Question for Jan 20, 2010

X was a scientist directly responsible for the creation of the Manhattan Project. In 1939, he drafted a letter to Roosevelt, explaining the possibility of nuclear weapons, and recommending the creation of a nuclear program to counter Nazi work on such weapons. He had earlier conceived the idea of nuclear fission when in London, partly inspired by the atomic bomb described in H. G. Wells's novel, "The World Set Free". The impetus for the idea was an article by Ernest Rutherford, who dismissed the possibility of using atomic energy for practical purposes, which annoyed X. Legend states that he conceived the process of a chain reaction while waiting for the traffic lights to change at Southampton Row, something he had never done before. Horrified by the destruction caused by nuclear weapons, X switched to molecular biology after the war. Identify X.