Viruses are the smallest microorganisms that can only replicate inside host cells. They consist of nucleic acid surrounded by a protein coat. Viruses are obligate intracellular parasites and are not considered living organisms as they cannot reproduce, grow, or metabolize on their own. The virus reproduction process involves the virus attaching to and entering a host cell, releasing its nucleic acid, using the host cell to produce more virus nucleic acid and proteins, assembling new virus particles, and bursting out to infect more cells. Common viral diseases include influenza, mumps, chickenpox, smallpox, polio, rabies, German measles, and viral meningitis.

1. MICRO ORGANISMS
VIRUSES
Diversity, Change and Continuity

2. MICROBES/MICRO ORGANISMS
• Some organisms are so small, that it can’t be
seen with the naked eye
• This group includes:
• VIRUSES: Smaller than a cell and does not
form part of the 5 Kingdom system that
classifies living organisms

3. • • The basic characteristics and structure of
viruses
• • Viruses consist of a central core of nucleic
acid either DNA or RNA.
• • They have a capsid or protein coat around
the nucleic acid
• • Viruses have various shapes that consist of
symmetrical faces

4. Viruses do not perform any metabolic reactions
of living organisms (no cellular respiration)
• Viruses cannot reproduce unless they have
a host
• Viruses are parasitic on plants, animals,
protistsa and bacteria
• Most viruses are pathogens and can cause
diseases in their host

6. • They contain either DNA or RNA but never both.
• They are OBLIGATE INTRA-CELLULAR PARASITES –
can only replicate within a living host cell
• Viruses are host or tissue specific
• They are PATHOGENS (disease causing
organisms)
• Viruses REPRODUCE by changing the genetic
material of the host cell to its own

7. WHY ARE VIRUSSES REGARDED AS NON-LIVING?
• They can’t feed
• They can’t respire
• They can’t reproduce, only multiply
• They can’t excrete
• They show no form of locomotion
• They show no sensitivity
• They can’t grow

8. • They have no nucleus
• They have no cytoplasm
• They have no organelles

9. VIRUSES ARE ROD SHAPED OR SPHERICAL
• Some viruses that
attack bavteria are
more complex in
structure:
• Spherical head
• Rod-shaped
• Known as a
BACTERIOPHAGE

10. REPRODUCTION OF VIRUSES
1. The virus approaches a cell
2. Virus attaches to receptor on cell membrane of host
cell
3. Virus engulfed by endocytosis
4. Virus wall disintegrates
5. Virus releases its nucleic acid
6. A new virus nucleic acid is formed from nucleic acid of
host cell
7. New virus with protein shield is formed
8. Host cell bursts open and new viruses are released
9. Host cell destroyed in the process

11. 1. Virus
approaches the
host cell
2. The virus
attaches itself
to the host cell 3. The virus
releases its nucleic
acid
4. Virus uses the host cell’s
nucleic acid to replicate its
own
5. New protein shields
(capsids) are formed around
nucleic acids
6. New viruses are
released as host cells
burst open and is
destroyed

12. Microbes can cause disease
The diseases and their severity caused by microbes depend on the type of microbe.
Some of these are listed below.
VIRUSES
Influenza (flu)
Mumps
Chickenpox
Smallpox s
Polio
Rabies
German measles
Viral
meningitis