Viruses

Properties of viruses

  • no membranes, cytoplasm, ribosomes, or other cellular components
  • they cannot move or grow
  • they can only reproduce inside a host cell
  • they consist of 2 major parts - a protein coat(capsid), and hereditary material (DNA or RNA)
  • they are extremely tiny, much smaller than a cell and only visible with advanced electron microscopes

 

Review the structure of DNA

Shape of a double helix
Base pairs held together by hydrogen bonds (weak)
Adenine <-> Thymine
Guanine <-> Cytosine

RNA - single stranded, contains no thymine

 

Parasitic Nature

  • Obligate intracellular parasites
  • Specific to their hosts (human, dog, some can cross species)
  • They can only attack specific cells , the common cold is a virus that specifically attacks cells of the respiratory track (hence the coughing and sneezing and sniffling). HIV virus specifically attacks white blood cells

Viral Reproduction

Lytic cycle = reproduction occurs, cells burst
Lysogenic cycle = reproduction does not immediately occur (dormancy)

Virulent = viruses that undergo both cycles

 

Viruses multiply, or replicate using their own genetic material and the host cell's machinery to create more viruses. Viruses cannot reproduce on their own, and must infect a host cell in order to create more viruses.

1. Attachment

2. Penetration & Integration- the virus is engulfed by the cell

3. Biosynthesis - viral components are made (protein coat, capsid, DNA/RNA)

4. Maturation - assembly of viral components

5. Release - viruses leave host cell to infect new cells (often destroys host)

The following image outlines a typical cycle of a bacteriophage called Lambda: (Bacteriophages are viruses that infect bacteria)

 

Retroviruses -- RNA viruses that have a DNA stage

Human Immunodefiency Virus - causes AIDS

  • Retrovirus (RNA inside a protein coat)
  • Reverse Transcriptase makes DNA from the virus RNA
  • DNA inserts into host DNA
  • Proteins are assembled from the DNA code
  • Viruses assembled from the proteins
  • Viruses released from the cell
  • Animated Retrovirus

Emerging Viruses

  • illnesses not previously known
  • AIDS, West Nile Virus, SARS, Ebola, Bird Flu
  • Could be mutations of known viruses
  • Could be viruses exposed when knew areas were developed
  • Could have jumped species

Related to Viruses

Viroids - even smaller than viruses, consist of RNA strands that lack a protein coat
Prions - "rogue protein", believed to be the cause of Mad Cow Disease, also may causes Kuru in cannibal tribes

 

Viral Images -- Electron Microscopes