Viruses are small infectious particles that contain genetic material and reproduce by inserting their DNA or RNA into host cells. They are not considered living organisms as they do not have cells, lack metabolism, and are obligate intracellular parasites. Viruses can cause diseases in humans and other organisms. They reproduce through lytic and lysogenic cycles and can be transmitted through various methods like contact, air, body fluids, or insect bites depending on the virus. While viruses cause disease, they can also be used to deliver beneficial genes or drugs through viral vectors in applications like gene therapy or chemotherapy. Other infectious particles like viroids and prions also lack cells but can damage crops or brains.

1. VIRUSES

2. Viruses
Yellow fever virus. The
first human virus to be
discovered, in 1901.

3. Viruses
A seventh kingdom of life?
1. Eubacteria
2. Archaea
3. Protista
4. Fungi
5. Plants
6. Animals
7. (Viruses, Viroids, and Prions?)

4. Think. Pair. Share.
What is a virus?
Can you think of any examples of viruses?
Are viruses living or non-living?

5. Think. Pair. Share.
Which Domain do you think viruses are in?
Which Kingdom do you think viruses are in?
The answers are coming up!

6. Examples of Viruses
• Influenza
• HIV
• Chicken pox
• H1N1
• Rabies
• Ebola
• T4 Bacteriophages

7. Viruses
A virus is a small infectious particle containing genetic
material (DNA or RNA) within a protein capsule (capsid).

8. Viruses
Viruses reproduce by inserting their genetic material
into host cells and "taking them over". This makes them
intracellular parasites.
The general name given to a single virus particle that is
not infecting a cell is "virion".

9. Viruses
A virus is an obligate parasite; it requires a host
cell to reproduce.

10. Viruses
They are responsible for many diseases.
Epidemic: Large-scale disease outbreak in a region.
Pandemic: Multi-region or global disease outbreak.
AIDS

11. Viruses
There are many different ways viruses can be
transmitted:
Disease
rabies
HIV
Transmission Method
bites (saliva  blood)
body fluid exchange
– excluding saliva
influenza, chicken pox, airborne/direct contact
common cold
measles, mumps
direct contact

12. Viruses
Viruses are non-living and are not part of the traditional
taxonomic ranks.
Virus
Think. Pair. Share.
Which properties of living things do viruses not have?

13. Viruses
Viruses are acellular; they have no membranes or cytoplasm.
Viruses have no metabolism they can not grow or divide.
Viruses do not require nutrients or energy. They do not create
waste.

14. Differences Between Viruses and Cells
Genetic material: Viruses
contain RNA or DNA
enclosed by protein, not
a membrane.
Organelles:
None.
Life cycle: Is not
alive – has no real
"life cycle"

15. Differences Between Viruses and Cells
Size: Viruses are
much smaller
than cells.
Metabolism: Viruses
have no metabolism.

16. Viruses
Viruses can be classified into their own
orders/families/genera/species based on certain properties:
- size
- shape
- genetic material
- host cells
- contains DNA or RNA
- single-stranded or double-stranded genetic material
- presence of absence of an envelope

17. Virus Shapes - Cylindrical

18. Virus Shapes - Polyhedral

19. Virus Shapes - Spherical

20. Virus Shapes – Irregular (Phage)
This bacteriophage (phage for short) is a virus that infects bacteria only.
capsid (entire top part)
nucleic acid
tail (entire bottom part)
sheath
tail fibres
cell membrane
viral RNA/DNA injected into host cell

21. Virus Shapes
Tail fibres and spikes are used to attach to host cells.
Capsids and capsomeres are protective protein shells
for the viruses.
Envelopes are made from dead host cell membranes
and allow the virus to "pretend" to be host cells and
avoid detect by the immune system.

