Dr. Brenner made a decisive contribution to solving the code with an ingenious series of experiments in which he altered the DNA of a virus that attacks bacteria.

He showed that by making a series of three mutations, the virus would first lose, then regain, its ability to make a certain protein, as if the cell’s reading of the DNA “tape” had come back into correct phase. The experiment showed that DNA is a triplet code, with each group of three DNA letters specifying one of the 20 kinds of amino acids of which proteins are composed. Dr. Brenner gave these triplets the name codon.

Other researchers were then able to figure out which codon specified each of the 20 amino acids. It fell to Dr. Brenner to identify two of the three triplets that signal “Stop” to the cell’s protein-making machinery.

Dr. Brenner was also the first to conclude that there must be some means for copying the information in DNA and conveying it to the cellular organelles that manufacture proteins. That intermediary, now known as messenger RNA, was discovered in 1960 in an experiment devised by Dr. Brenner and others.

With the fundamental problems of molecular biology solved, as they saw it, Dr. Brenner and Dr. Crick looked for new areas of inquiry. Dr. Brenner decided to approach the brain, but he realized he needed a simpler animal to study than the fruit fly, a standard organism used in laboratories.

He settled on Caenorhabditis elegans, or C. elegans, a tiny, transparent roundworm that dwells in the soil, eats bacteria and completes its life cycle in three days. That worm has spun off many developments, starting with the decoding of the human genome.