What did Richard Feynman contribute to molecular biology?
According to Gleick, Feynman spent the summer of 1960 in Delbr&uuml;ck's lab at Caltech and discovered intragenic supression. 

This is where the expression of a gene which has been knocked out by a mutation may be restored by a second mutation within the same gene.

Fenynman worked with the rII mutant of phage T4 and was looking for back mutations in E.coli strain K. 

He noted some unusual plaques which he labelled idiot r's. 

Feynman correctly deduced that these revertants were due to a second mutation which cancelled the effect of the first, but he did not publish the result, possibly because he had no molecular explanation for what was going on. 

The Delbr&uuml;ck lab referred to these revertants as Feyntrons.

Cricks and co-workers in Cambridge, UK, correctly explained the phenomenon.  If, for example, a mutation caused a gene to be read out of sequence (a frameshift mutation) then a second mutation within the same gene could put the message back 'in phase', thus cancelling out the effect of the first mutation. 

All the above is taken from James Gleick (1992) Genius.  Richard Feynman and modern physics, a great book.