VIRUSES CH 20

made of nucleic acid & a protein coat. not living. Viruses are ... biological particles. made of nucleic acid & a protein coat. not living.

Viruses have... No growth No homeostasis No metabolism

virus means “poison” They do mutate! They have DNA or RNA Reproduction occurs only within a HOST.

Viruses have structure They come in many shapes.

Viruses are grouped into families based on their nucleic acid type, capsid structure, presence or absence of an “envelope”.

Viruses cause diseases smallpox hepatitis B AIDS shingles, chicken pox Cancer Ebola

VIRUS REPRODUCTION Lytic Cycle Lysogenic Cycle

Lytic Cycle Absorption (Attachment): Virus attaches itself to the cell.

Entry: Enzymes weaken cell wall; nucleic acid is injected into cell, leaving the empty capsid outside the cell. Many viruses actually enter the host cell intact.

Replication: Viral DNA takes control of cell activity.

Assembly: All metabolic activity of the cell is directed to assemble new viruses.

Release: Enzymes disintegrate the cell in a process called lysis, releasing the new viruses.

Lysogenic Cycle The virus attaches itself and injects its DNA into the cell The viral DNA incorporates itself to the host DNA, becoming a new set of cell genes called a prophage.

When the host cell divides, this new gene is replicated and passed to new cells. This causes no harm to the cell, but may alter its traits.

Now there are two possibilities: The prophage survives as a permanent part of the DNA of the host organism

Some external stimuli can cause the prophage to become active, using the cell to produce new viruses