Chapter 18: Genetics of Viruses and Bacteria Concept 18.2 is not required knowledge for the AP Exam and has been intentionally omitted. Overview: Microbial Model Systems viruses called bacteriophages can infect and set in motion a genetic takeover of bacteria E. coli and its viruses are called model systems because of their frequent use by researches in studies that reveal broad biological principles beyond their value as model systems, viruses and bacteria have unique genetic mechanisms that are interesting in their own right bacteria are prokaryotes with cells much smaller and more simply organized than those of eukaryotes viruses are smaller and simpler still Concept 18.1: A virus has a genome but can reproduce only within a host cell