18.2 Viral Structure and Reproduction KEY CONCEPT Viruses exist in a variety of shapes and sizes. 18.2 Viral Structure and Reproduction Viruses differ in shape and in ways of entering host cells. • Viruses have a simple structure. – genetic material – capsid, a protein shell – maybe a lipid envelope, a protective outer coat enveloped (influenza) capsid nucleic acid lipid envelope helical (rabies) Surface proteins capsid nucleic acid surface proteins lipid envelope polyhedral (foot-and-mouth disease) surface proteins capsid nucleic acid 18.2 Viral Structure and Reproduction • Bacteriophages infect bacteria. capsid DNA tail sheath tail fiber 18.2 Viral Structure and Reproduction • Viruses enter cells in various ways. – bacteriophages pierce host cells colored SEM; magnifications: large photo 25,000; inset 38,000x 18.2 Viral Structure and Reproduction • Viruses enter cells in various ways. – viruses of eukaryotes enter by endocytosis 18.2 Viral Structure and Reproduction • Viruses enter cells in various ways. – viruses of eukaryotes also fuse with membrane 18.2 Viral Structure and Reproduction Viruses cause two types of infections. • A lytic infection causes the host cell to burst. host bacterium Event 1 Event 4 The host bacterium breaks apart, or lyses. Bacteriophages are able to infect new host cells. The bacterophage attaches and injects it DNA into a host bacterium. Event 2 The viral DNA forms a circle. Event 3 The viral DNA directs the host cell to produce new viral parts. The parts assemble into new bacteriophages. The virus may enter the lysogenic cycle, in which the host cell is not destroyed. 18.2 Viral Structure and Reproduction • A lysogenic infection does no immediate harm. – Event 1 was injection - shown on previous slide. The prophage may leave the host’s DNA and enter the lytic cycle. Event 2 The viral DNA is called a prophage when it combines with the host cell’s DNA. Triggered by environmental factors such as stress. Event 3 Event 4 Many cell divisions produce a colony of bacteria infected with prophage. Although the prophage is not active, it replicates along with the host cell’s DNA. 18.2 Viral Structure and Reproduction Viruses - The Bad House Guests • Using the analogy of viral infections resembling houseguests described on page 515, explain which describes a lytic infection and which describes a lysogenic infection. – Lytic infection is like the bad house guest that eats all the food and blows up the house on the way out because a lytic virus destroys the host cell in the end. – A lysogenic infection is like the houseguest that never leaves. – Neither is good for the host because in the end they can both be harmful. 18.2 Viral Structure and Reproduction Interpreting Models – Lytic and Lysogenic cycle D A C F H B E G 18.2 Viral Structure and Reproduction Two possible paths for a lysogenic viral infection. • Viruses in the lysogenic phase are sometimes called dormant or hidden because while they are part of the host cell’s DNA, they are not active or infectious. What are the two possible paths this type of virus can take after the viral DNA becomes incorporated? – They can stay “hidden” or dormant and remain incorporated in the host cell’s DNA. It will not affect the host but will be copied every time the cell divides. – A “trigger” such as stress can cause the virus to become activated and enter the lytic cycle. When this happens it can now destroy the host cell. – Shingles is an example of this type of virus.