Virus Notes

Basic Definition• Viruses: Submicroscopic, parasitic, acellular entity

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• 0.000000001m or 10-9

Submicroscopic parasitic acellular

Comparing the size of a virus, a bacterium, and an animal cell

0.25 m

Virus

Animalcell

Bacterium

Animal cell nucleus

Viral Shapes and structure

• Although viruses can have several shapes, all have at least two parts:– An outer capsid made of proteins. – Genetic material (DNA or RNA – never both)18 250 mm 70–90 nm (diameter) 80–200 nm (diameter) 80 225 nm

20 nm 50 nm 50 nm 50 nm

(a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4

RNA

RNACapsomereof capsid

DNACapsomere

Glycoprotein Glycoprotein

Membranousenvelope

Capsid DNA

Head

Tail fiber

Tail sheath

What is a bacteriophage?

• A virus that invades bacteria. It consists of a DNA core and a protein coat DNA

Protein coatT4 bacteriophages infecting an E. coli cell

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 Infection2. Lysogenic Infection

What are the steps of a lytic infection?

Step 1: Attachment of virus to host cell

Step 2: Injection of viral DNA into cell

Step 3: Replication of viral DNA and Synthesis of Protein Capsule using cellular “machinery” – cellular enzymes, ribosomes, etc.

Step 4: Assembly of new viruses inside host cell

Step 5: New viruses “lyse” the host cell and are released for further infection

Characteristics of Lytic Infections

1. Fast acting

2. Symptoms emerge within 24 – 48 hours

3. Examples – influenza, west-nile

The Lysogenic InfectionStep 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 called reverse transcriptase.• The viral DNA then integrates into a

chromosome.

(Just kidding… here’s a real retrovirus!)