Review What do viruses depend on for their reproduction Review Describe each of the two paths viruses may follow once they have entered a cell Compare and Contrast Compare the structure of a virus to the structure of both a prokaryotic cell and a eukaryotic cell. Use a graphic organizer of your choice to organize the information

Ch 20 Viruses and Prokaryotes

Viruses Nonliving particle made of proteins, nucleic acids, and sometimes lipids Can reproduce only by infecting living cells Wide variety of size and structure Very small Need electron microscope to see.

Contain a few to a few hundred genes. Capsid Protein coat surrounding a virus Proteins on their surface membrane that trick receptor cells on host cell into accepting its genetic material Contain a few to a few hundred genes.

Most viruses infect only a very specific kind of cell Bacteriophages Viruses that infect bacteria Plant viruses infect plant cells Most animal viruses infect only certain related species of animals.

Lytic Infections Virus enters a bacterial cell Makes copies of itself Causes the cell to burst, or lyse Bacteriophage T4.

The virus injects its DNA into the cell.

Cell then begins to make messenger RNA (mRNA) from the viral genes.

Viral mRNA is translated into viral proteins that chop up the cell’s DNA.

Host cell’s metabolic system makes copies of viral nucleic acid and capsid proteins.

Viral nucleic acid and capsid proteins are then assembled into new virus particles.

Host cell lyses, releasing hundreds of virus particles that go on to infect other cells.

Lytic virus is similar to an outlaw in the Wild West Outlaw eliminates the town’s existing authority.

Outlaw demands to be outfitted with new equipment from the local townspeople.

Outlaw forms a gang that leaves the town to attack new communities.

Lysogenic Infection Host cell is not immediately taken over.

Viral nucleic acid is inserted into the host cell’s DNA Prohage Bacteriophage DNA that becomes embedded in the bacterial host’s DNA.

Viral DNA is then copied along with the host DNA without damaging the host.

Viral DNA multiplies as the host cells multiply Each generation of daughter cells derived from the original host cell is infected.

Prophage may remain part of the DNA of the host cell for many generations.

Influences from the environment (radiation, heat, etc) trigger the prophage to become active.

Removes itself from the host cell DNA, directs the synthesis of new virus particles, and now becomes an active lytic infection.

Cold viruses attack with a very simple, fast-acting infection Capsid settles on a cell, typically in nose, and is brought inside, where a viral protein makes many new copies of the viral RNA.

Host cell’s ribosomes mistake the viral RNA for it’s own and translates it into capsids and other viral proteins.

New capsids assemble around the viral RNA copies Host cell releases hundreds of new virus particles to infect other cells within 8 hours.

HIV Human immunodeficiency virus (HIV) Acquired immune deficiency syndrome (AIDS) Retrovirus Genetic information is copied from RNA to DNA.

Enter cell and makes a DNA copy of its RNA.

Copy inserts itself into the DNA of the host cell.

HIV is similar to a lysogenic infection Viral DNA may remain inactive for many cell cycles before making new virus particles and damaging the cells of the host’s immune system.

Viruses must infect living cells in order to grow and reproduce, taking advantage of the nutrients and cellular machinery of their hosts After infecting living cells, viruses can reproduce, regulate gene expression, and even evolve.