This document provides an overview of viruses, including their history of discovery, characteristics, components, shapes, classification, bacteriophages, replication cycles, enveloped viruses, and other related infectious agents like viroids and prions. It discusses key scientists and experiments that contributed to the understanding of viruses. The replication cycles of lytic and lysogenic bacteriophages as well as enveloped DNA and RNA viruses are described.

1. Viruses
Chapter 3

2. Learning objectives
i. Describe the history of virus discovery.
ii. Contrast the characteristic of a virus as
compared to a living cell.
iii. Understand the origin of virus.
iv. Explain virus component and shape.
v. Characterize a bacteriophage.
vi. Contrast bacteriophage lytic and lysogenic
life cycle.
vii. Describe the life cycle of an enveloped DNA
and RNA virus.
viii.State the characteristic of viriods and
prions.

3. History of virus discovery
In the late 1800s,
botanists had been trying
to find the cause of
tobacco mosaic disease.
In 1892, D. IWANOWSKI
tried to filter the sap of
infected tobacco plants
(Filter capable of
removing particles the
size of all known
bacteria).

4. Experiment by Iwanoski

5. History of virus discovery
The filtrate was FULLY CAPABLE
of producing the ORIGINAL
DISEASE in new hosts.
Nothing could be seen in the
filtrates using the most powerful
microscopes, nor could anything
be cultivated from the filtrates.
Iwanowski concluded that the
bacteria was so small / or they
made a filterable toxin.

6. History of virus discovery
A Dutch botanist named Martinus
Beijerink ruled out the filterable toxin
conclusion because the filtered sap
are capable of causing undiluted
infection.
The agent cannot be cultivated on
nutrient media (need a host)
In 1935, Stanley
discovered this agent
after crystallization

7. Virus characteristics
Viruses are not classified into any of the
biological classification system.
They lie in the threshold of life and
nonlife.

8. Virus characteristics
Non-Life Life
They are acellular, They could only
with no cell nucleus, reproduce within the
organelles or living cells that they
cytoplasm. Therefore, infect. They use their
they do not have the genetic information to
components necessary force the host cell to
to carry metabolic replicate themselves
activities (obligate intracellular
independently. parasite) .
Viruses cannot move
and reproduce on
their own.

9. Virus size Viruses are smaller
than bacteria.
Viruses are too small
even to be seen by a
light microscope.
The biggest size virus is
about 240-300nm
(1/10 of red blood
cells/ size of the
smallest bacteria)
The tiniest virus is
20nm – smaller than a
ribosome

10. Virus origin According to a hypothesis,
viruses are bits of nucleic
acid that ‘escaped’ from
cellular organism.
Some traces are from
animal cells, plant cells
and bacterial cells.
Their multiple origins
explain why viruses are
species-specific.
However, some other have
broader range of host cells

11. Virus component
Virus consist of only
I) Nucleic acid (DNA or RNA) The DNA /RNA
could be single or double stranded.
II) A capsid or a protein coat which functions in
protecting the genetic material during the viral
infection process.
III) In some viruses, there is an outer envelope
that encloses the coat, and is made of parts of
the previously infected cells.
(A complete virus that consist of the genetic material,
the protein coat and an envelope is called the virion)

12. Virus Component

13. Virus shape
Virus shape can be based on the capsid
i. Helical (rod-shaped) e.g. tobacco
mosaic virus
ii. Polyhedral / Icosahedral (many-
sided shaped) e.g. adenovirus.
iii. Complex combination of both by
having structures like tail (helical
and polyhedral) e.g. bacteriophage
iv. Most enveloped virus have spherical
shape e.g. influenza virus

15. Virus classification
Before, viruses are classified according to the type of
host that they infected.
The current system reflect phenotypic
characteristics.
The Baltimore classification system distinguish
viruses based on their
- Method of replication
- Genome type (DNA or RNA)
The International Committee on Taxonomy of
Viruses devised and implemented several rules on
the naming and classification of viruses early in the
1990s.
It started at the level of order and ends at genus

16. 21 virus families that infect humans

17. Bacteriophage
Much of the knowledge comes from
studying bacteriophage, because they can
be cultured easily within living bacteria.
Bacteriophage possess dsDNA inside their
capsid (protein head). The capsid functions
as protection of their genetic material.
Their tail fibers are the base used to attach
themselves to bacterial host cell
The tail is the channel for their genetic
material to be injected to the host cell.

