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, 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

Virus Structure

 

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

 

Viral Replication (see page 404-405)

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 - the virus is engulfed by the cell (Cell can enter Lysogenic or Lytic Cycle)

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:

Bacteriophage - viruses that infect bacteria.

Animation of a bacteriophage

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

See Animation of HIV infection | HHMI Animation (browse for Viral Infection) | HIV (Learner.org) | Hopkins Institute

Assignments: HIV Coloring

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