the specific values of information in one or more spots on the strand. An enzyme might add a value of 1 to the end of a strand if either of two specific values in the strand were 1. By orchestrating many such operations, researchers can use the enzymes to perform sophisticated logical and mathematical computations. When the chemical reactions are complete, researchers analyse the strands to find the answer - for instance, by locating the longest or the shortest strand and decoding it to determine what answer it represents.