Viruses Ch. 18-2

General Characteristics 1/100 the size of bacteria 1/100 the size of bacteria –Only seen with electron microscope Non-living Non-living –Need a host to reproduce Latin for “poison” Latin for “poison” Disease-causing Disease-causing

3 A virus is surrounded by a capsid (protein coat) which determines the shape of the virus. The capsid contains nucleic acids (either DNA or RNA). Tail fibers for attachment to host cell.

Attachment Protein coat shape matches cell membrane receptor Protein coat shape matches cell membrane receptor –Viruses can only attach to certain host cells

Two Modes of Entry 1.) Inject DNA into host cell 2.) Entire virus enters host cell –“Bursts” and releases DNA

6 VIRAL REPLICATION Viruses replicate in one of two ways: Lytic Lytic Lysogenic Lysogenic

1.) Lytic Cycle Virus Attachment & entry Virus Attachment & entry Replication Replication –Viral DNA takes over Assembly Assembly –Copies DNA & Proteins Lysis Lysis –New viruses burst out of host cell

2.) Lysogenic Cycle Attachment and Entry Attachment and Entry Provirus (prophage) Insertion Provirus (prophage) Insertion –Viral DNA put into host cell chromosome Cell Division Cell Division –Virus DNA copied with rest of cell’s –Lies dormant Lytic Cycle (environment triggers it) Lytic Cycle (environment triggers it) –Replication, Assembly, Lysis

Reproduction – Lysogenic Cycle

Viral Diseases: 4-ways of transmission of disease: 4-ways of transmission of disease: –1.) Air-borne (Chicken Pox) –2.) Vectors (West Nile Virus) –3.) Direct contact (Cold Sores) –4.) Food-Borne (Polio Virus) 10

Viral Disease Continued… Symptoms of infections due to lysis Symptoms of infections due to lysis Recurring diseases have lysogenic cycles Recurring diseases have lysogenic cycles –Sit dormant –Ex.) Cold sores

Viruses and Disease Medicines are ineffective because viruses are too simple Medicines are ineffective because viruses are too simple –Treat symptoms only Vaccines help your body prepare for virus attack Vaccines help your body prepare for virus attack –Knows what the enemy looks like