22. Infectious Cycles
A virus infects a cell by inserting its nucleic acid (DNA or RNA) into it.
The viral nucleic acid then enters 1 of 2 cycles:
Step 1
Step 2
Step 3
Step 4
Step 5
Step 6
Lytic = steps 1, 2, 5, 6
Lysogenic = steps 1, 2, 3, 4 (5 & 6 occur later)

23. The Lysogenic Cycle
Lysogenic cycle: The host cell survives, but when it
divides, the virus DNA or RNA is also duplicated.
Lysogenic Cycle

24. The Lysogenic Cycle
There are 4 steps:
Viral
DNA/RNA
Host Cell
DNA
A) Attachment
& Entry

25. The Lysogenic Cycle
There are 4 steps:
Provirus
B) Integration – viral nucleic acid
becomes part of host nucleic acid
Provirus: a viral section of DNA that has become part of
the host cell's DNA

26. The Lysogenic Cycle
There are 4 steps:
C) Multiplication of genome

27. The Lysogenic Cycle
There are 4 steps:
D) Cell
division

28. 1) Attachment
virus
viral DNA
host
DNA
host cell
5) Lysis &
Release
- Host cell
bursts
open and
viruses
escape.
2) Insertion
- Virus injects its
nucleic acid
into the
host cell.
The Lytic
Cycle
3) Replication
4) Assembly
- New virus particles are
assembled by the host.
- The host's
metabolism
replicates the viral
parts and RNA/DNA.

29. The Lytic Cycle
The lytic cycle eliminates the integration and induction of viral DNA
Step 1
Step 2
Step 3
Step 4
Step 5
Step 6
Lytic = steps 1, 2, 5, 6
Lysogenic = steps 1, 2, 3, 4 (5 & 6 occur later)

30. Infectious Cycles
A virus can switch
back and forth
between cycles
and a virus may be
in lysogeny for
many years.
Lytic
Cycle
Lysogenic
Cycle

31. Infectious Cycles
Sometimes newly formed viruses take sections host DNA with
them.
Transfer of genes using a virus is called transduction.

32. Vaccinations
Our immune systems can create a memory for
infections that we have previously fought-off.

33. Vaccinations
Previously encountered pathogens (like viruses or bacteria) are much
more easily destroyed.
Pathogen
Pathogen

34. Vaccinations
Previously encountered pathogens (like viruses or bacteria) are much
more easily destroyed.
Pathogen
Pathogen

35. Vaccinations
A vaccination is a weakened pathogen that our immune systems can
easily fight-off. After our bodies have learned how to combat that
particular pathogen, repeat infections are more easily destroyed.

36. Think. Pair. Share.
How might a virus be used to benefit humans?

37. Viruses - Benefits
We can now craft viruses that contain beneficial genes or drugs
within their capsids (called viral vectors).

38. Viruses - Benefits
These vectors enter the target cells and deliver their DNA to
create a helpful provirus:
Vector Contents
Drugs
Copies of beneficial genes
Novel (new) genes
Application
Ex. Chemotherapy for cancerous
cells.
Gene therapy (replacement or
addition of new or better genes).
Creation of GMOs (genetically
modified organisms, like plumper
fruits or leaner livestock).

39. Viroids
A viroid is an infectious piece of RNA. It does not code for
anything and has no protective structure like a membrane or
capsid.

40. Viroids
These properties make them no less destructive to vulnerable
species.
Normal Potatoes
Infected Potatoes
The potato spindle tuber viroid (PSTVd) greatly damages potato
crops.
The PSTVd molecule

41. Prions
A prion is an infectious protein. It has no outer structure, but
damages other proteins it comes into contact with. The damaged
proteins may become prions.

42. Prions

43. Prions

44. Prions

45. Prions

46. Prions

47. Prions

48. Prions

49. Prions
Bovine spongiform encephalopathy (BSE, mad cow disease) is
caused by prions. The brain losses mass and becomes "spongy"
due to destroyed/damaged proteins.
Brain of animal
with BSE
Brain of healthy
animal

50. Prions
Creutzfeldt-Jakob disease (CJD) is caused by prions in human
brains. It starts with memory loss and dementia, leading to loss of
muscle control and death.