Viruses Chapter 10.17

What you need to know! The components of a virus. The differences between lytic and lysogenic cycles.

What’s a Virus? Not a living cell but an infectious particle Obligate intracellular parasite Contains: nucleic acids protein coat

Viruses are Tiny

Protein Coats-Capsids Capsids are made from proteins called capsomeres Capsids have many different shapes depending on the virus: Rod shape, Helical, Polyhedral, Icosahedral Some viruses will have membrane left from the host cell

Tabacco Mosaic Virus Helical capsid with RNA

Adenovirus Respiratory virus in animal Polyhedral capsid with glycoprotein spikes

Influenza virus Membrane envelope from host studded with glycoproteins

Bacteriophages Viruses that infect bacteria icosahedral shape of a phage resembles a lunar landing probe

Viral Genome DNA double helix Single stranded DNA Double stranded RNA Single stranded RNA

Viral Reproduction Viruses are obligate intracellular parasites Isolated viruses cannot reproduce They lack the ribosomes and enzymes for making proteins Viruses can only infect limited range of host

Lytic Cycle Virus infects host cell Cell constructs virus Cell dies and releases the virus

Lysogenic Cycle Virus infects host cell Virus nucleic acid hides inside host DNA A stimulus triggers the virus into the Lytic Cycle

RNA as genetic material Either used directly as mRNA Or retroviruses deliver an enzyme called “Reverse Transcriptase” that converts RNA into DNA HIV Due to their single strand, RNA viruses mutate a lot

HIV – enveloped RNA Virus Attaches only to T-cells (WBC) Translates RNA into DNA, integration into cellular DNA called provirus Provirus DNA is transcribed to make new HIV viruses that leave cell Provirus never leaves cell

Immune Response and Vaccinations Body produces antibodies that fit specific virus structure Vaccination are treated versions of the virus that don’t infect and allow the immune system to develop antibodies Example: measles , small pox, Hep B, polio