This document provides an overview of viruses, including: - The history of virus discovery from Iwanowski's experiments in 1892 showing that the cause of tobacco mosaic disease was able to pass through filters that removed bacteria. - Characteristics of viruses that distinguish them from living cells, including being acellular and only able to reproduce within host cells. - The components of viruses, which include nucleic acids and protein capsids, with some viruses also having envelopes. - The replication cycles of bacteriophages and how they can either undergo lytic or lysogenic cycles, and the replication processes of enveloped DNA, RNA, and retroviruses within host cells. - Emerging viruses

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

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

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

10. 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

12. 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

13. 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)

14. Virus Component

15. 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

18. 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

19. 21 virus families that infect humans

20. 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.

21. Bacteriophage

22. 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.

23. 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.

24. Bacteriophage Lytic Cycle

25. 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.

31. 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.

32. Virus lysogenic cycle

33. Bacteriophage replication cycle

34. Reproductive Cycles of Animal
Viruses
• There are two key variables used to classify
viruses that infect animals:
● DNA or RNA?
● Single-stranded or double-stranded?

35. 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.

36. 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.

37. 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.

38. 1. After entering the cell, the viral
DNA uses host nucleotides and
enzymes to replicate itself
2. The viral DNA use other host
resources to produce its capsid
proteins by transcription and
translation
3. The new viral DNA and capsid
protein assembled into new virus
particle, which leave the cell
1 2
3
Self-assembly of new
virus particles and
their exit from cell

40. 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

41. 1. Glycoprotiens bind to specific
receptor on the surface of the host
cell
2. Capsid and viral genome enter the
cell
3. Cellular enzyme remove the capsid
4. The viral genome functions as a
template for making complementary
RNA strands which have two
functions
5. Serves as templates for making new
copies of genome RNA
6. Serves as mRNA which will be
translated into both capsid proteins
and glycoprotein for the viral
enveloped
7. Vesicle transport (ER) the
glycoproteins to the cell’s plasma
membrane
8. A capsid assembled around each
viral genome molecule
1
2
3
4
5 6
7
8

43. 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

44. 1. The virus fuses with the cell’s plasma
membrane. The capsid proteins are removed,
releasing the viral genome.
2. Reverse transcriptase catalyzes the synthesis of
a DNA strand complimentary to the viral RNA.
3. Reverse transcriptase catalylzes the synthesis of
the second DNA strand complimentary to the
first.
4. The double stranded DNA is incorporated as a
provirus into the cell’s DNA (chromosomal DNA)
5. Proviral gene are transcribed into RNA
molecules.
6. RNA transcribed from the provirus serves as
mRNA for translation into HIV proteins and as
genome for the next viral generations.
7. Capsids are assembled around viral genomes
and reverse transcriptase molecules.
8. The new viruses bud off from the host cell.
1
2
3
4
5
6
7
8

47. 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

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

49. 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.

50. 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.

52. THANK YOU…