18.2 Viral Structure and Reproduction Introduction to Viruses EHS Biology – Chapter 20.1 HIV Virus infected White Blood Cell

Are Viruses Alive? List as many characteristics of living things as possible. Collaborate with a neighbor to decide if you think viruses are living!

No, viruses are not considered living. They are not made of cells! Cannot reproduce on their own - Use the host cell to make new viruses.

Pathogen - disease causing agent Examples: Viruses, Bacteria, Prion, Viroid, Fungus

Viral Structure Very simple structure. Genetic material – RNA or DNA Capsid (Protein Shell) Phospholipid protective outer coat (Most viruses) capsid nucleic acid lipid envelope surface proteins capsid nucleic acid lipid envelope Surface proteins capsid surface proteins nucleic acid helical (rabies) polyhedral (foot-and-mouth disease) enveloped (influenza)

Bacteriophage: Viruses that infect bacteria. B. Capsid A. DNA C. Tail D. tail fiber

Entering the Cell colored SEM; magnifications: large photo 25,000; inset 38,000x 1.The virus pierces host cells membrane and inserts its genetic material.

2.Viruses are brought into the cell through endocytosis. Entering the Cell

3.Viruses of eukaryotes also fuse with membrane Entering the Cell

host bacterium The bacteriophage attaches and injects it DNA into a host bacterium. The host bacterium breaks apart, or lyses. Bacteriophages are able to infect new host cells. The viral DNA directs the host cell to produce new viral parts. The parts assemble into new bacteriophages. The viral DNA forms a circle. Types of Viral Infection Pattern Lytic infection - causes the host cell to burst. The virus may enter the lysogenic cycle, in which the host cell is not destroyed

Lysogenic infection – Doesn’t immediately harm the host. The viral genetic material combines with the host DNA The viral genetic material is replicated along with the host cell’s DNA. Many cell divisions produce a colony of bacteria infected with the viral genetic material. The viral genetic material may leave the host’s DNA and enter the lytic cycle Types of Viral Infection Pattern

Lytic and Lysogenic Cycle

Viroid - Simple Infectious Agents Short strands of circular, single-stranded RNA. Generally infect plants. 1 st identified = Potato spindle tuber viroid

Prions - Infectious Proteins Slow-acting, virtually indestructible infectious misfolded proteins Can induce normal proteins to fold improperly. Cause nervous tissue death by colleting in neurons. Examples: Scrapie in sheep Mad cow disease Creutzfeldt-Jakob disease in humans