The Viruses

on 15.9.05 with 0 comments

Definition of a Virus
  • Viruses are segments of nucleic acid enclosed in a protein coat.

Properties of Viruses

  • Small size: range ~0.02 - 0.3 micrometers

Various morphologies

  • polyhedral
  • helical
  • spherical
  • filamentous
  • complex

Obligate intracellular parasites

  • Lack membranes and a means to generate energy

  • Lack metabolism

  • Lack ribosomes and a mechanism for their own protein synthesis

  • Do not grow in size

  • Viruses can only reproduce inside of a host cell.

Structure of Viruses

  • The viral genome is DNA or RNA.

  • Most bacterial viruses contain double-stranded DNA.

  • HIV is a retrovirus (an RNA virus)

Classification of Viruses

  • Type of nucleic acid

  • Manner of nucleic acid replication

  • Size and morphology

  • Additional structures such as envelopes and tails

Viruses Pathogenic to Humans

  • Poxvirus group—relatively large, DNA viruses. Mostly affect skin. Ex. Small pox, cowpox

  • Herpesvirus group—medium-sized, 20 sided DNA virus. Can cause lifetime infection. Ex. Cold sores, shingles, chicken pox

  • Adenovirus group—medium-sized, spiked DNA virus. Ex. conjunctivitis

  • Papovavirus group—small with circular DNA. Ex. Human Papilloma Virus (HPV) and Warts

  • Myxovirus group—medium-sized helical RNA viruses. Ex. Influenza

  • Paramyxovirus group—similar to above, but larger. Ex. measles, mumps

  • Rhabdovirus group—RNA virus. Ex. rabies

  • Arbovirus group—carried by arthropods. Ex. Yellow fever

  • Picornavirus group—small RNA viruses divided into several groups: enteroviruses (ex. Polio), rhinoviruses (ex. Common cold). Also includes hepatitis.

  • HIV—Human immunodeficiency virus, often latent retrovirus

Two types of infections:

1. Lytic infection: phage replicates its DNA and lyses the host cell

2. Lysogenic infection: phage DNA is maintained by the host cell, which is only rarely lysed

Lytic infections

  • The bacteriophage always lyses its host cell.

Life Cycle of a Lytic Phage

Step 1 Adsorption: virus attaches to the cell wall surface

Step 2 Penetration: entry of the viral DNA

Step 3 UNCOATING--bare nucleic acid enters cell and protein coat is left behind

Step 4 Replication of nucleic acid-- bacteria is taken over and begins to reproduce viral DNA.

Step 5 Maturation---The viral nucleic acid becomes enclosed in the virus coat (capsid)

Step 6 Release—mature viruses are released from the cell.

Lysogenic Infections

  • Lysogenic phages rarely lyse their host cell
  • Lysogenic phages are also called temperate phages

Life Cycle of a Lysogenic Phage


Lysogenic infection begins like a lytic infection with adsorption of the virus and penetration of the viral DNA

Cells divide normally and appear normal

Each time the bacteria divides, it reproduces the prophage chromosome

Occasionally (1 in 10,000 cell divisions) the prophage detaches from the host bacterial chromosome and enters into a lytic cycle which produces mature infectious bacteriophage.

Category: Microbiology Notes



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