Homologous recombination is not restricted to eukaryotic organisms. Viruses and bacteria also recombine their genomes. Studies of HR in these biological systems have provided important information about the physical nature and enzymology of HR. Recall from Chapter 8 that λ phages produce plaques on Escbericbia coli lawns. Plaque morphology is determined by several λ phage genes, which can be used for recombination studies.

Infection of E. coli with a pair of λ phages that differ in plaque morphology gives parental and recombinant plaque types. Matthew Meselson and Jean Weigle coinfected E. coli with a pair of λ phages that differed in genetic markers and in DNA density. One λ phage was used to infect bacterial cells cultured in a "light" medium that contained nutrients with normal isotopes 12C and 14N, while the other λ phage was used to infect bacterial cells cultured in a "heavy" medium that contained nutrients with the heavy isotopes 13C and 15N.

The λ phages released when the cells in each culture lysed were used to coinfect E. coli cultured in "light" medium. The cell lysate containing the progeny λ phages was centrifuged in a cesium chloride gradient, which separated the λ phages based on density. The λ phages were collected and then tested for parental or recombinant genotype. The major conclusion of this experiment was that recombinant DNA can be formed by breakage and rejoining as indicated by the appearance of genetic recombinants that had both heavy and light DNA.