Viruses

Learning Objectives  Explain how viruses reproduce.  Explain what happens after a virus infects a cell.

The Discovery of Viruses 1892 Dmitri Ivanovski 1892 Dmitri Ivanovski 1897 Martinus Beijerinck 1897 Martinus Beijerinck 1935 Wendell Stanley 1935 Wendell Stanley

Virus Reproduction A virus is nonliving. Viruses can reproduce only by infecting living cells.

Virus Structure and Composition Capsid: protein coat surrounding a virus

Viral Infections Viruses use their genetic information to reproduce inside living cells. Lysogenic infection Lytic infection

Lytic Infections The virus injects DNA into a bacterium. Viral genes are transcribed by the host cell. The bacterium makes new viral proteins and nucleic acid. The proteins and nucleic acids assemble into new viruses. Viral enzymes lyse the bacterium’s cell wall. The new viruses escape.

Lytic Infections Analogy A lytic virus is similar to the Wild West of the American frontier. The host cell’s DNA is chopped up. Virus uses host cell to make viral DNA and viral proteins. The host cell bursts, releasing hundreds of virus particles.

Lysogenic Infections The viral DNA inserts itself into the bacterial chromosome. Prophage The prophage may replicate with the bacterium for many generations. The prophage can exit the bacterial chromosome and enter a lytic cycle. The virus injects DNA into the bacterium.

An RNA Virus: The Common Cold Once the cold virus has penetrated the host’s cells, it uses the host’s cellular machinery to replicate itself. Cytoplasm The virus makes many copies of its RNA. The copies are translated by the host into new viral parts. The parts assemble into new viruses and burst from the host cell.

An RNA Virus: HIV HIV makes a DNA copy of itself that inserts into the host’s DNA. There, it may remain inactive for many cell cycles. Cytoplasm A DNA copy of the viral RNA is made. The copy is inserted into the host’s genome. It is later transcribed and translated into new viral parts. The parts assemble into new viruses and burst from the host cell. DNA

Viruses and Cells

Summary of Viruses Viruses reproduce by infecting living cells. Some viruses replicate immediately; others initially persist in an inactive state within the host. Lysogenic infection Lytic infection

Student Worksheet Answers