Viruses

Tiny 1/2 to 1/100 the size of smallest bacterium Nonliving Do not fulfill the criteria for life Do not carry out respiration, grow or move Can only reproduce inside a host cell Tiny 1/2 to 1/100 the size of smallest bacterium Nonliving Do not fulfill the criteria for life Do not carry out respiration, grow or move Can only reproduce inside a host cell Virus means poison in Latin

Viral Structure Have a nucleic acid (DNA or RNA) Surrounded by a protein coat called a capsid Shape of viral coat matches the shape on its specific host cell

Shape important to infection

Lytic Cycle

Lytic Cycle Cont. Virus attaches to a host cell Injects its nucleic acid into host Host DNA is destroyed Viral genes are copied Viral particles are assembled Host breaks open, viruses are released Virus attaches to a host cell Injects its nucleic acid into host Host DNA is destroyed Viral genes are copied Viral particles are assembled Host breaks open, viruses are released

Lytic Cycle

Lysogenic Cycle

Lysogenic Cycle Cont. Attaches to host, injects DNA, now becomes a prophage Normal activities of the cell continue Prophage replicates with host cell Emerges out of host DNA to begin lytic cycle Or, can remain in this cycle and continue to replicate with host cell Attaches to host, injects DNA, now becomes a prophage Normal activities of the cell continue Prophage replicates with host cell Emerges out of host DNA to begin lytic cycle Or, can remain in this cycle and continue to replicate with host cell

Lysogenic Cycle

Prophage Viral DNA inserted into the host cell chromosome Does not interfere with normal functioning of the cell Replicates along with host cell DNA May remain dormant for years Prophage can enter the lytic cycle at any time, and no one knows what causes this. Viral DNA inserted into the host cell chromosome Does not interfere with normal functioning of the cell Replicates along with host cell DNA May remain dormant for years Prophage can enter the lytic cycle at any time, and no one knows what causes this.

Examples of Lysogenic Viruses Herpes Chicken Pox HIV Herpes Chicken Pox HIV

Retroviruses Many viruses, such as HIV, have RNA as their nucleic acid A molecule of reverse transcriptase is injected along with the nucleic acid Reverse transcriptase copies viral RNA into DNA

Viroids and Prions Viroids – single stranded RNA molecules that have no surrounding capsid; causes plant diseases Prions – do not contain a nucleic acid, only a protein; causes diseases in animals Viroids – single stranded RNA molecules that have no surrounding capsid; causes plant diseases Prions – do not contain a nucleic acid, only a protein; causes diseases in animals

Polio

Influenza Virus Virus that causes the flu

Small Pox

TMV Tobbaco Mosaic Virus

Review Question #1 Why is a virus not considered to be alive?

Review Question #2 What is the difference between a lytic and lysogenic cycle?

Review Question #3 Why can most viruses infect only a few kinds of cells?

Review Question #4 Describe the state of a herpes virus in a person who had cold sores several years ago but who does not have any now.