1962: Ris and Plaut use electron microscopy to
demonstrate that chloroplasts contain DNA.
1969: Ris, Hans; Westmoreland, and Szybalski
develop an electron microsopic method for
physically mapping genes in lambda phage.
They photograph heteroduplex DNA by annealing
the - strand with the + strand, the heteroduplex
containing deletions, insertions, substitutions,
and inversions.