12.1 Introduction Type of Unit Genome structure Mode of Propagation Cneq山 ences Lytic phage ds- or ss-DN A or RNA Infects susceptible host Usually kills host li near or circular Lysogenic phage ds-DNA Linear sequence in Im munity to infection host chromo some Plasmid ds-DNA cirde Replicate s at defined Im munity to plasmids May be transmissible In same group Episome ds-DNA circle Free circle or linear integrated May transfer host DNA Figure 11.2 Several types of independent genetic units exist in bacteria 消当Figure 11.2 Several types of independent genetic units exist in bacteria. 12.1 Introduction