Too late to be ennobelled?

Last night on the BBC World Service I got gripped by the story of Florence Bell (1913 to 2000) who in 1938 went to Leeds, drawing upon her skills as a

. She discovered DNA thus, of course, transforming our understanding of the make up of the human body. Her supervisor took the view that women lacked the vital spark which enabled achieving this sort of breakthrough.

However, Florence's work was published in NATURE and that was that. The interesting point is that she never received a Nobel prize or equivalent (is there an equivalent?). Florence was a modest woman and merely had Housewife to describe her on her tombstone. Is it too late to award her a Nobel prize posthumously? If not, why not?