18. Bacteriophage

19. Bacteriophage replication cycle

20. Bacteriophage replication
There are two types of bacteriophage
replication, LYTIC and LYSOGENIC
cycles.
In a lytic cycle, the virus destroys the
host cell. It is a rapid process where
the host cell undergoes lysis.
In a lysogenic cycle, the viral genome
usually becomes integrated into the
host cell.

21. Virus lytic cycle
There are five steps in a typical
bacteriophage lytic reproduction,
i. Attachment-A virus will attach to a
suitable host cell
ii. Penetration- The whole virus or only
the genetic material (nucleic acid) will
penetrate the cell’s cytoplasm. A
bacteriophage capsid remains on the
outside of the bacterial cell whereas
many viruses that infect animal cell
enter a host cell intact.

22. Bacteriophage Lytic Cycle

23. Virus lytic cycle
iii.Replication and synthesis - The viral
DNA/RNA directs the host cell to
produce many copies of viral nucleic
acids and proteins necessary for its
replication.
iv. Assembly - The viral nucleic acids
and proteins are assembled together
to form new infectious particles.
v. Release - Newly generated viral
particles are released from the host
cell.

25. Virus lysogenic cycle
The infection will enter a latent period. The
host cell is not killed in this process, but
the viral nucleic acid will undergo genetic
recombination with the host cell’s
chromosome. This integrated structure is
called a prophage.
When the bacterial DNA replicates, the
prophage also replicates.
Certain external condition such as UV light
and x-rays cause viruses to revert to a lytic
cycle and then destroy their hosts.

26. Virus lysogenic cycle

27. Replication of an enveloped DNA
virus
Enveloped virus has a different way of infecting
eukaryotic cells.
After attachment to a host-cell receptor, some
enveloped viruses fuse with the animal cell’s
plasma membrane. The viral capsid and nucleic
acid will then be released into the animal cell.

28. Enveloped virus penetration step
Some virus enter the cell through endocytosis.
In this process, the plasma membrane of the
animal cell invaginates to form a membrane-
bounded vesicle that contains a virus.

29. Replication of an enveloped DNA
virus
The viral DNA will be replicated and
transcribed by the host cell.
After the viral genes are transcribed, the viral
structural proteins are synthesized through
translation outside nucleus.
The new virus particles are then assembled.
Enveloped viruses obtain their glycoprotein
spikes on the envelopes by picking up a
fragment of the host plasma membrane as
they leave the infected cell.

30. Replication of an enveloped RNA
virus
The viral genome (single stranded RNA)
function as a template for synthesis of
complementary RNA strand
Some complementary strands became
mRNA that will translated
Viral genome RNA are made using
complementary strands
After translation, assembly and release step
be done in proper sequence

31. Replication of an RNA virus (retrovirus)
Virus attach through specific glycoprotein and
enter through endocytosis
Digestion of capsid through cellular enzyme
Viral RNA ia a template for complementary
DNA sense by reverse transcriptase
Second DNA strand will be synthesized by
reverse transcriptase
Ds DNA incorporated with the cell’s DNA as a
provirus
The genes are replicated, transcribed and
translated to build the components for the
RNA virus assembly before being released

32. H1N1 virus
H1N1 virus isolated
from patients found
that it is made up of
genetic elements
from four different
flu viruses – North
American Mexican
influenza, North
American avian
influenza, human
influenza, and swine
influenza virus

33. Viruslike agents
Viruses is considered as the
smallest living / nonliving microbe.
However, there are even smaller
infectious agents found – viroids
and prions.

34. Viroids
In 1961, an infective agent in potatoes has
been discovered. The agent is called viroid
and it is smaller than viruses with no
protein coats.
Viroids are infectious RNA particle that
may cause plant diseases by interfering
with mRNA processing.

35. Prions
Prions are infectious particles made of protein.
Research indicates that prions are normal
proteins that become folded incorrectly.
Prions could cause neurological degenerative
diseases such as mad cow disease and Scrapie.