1) Viruses are non-living infectious particles that contain genetic material and a protein coat called a capsid. 2) Viruses can only replicate inside a host cell by injecting their genetic material and using the host cell's machinery. 3) Viruses exist in two states - as active viruses when infecting a host cell, or dormant virions when not in contact with a host.

1. CHARACTERISTICS OF VIRUSES

2. A virus is an infectious agent made up of
nucleic acid (DNA or RNA) wrapped in a
protein coat called a capsid.
Viruses have no nucleus, no organelles, no cytoplasm or
cell membrane—Non-cellular
vs
This is why it does NOT belong to any kingdom.

3. Are Viruses Living or Non-
living?
 Biologists consider viruses to be non-living because:
 Are not cells
 Do not grow or respond to their surroundings
 Cannot make food, take in food, or produce wastes
 Viruses do not respond to stimuli.
 They can only multiply if in another living cell

4. Viruses have either DNA or RNA but NOT both.
Viruses with RNA that transcribe
into DNA are called retroviruses.
HIV Infected Cell
A flea is a parasite to a dog
and is harmful to the dog.
Viruses are parasites—an
organism that depends entirely
upon another living organism (a
host) for its existence in such a
way that it harms that organism.

5. What are Viruses?
Definition-
 Viruses are non-cellular particles made up of genetic
material and protein that can invade living cells.

6. Discovery of Viruses
Beijerinck (1897)
coined the Latin name
“virus” meaning poison
Ivanovski and Beijerinck

7. Tobacco Mosaic Virus
Wendell Stanley (1935)
discovered viruses were
made of nucleic acid and
protein

8. Smallpox
Edward Jenner (1796)
developed a smallpox
vaccine using milder
cowpox viruses
Smallpox has been
eradicated in the world
today

9. How Big is a Virus?
 Viruses are very small – smaller than the smallest cell.
Porcine circovirus type 1 has a capsid diameter of only 17nm
i.e. 0.017µm
Mimivirus: Protein filaments measuring 100 nm project from the
surface of the capsid, bringing the total length of the virus up to
600nm (0.6µm) and a capsid diameter of 400nm (0.4µm).
It is the third-largest virus, preceded by the recently discovered
Megavirus chilensis and Pandoravirus.

11. Characteristics
 Non living structures
 Non-cellular
 Contain a protein coat called the capsid
 Have a nucleic acid core containing DNA or RNA (one or the
other - not both)
 Capable of replication only when inside a HOST cell
 Exist in two distinct states: a virus (when active) and a virion
(when dormant and not in contact with a host cell).
 Can also remain dormant within an organism (latency).
 A viriod (NOT VIRION) is an infectious RNA particle that
resembles a virus – but is smaller.

12. Prions has protein only, no DNA or RNA (cause of
mad cow disease and Creutfeldt-Jacob disease in
humans)—affects the brain and is always fatal
No DNA or RNA!
Prions are made up of harmless proteins that are found in
mammals and birds. But these proteins are in abnormal
form and once they enter human brain, they are capable of
severe brain infections. Normally these prions are ingested
but they also get formed through mutation of a gene that
contains this protein.

13. Characteristics
 Some viruses are
enclosed in an protective
envelope
 Some viruses may have
spikes to help attach to
the host cell
 Most viruses infect only
SPECIFIC host cells
CAPSID
ENVELOPE
DNA
SPIKES

14. Certain viruses can only attack certain
cell types. They are said to be specific.
It’s like the pieces of a puzzle. The
ends have to match up so only
certain pieces fit.
Surface Markers
Receptor Sites
Example: The rabies virus only attacks brain or nervous cells.
Virus
Cell

15. Surface
Markers
Receptor
Sites
Virus
A virus recognizes cells it can infect by matching
its surface marker with a receptor site on a cell.
Cell

16. Outside of host cells,
viruses are inactive
Viruses cause many
common illnesses/
diseases
Some viruses may
cause some cancers
like leukemia
EBOLA VIRUS
HIV VIRUS
MEASLES

17. What do Viruses look like?
 Viruses are unusual and different from other things in
nature.
 Viruses come in a variety of shapes
Some may be helical shape like the Ebola virus
Some may be polyhedral shapes like the influenza virus
Others have more complex shapes like bacteriophages

