Viruses

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

The Discovery of Viruses

1892Dmitri Ivanovski1892Dmitri Ivanovski

1897Martinus Beijerinck1897Martinus Beijerinck

1935Wendell Stanley1935Wendell 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

Lysogenic infection

Lytic infectionLytic

infection

Lytic Infections

The virus injects DNA into a bacterium.

The virus injects DNA into a bacterium.

Viral genes are transcribed by the host cell.

Viral genes are transcribed by the host cell.

The bacterium makes new viral proteins and nucleic acid.

The bacterium makes new viral proteins and nucleic acid.

The proteins and nucleic acids assemble into new viruses.

The proteins and nucleic acids assemble into new viruses.

Viral enzymes lyse the bacterium’s cell wall. The new viruses escape.

Viral enzymes lyse the bacterium’s cell wall. The new viruses escape.

Lytic Infections AnalogyA 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.

The viral DNA inserts itself into the bacterial chromosome.

Prophage

The prophage may replicate with the bacterium for many generations.

The prophage may replicate with the bacterium for many generations.

The prophage can exit the bacterial chromosome and enter a lytic cycle.

The prophage can exit the bacterial chromosome and enter a lytic cycle.

The virus injects DNA into the bacterium.

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: HIVHIV 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