Common plasmids are simple DNA molecules which contain a few genes and regulatory elements. Most viral genomes are more complex. For example, the genome of phage lambda contains approximately 50 genes. About 4,000 genes are present in the E. coli genome while there is approximately 1,000 times more DNA in the genome of a mammal. This progression in genome complexity is the topic of this exercise. Here, students compare the electrophoretic patterns of restriction digests of a plasmid, phage lambda DNA, and cow DNA from thymus and kidney as shown in the figure below. The exercise serves as a good introduction for determining the size of DNA molecules and provides an appreciation for the complexity of genomes from different organisms.