18.  Viruses can have different shapes:
 Polyhedral
cubical
e.g. adenovirus
 Helical
spiral cylinder
e.g. tobacco virus
 Complex
tadpole-like
e.g. bacteriophage

19. Types of Viruses:
Helical Viruses

20. Polyhedral Viruses

21. Complex Viruses

24. Viruses can be placed in one of
the seven following groups:
I: dsDNA viruses (e.g. Adenoviruses, Herpesviruses, Poxviruses)
II: ssDNA viruses (+)sense DNA (e.g. Parvoviruses)
III: dsRNA viruses (e.g. Reoviruses)
IV: (+)ssRNA viruses (+)sense RNA (e.g. Picornaviruses,
Togaviruses)
V: (−)ssRNA viruses (−)sense RNA (e.g. Orthomyxoviruses,
Rhabdoviruses)
VI: ssRNA-RT viruses (+)sense RNA with DNA intermediate in
life-cycle (e.g. Retroviruses)
VII: dsDNA-RT viruses (e.g. Hepadnaviruses and Caulimoviridae)

25. R

27. Viral Taxonomy
 Order names end in -virales
 Family names end in –viridae
 Subfamily -virinae
 Genus names end in -virus
 Viral Species: A group of viruses sharing the same
genetic information and ecological niche (host).
 Common names are used for species
 Subspecies are designated by a number

28. Herpes Virus
SIMPLEX I and II

29. Adenovirus
COMMON COLD

30. Influenza Virus

31. Papillomavirus – Warts!

32. HOST SPECIFICITY
 All kingdoms can be infected by viruses
 Viruses are kingdom specific but they may or may
not be species specific
 Spread is specific to the type of virus

33. PARASITISM
 Viruses are parasites.
 i.e. it depends upon another living
organism for its existence in such a
way that it harms that organism.

35. Viral Replication

36. Capsid
Nucleic Acid
Bacteriophage—a
virus that infects
bacteria (bacteria
is the host)

37. Capsid (protein coat)
– inside contains either
RNA or DNA
Bacteriophage—viruses that infect bacteria

39. 1) Adsorption
Virus approaches a cell.
2) Penetration
Virus attaches to the cell, injecting nucleic
acid into the cell. Capsid left outside cell.
3) Latent phase
Virus multiplies its nucleic acid using materials
from the host cell.
4) Lysis
Protein coats form around strands of nucleic
acid. The cell releases viruses.

41. Lytic Cycle

42. Lysogenic Cycle
 All phage species can undergo a lytic
cycle
 Phages capable of only the lytic cycle
are called virulent
 The alternative to the lytic cycle is
called the lysogenic cycle: no progeny
particles are produced, the infected
bacterium survives, and a phage DNA
is transmitted to each bacterial
progeny cell when the cell divides
 Those phages that are also capable of
the lysogenic cycle are called
temperate

43. Lysogenic Cycle

44. Cylces
 Lysogenic Cycle
 Viral DNA
 May stay inactive in host for long periods of time
 Long lasting
 Example Mono or chickenpox
 Lytic Cylce
 Short and can be over come
 Example flu virus

45.  Host cells are affected in three ways:
 The host cells may be destroyed. They may swell and burst, e.g. as with
nerve cells infected with the rabies virus.
 The host cells may not be able to function correctly, e.g. ciliated
epithelial cells infected with the influenza virus.
 The virus can interact with the host cell’s chromosomes causing a
mutation, e.g. warts and cancer cells.
 The immune system becomes activated in order to fight the
infection. This may lead to fever, tiredness or an opportunistic
disease.

46. Vaccine
 Is a weaken form of the virus
 To expose your immune system to the virus which will allow
your body to better fight off the virus when exposed to the full
blow virus.

48. RNA or DNA core (center),
protein coat (capsid)
Copies itself only inside
host cell--REPLICATION
DNA or RNA
NO
NO
NO
NO
Cell membrane, cytoplasm,
genetic material, organelles
Asexual or Sexual
DNA and RNA
YES—Multicellular Organisms
YES
YES
YES
Structure
Reproduction
Genetic Material
Growth and
Development
Response to
Environment
Change over time
Obtain and
Use Energy

49. How many characteristics
of life do viruses possess?
*Genetic Material
Are viruses living?

50. Assignment 1
• Describe the life cycle of a temperate phage
with a labelled diagram.
• Compare viruses, virions, viroids and prions.