Virus Notes Basic Definition Viruses parasitic acellular entity

Virus Notes

Basic Definition • Viruses: parasitic, acellular entity Submicroscopic parasitic Viruses Submicroscopic, composed of a nucleic acid core surrounded by a protein coat. – Below the resolution of a microscope – Relies on a host – Does not have the properties of cellular life – Viruses are measured in nanometers • 0. 00001 m or 10 -9

Comparing the size of a virus, a bacterium, and an animal cell Virus Bacterium Animal cell nucleus 0. 25 m

Capsomere of capsid Viral Shapes and structure RNA Capsomere DNA Membranous envelope Head Capsid Tail sheath RNA • Although viruses can have several shapes, all DNA have at least two parts: Tail fiber – An outer capsid made of proteins. Glycoprotein – Genetic material (DNA or 80– 200 RNAnm –(diameter) never both)80 225 nm 18 250 mm 70– 90 nm (diameter) 20 nm (a) Tobacco mosaic virus 50 nm (b) Adenoviruses 50 nm (c) Influenza viruses 50 nm (d) Bacteriophage T 4

What is a bacteriophage? T 4 bacteriophages infecting an E. coli cell Protein coat • A virus that invades bacteria. It consists of a DNA core and a protein coat DNA

Why aren’t viruses considered alive? • Viruses are acellular (not cells) • Viruses have no organelles to take in nutrients or use energy. • Viruses cannot make proteins. • Viruses cannot move. • Viruses cannot replicate on their own.

Virus Reproduction Viruses reproduce by infecting other cells. Two types of viral infections: 1. Lytic Infection 2. Lysogenic Infection

What are the steps of a lytic infection? Step 5: New viruses “lyse” the host Step 3: Replication of viral cell and are released for further DNA and Synthesis of to host ce Step 1: Attachment of virus infection Protein Capsule using cellular “machinery” – cellular enzymes, ribosomes, etc. Step 4: Assembly of new viruses Step 2: Injection of viral DNA into cell inside host cell

Characteristics of Lytic Infections 1. Fast acting 2. Symptoms emerge within 24 – 48 hours 3. Examples – influenza, west-nile

The Lysogenic Infection Step 1: Virus attaches and inserts its DNA inside host Step 2: Viral DNA attaches to the host DNA (prophage DNA) Step 3: The viral DNA lies “dormant” and the viral DNA replicates each time the cell divides Step 4: Stress or other “factors” causes the infection to progress to the “lytic” phase

Characteristics of Lysogenic Infections 1. Slow Acting - Viral DNA can lie “dormant” for many years, hidden within prophage DNA 2. The host are “symptom-free” during dormancy 3. Infection is fast acting when the infection progresses to the lytic phase 4. Example: HIV

What is a retrovirus? • A retrovirus is a virus with RNA rather than DNA for its genetic material. • These viruses carry an enzyme to create DNA from their RNA. • The viral DNA then integrates into a chromosome. (Just kidding… here’s a real retrovirus!)