LESSON 4

• VIRUSES


  are microscopic organisms that can infect hosts, like humans, plants or animals.
• CHARACTERISTICS OF VIRUSES

  •Viruses are 100 to 1,000 times smaller than the cells in your body.

  •Viruses are made up of genetic material (RNA or DNA) and a protective protein coating (caspid).

  •Similar to parasites - they need a host to reproduce.
• GENERAL STRUCTURE OF A VIRUS

  The genetic material of the virus is surrounded by a protein capsid which protects it from the environment and the host immune system. The capsid (envelope) has "spikes", which represents attachment (envelope) proteins, sticking out of it. These attachment proteins help the virus attach to and infect a host cell.
• helical


  These look like long rods. They can be rigid or flexible. The helical virus shown above is a tomato mosaic virus. It causes the leaves and fruit of tomato plants to appear blotchy.
• polyhedral


  These are many-sided viruses.  Their capsids can have different number of sides. Most have 20 triangular sides and 12 ventricles, or corners. The polyhedral virus shown is the adenovirus. It causes respiratory illnesses.
• enveloped


  These are shape like spheres and have a protein, fat or carbohydrate coat over their capsid. The image above shows an influenza virus. It causes respiratory illnesses like coughs and sore throats
• complex


  These have complicated structures. Their capsids can be attached to structures that look like legs. These are called tail fibers. The virus shown is a bacteriophage. It infects bacteria.
• LYTIC CYCLE


  The DNA of the virus doesn't integrate into the host DNA

  DNA replication of virus takes place independently from the host DNA replication.

  Occurs within a short period of time

  The cellular mechanism of the host cell is totally undertaken by the viral genome
• LYSOGENIC CYCLE

  The DNA of the virus integrates into the host DNA

  DNA replication of the virus takes place along with the host DNA replication

  Takes time

  The cellular mechanism of the host cell is somewhat disturbed by the viral genome
• BACTERIOPHAGE


  A type of virus that infect bacteria and archaea. It multiply through destroying their host cells by making use of some or all of the host biosynthetic machinery.
• Ernest Hanbury Hankin


  first to observe the bactericidal activity of bacteriophage .
• William Twort


  discovered bacteriophage in 1915.
• D’Hérelle


  coined the term ‘bacteriophage’ in 1917.
• Bacteriophages are classified based on their nucleic acid content and morphological characteristics.
• STRUCTURE OF BACTERIOPHAGE

  1.Capsid Head - It encapsulates or protects the viral genome.

  2. Tail - serves as a passageway for the genetic material to pass from the capsid to the host bacteria.

  3. Tail fibers - The bacteriophage attaches to the bacteria with the help of these tail fibers.
• Proteobacteria


  Discovered by Carl Woese in the1980s and it is the largest phylumwithin the bacteria domain.
• Non-Proteobacteria


  These bacteria are photoautotrophs and chemoheterotrophs. They can either be classified as gram-positive or negative.
• Archaea


  are unicellular prokaryotes that comprise the domain of the same name. These microorganisms physically resemble the bacteria but

  are genetically distinct from the latter.