Structure of Viruses All viruses are composed of: – Nucleic acids: Virus’s genetic material DNA or RNA Code solely for reproduction – Capsid: Protein coat.

Structure of Viruses All viruses are composed of: – Nucleic acids: Virus’s genetic material DNA or RNA Code solely for reproduction – Capsid: Protein coat 1 or 2 layers thick Determines shape Determines infection process Houses nucleic acids

Structure of Viruses Some viruses: – Viral envelope: Outside of protein coats Made of phospholipids (like a membrane) Flu and HIV

Viral Shapes Determined by capsid (protein coat)

Viral Shapes Polyhedral: polio

Viral Shapes Cylindrical: tobacco mosaic

Viral Shapes Studded capsid: HIV, influenza

Viral Shapes Polyhedral head with protein tails: bacteriophage

Viral Attachment A cell’s membrane won’t let just any ol’ particle in

Viral Attachment Amazing forms viruses take mimic molecules a host cell wants and needs to take in

Viral Attachment Amazing forms viruses take mimic molecules a host cell wants and needs to take in Viruses “trick” the host cell into letting the virus in

Viral Attachment Specificity of membrane receptors forces viruses to be specific shapes Usually a virus can infect only a single species – Sometimes single cell type within single species

Viral Reproduction After attaching virus incorporates its nucleic acid into the host cell (method depends on viral shape) – Inject just the nucleic acid – Whole virus barges in inside a vacuole which then bursts

Viral Reproduction Lytic cycle Lysogenic cycle Retroviruses

Lytic Cycle 1. Virus attaches to host 2. Inserts nucleic acid 3. Destroys host’s DNA 4. Reprograms metabolism Make viral protein coats Copy viral genes 5. New viruses constructed 6. Host cell bursts (lyses), killing host cell

Step 1: virus attaches Step 2: virus inserts nucleic acid Step 4: virus takes over cell’s metabolism and viral parts are constructed Step 5: viral parts are assembled into new viruses Step 6: host cell bursts, releasing newly-constructed viruses into the environment to infect other host cells Step 3: host cell’s DNA is destroyed

Lytic Cycle Viruses that cause host cell to burst are referred to as virulent viruses

Lysogenic Cycle 1. Virus attaches to host 2. Inserts nucleic acid 3. Viral DNA incorporates itself into host’s DNA Provirus No reprogramming of metabolism 4. Provirus reproduced when host cell reproduces 5. Lysogenic phase continues 6. Provirus emerges and enters lytic cycle

Step 1: virus attaches Step 2: virus inserts nucleic acid Step 3: viral DNA is incorporated into host cell’s forming provirus Step 4: Provirus is replicated when host cell replicates Step 5: lysogenic phase continues

Step 1: virus attaches Step 2: virus inserts nucleic acid Step 3: viral DNA is incorporated into host cell’s forming provirus Step 4: Provirus is replicated when host cell replicates Step 5: lysogenic phase continues Step 6: provirus pops out of DNA and enters lytic cycle

Step 1: virus attaches Step 2: virus inserts nucleic acid Step 3: viral DNA is incorporated into host cell’s forming provirus Step 4: Provirus is replicated when host cell replicates Step 5: lysogenic phase continues LYTIC CYCLE Step 6: provirus pops out of DNA and enters lytic cycle

Lysogenic Cycle Viruses that do not burst host cell are referred to as temperate viruses

Lysogenic Cycle Classic lysogenic virus: Herpes Simplex I – Causes cold sores – Virus is always present in cells – Symptom-free during lysogenic cycle – Cold sores appear during lytic cycle

Lysogenic Cycle Viruses that bud: influenza – Release from host cell without bursting – Form of exocytosis

Only contain RNA (not DNA) RNA is made from DNA but not the other way around How can a virus that contains RNA take over the host cell’s DNA?

Retroviruses Retroviruses also inject reverse transcriptase – Enzyme – Makes double-stranded DNA copy of viral RNA to incorporate into host cell’s DNA

Retroviruses Retroviruses also inject reverse transcriptase – Enzyme – Makes double-stranded DNA copy of viral RNA to incorporate into host cell’s DNA Lysogenic cycle then proceeds normally

Retroviruses Viruses that are quietly active: HIV – HIV is a retrovirus – HIV enters then produces only small numbers of viruses within host cell (lysogenic-type of cycle) – Viruses are released by budding (without bursting the cell) – Eventually it will enter a lytic cycle causing the disease: AIDS

Viral Diseases Viruses cause illness by attacking and destroying cells Illnesses caused by viruses include: – Cold, flu, smallpox, AIDS, chickenpox, measles, Hepatitis, West Nile

Control of viruses Vaccination: individuals are infected with active, but relatively harmless strain of virus – Makes some people sick – Smallpox virus, flu nothing Antibiotics do nothing for a person infected with a virus!