I. General characteristics of viruses:

1. What are viruses?

Obligate intracellular parasites

2. Components of a virus:

  • A nucleic acid core that contains RNA or DNA but not both;
  • A protein capsid that is made with capsomeres.
  • Some have envelope that is aquired from host cell membrane. Spikes on the envelope are the recognition apparatus. There are enveloped and nakedviruses.

3. Size and shapes:

  • 20 to 300 nm in diameter
  • Helical – tobacco mosaic virus
  • Spherical – Poxvirus
  • Polythecal–Picornavirus; icosahedral= 20 triangle
  • A complex virus contains more than one basic structure, such as the phage.

4. Viruses vary in host range and viral specificity.

II. Classification:According to size, morphology, and nucleic acid in the core (PLEASE SEE THE TABLE).

  • Family – Genera – Species
  • RNA viruses: + strand, - strand and dsRNA
  • DNA viruses: Mostly dsDNA, and ssDNA

III. Viral life cycle:

1. Five stages (steps) of viral infection: Absorption, penetration, biosynthesis, maturation and release.

2. Lysogeny and lytic cycles of phage: In lysogenic cycle, phage DNA incorporated into the host genome, become prophage. It can exit the lysogenic state and re-enter the lytic cycle.

3. The growth curve of a phage includes an eclipes period – the time following penetration through biosynthesis, and a latent period – the time after penetration up to release.

4. The number of phages produced in an infection can be determined by counting the number of plaques produced on a bacterial lawn. Each plaque represents a plaque-forming unit (PFU).

5. Life cycle of the animal viruses: On the surface of some viruses, proteins are used for attachment to the host membrane during absorption. Virus thus gains entry into the cell. Uncoating(loss of capsid) can occur at the entry or in the cytoplasm.

For DNA viruses, DNA is replicated, transcribed and translated as the host genes. ssDNAdsDNA  mRNA  protein

For RNA viruses, RNA can be copied into RNA, DNA or translated. ssRNAds RNAssDNAdsDNA  mRNA  protein

Some viral DNA (from DNA viruses or from retroviruses) is incorporated into the host genome becoming a provirus. Provirus or prophage can change the host virulence or antibiotic resistance etc, and prevent a second infection.

IV. Culture of animal viruses:

1. In live animals (HIV infects most commonly through needleprick)

2. In egg culture – embryonated eggs

3. In cell cultures, either a primary cell culture(1st generation cell)or a continuous cell lines(can divide in vitro).

V. Pathogenicity

1. Cytopathy such as synthetia

2. Disrupting the host genome: carcinogenesis and teratogenesis

VI. Viruslike agents:

  • Satellites: Pieces of RNAs unable to replicate without an unrelated helper virus or helper phage that encodes the capsid. Most are in plant.
  • Viroids: Small RNA molecule. Most are in plants.
  • Prions: Infectious protein, causing Creutzfeldt-Jacob disease, kuru, scrapie, mad cow disease, and chronic wasting disease.

What are required from the table?

Relate family name, DNA or RNA virus, and disease.

piccolo: Italian, means “very small”;

entero: Greek, means “intestine”;

hepato: Greek, means “liver”;

rhino: Greek, means “nose”;

para: Latin, means “near”;

myxo: Greek, means “mucus”;

rhabdo: Greek, means “rod”;

ortho:Greek, means “straight”;

adeno: Greek, means “gland”;

herpes: Greek, means “creeping”.