Figure 1. The E. coli lac operon. The lac operon contains three genes (lacZYA) whose products are needed for E. coli to utilize lactose as an energy source. These genes are regulated by operator (O) and promoter (P) sequences immediately adjacent to the genes. (a) When no lactose is present in the bacterial cell environment, repressor protein bound at the operator (O) prevents RNA polymerase from binding at the promoter (P). (b) When lactose is present, inducer (a lactose byproduct, represented as •) is produced, which can bind to the repressor. The repressor then no longer binds the operator, and the promoter is accessible to RNA polymerase.

Figure 2. Ligating, transforming and selecting plasmid DNA. (a) Linearized vector is ligated to a fragment of foreign DNA, forming a circular plasmid. When this plasmid is transformed into bacteria, the ampicillin resistance gene is expressed, producing an enzyme that degrades ampicillin. (b) When the bacteria are plated on to media containing ampicillin, cells containing the plasmid (and thus ampicillin-degrading enzyme) grow and form colonies (large, dark circles). Many of the cells do not contain the plasmid and fail to divide (represented as small, faint circles, although they would not be at all visible).

Figure 3. Bacteriophage life cycle. The bacteriophage, a capsule of proteins (called a phage particle) surrounding the phage DNA, anchors on to the surface of the bacterial cell (1) and injects its DNA into the cell (2). Once inside the bacterial cell, the bacteriophage DNA is transcribed into RNA (3), and also replicated to produce many copies of the phage DNA (4). The RNA is translated by the bacterial ribosomes (5), so that numerous phage particles, containing phage DNA, can be assembled (6). The phages then signal lysis of the bacterial cells, releasing a multitude of new phages (7), which can then infect neighbouring bacteria.