Figure 1. Plaques on an E. coli lawn. The transparent spots arise when bacteriophages enter E. coli cells, where they proliferate and lyse the cells. (Photograph: Svetlana Ber, Goettingen, Germany.)

Figure 2. Model of the bacteriophage T ₄. Left, the “attack” position. Constituents are: the hexagonal head made of protein and filled with DNA, the collar with six whiskers, the contractile sheath, the baseplate and six wedges. Right, the phage during infection. Contraction of the sheath allows injection of DNA through the internal tube. The size of the phage is about 200 nm. (Drawing: Anne Kemmling, Goettingen, Germany, after F.A. Eiserling. “Structure of the T ₄ Vision.” In: Ch.K. Mathews, E.M. Kutter, G. Mosig, P.B. Berget, Bacteriophage T ₄, ASM, Washington, D.C., 1983.)

Figure 3. Integration of phage DNA into the E. coli chromosome. The two circles, the E. coli DNA (large) and the phage DNA (small) associate in a region of homology, between the chromosomal regions gal and bio. Then both rings open up and interfuse to form a figure eight. Unfolding of this structure gives a larger circular DNA molecule carrying the integrated DNA molecule. (Drawing: Anne Kemmling, Goettingen, Germany.)