Viruses Learning Objectives Explain how viruses reproduce Explain

Viruses

Learning Objectives § Explain how viruses reproduce. § Explain how viruses cause infection.

The Discovery of Viruses 1892 Dmitri Ivanovski 1897 Martinus Beijerinck 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. Viral enzymes lyse the bacterium’s cell wall. The new viruses escape. The proteins and nucleic acids assemble into new viruses. The bacterium makes new viral proteins and nucleic acids.

Lytic Infections Analogy A lytic virus is similar to an outlaw. 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 prophage may replicate with the bacterium for many generations. The viral DNA inserts itself into the bacterial chromosome. Prophage The virus injects DNA into the bacterium. The prophage can exit the bacterial chromosome and enter a lytic cycle.

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. The virus makes many copies of its RNA. Cytoplasm The copies are translated by the host into new viral parts. The parts assemble into new viruses and burst from the host cell.

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

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