Electron micrographic autoradiograph of mating bacteria. The Hfr (high frequency recombination) cell, (also known as Hfr strain) is the large cell at the top of picture. It was labeled, previous to mating with the radioactive tracer H3-Thymine. The F-(minus) cell (at bottom) was unlabeled, and mating took place in cold medium. The autoradiographic grains indicate the presence of labeled DNA in the nucleus of the Hfr, and its transfer to the nucleus of the F- cell. Magnification 140,000x @ 8x10 inches.