Viruses exhibit great diversity in their structures and genomes. They range in size from 0.02 to 0.3 micrometers and have various morphologies including helical, polyhedral, and filamentous shapes. All viruses are obligate intracellular parasites as they cannot reproduce outside of a living host cell. The viral genome is comprised of DNA or RNA and is enclosed in a protein coat called a capsid. Viruses infect bacteria, plants, and animals and are classified based on attributes like nucleic acid composition and host range. The life cycles of viruses vary depending on whether they contain DNA or RNA genomes and can involve the production of mRNA, reverse transcription, integration into the host genome, and assembly of new viral particles.

1. Structure and
Diversity ofVirus
Farouk Faizal Al Husein

2. Viruses have one major characteristic in common: they
are obligate intracellular parasites.
Virology; the study of viruses
Viruses are UNABLE to grow and reproduce outside
of a living cell. No virus is able to produce its own
energy (ATP) to drive macromolecular synthesis.
However, in many other respects, they are a
highly diverse group.

3. Definition of a Virus
 Viruses are segments of nucleic acid enclosed
in a protein coat (virion / virus particle :
extracellular state)
Poliovirus

4.  Viruses are genetic elements that can
replicate independently of a cell’s
chromosomes but not independently of cells
themselves (intracellular state)
Definition of a Virus
a host (a place for initiating the
intracellular state)

5. Properties of Viruses
 Small size>range>0.02 - 0.3 micrometers
Picornavirus (“little RNA virus”) is
one of the smallest viruses, about
20 nanometers in diameter
Smallpox virus, one of the largest
viruses, about 300 nanometers, near
the resolution of the light microscope
• Size alone does not differentiate viruses & bacteria!
• smallest bacteria (e.g. Mycoplasma, Ralstonia pickettii)
are only 200-300 nm long.

6. Properties of Viruses
 Various morphologies
polyhedral
helical
spherical
filamentous
complex Ebola virus Rabies virus
Poliovirus Herpes virus Coronavirus Lassa virus

7. Properties of Viruses
 Obligate intracellular parasites
Bacteriophage T4, a virus that
Infects E. coli

8. Properties of Viruses
 Lack membranes and a means to generate
energy
HIV

9. Properties of Viruses
 Lack metabolic and biosynthetic enzymes
 Lack ribosomes

10. Properties of Viruses
 Do not grow in
size
Viruses grow by
independent
synthesis and
assembly of their
components
inside of a host
cell Human adenoviruses growing in the
nucleus of their host cell

11. Virion Structure
Nucleic Acid
Spike
Projections
Protein
Capsid
Lipid Envelope
Virion
Associated
Polymerase

12. Structure of Viruses

13. Virion Components
 Protein
 Structural proteins
 Membrane proteins
 Receptor recognition
 Enzymes
 Genomic nucleic Acid
 DNA
 RNA
 Lipid envelope
 Plasma membrane – Paramyxoviruses
 Nuclear membrane – Herpes viruses
 Golgi membrane - Bunyaviruses

14. Structure of Viruses
 The viral genome is DNA or RNA
 Most bacterial viruses contain double-stranded DNA
 Many animal viruses contain ds DNA or ssRNA

15. Structure of Viruses
 Most common morphologies are polyhedral
(icosahedral) and helical
Polyhedral virus
Helical virus

16. Structure of Viruses
 Some viruses have additional structures:
animal viruses may have envelopes and
“spikes”

17. Structure of Viruses
 bacterial viruses may have tails and related
structures
T4
virus

18. Classification of Viruses
Criteria: Baltimore
 Type of nucleic acid
 Size and morphology
 Additional structures such as envelopes and tails
 Host range > refers to the range of cells that can
be infected by the virus, most often expressed as
bacteria, plant and animal hosts

19. Classification of Viruses
Comparative size and shape of various groups of viruses representing
diversity of form and host range

20. Some Families of Bacteriophage

21. Some Families of Animal Viruses (continued)

22. 11

23. Virus Groups
 Some members possess large DNA genomes encoding a range of
enzymes involved in nucleic acid synthesis.
 Depending on virus group viruses show temporal regulation of protein
synthesis.
 Small DNA genomes with limited coding capacity.
 Some members of this group are dependant upon other viruses for their
replication.
1 dsDNA dsDNA mRNA Herpes simplex
virus
2 ssDNA Parvovirus
dsDNA mRNA
ssDNA

24. Virus Groups
 Viruses possessing RNA genomes all encode an RNA-
dependant RNA polymerase.
 RNA viruses show a higher mutation rate compared to DNA
viruses.
 Segmented genomes.
 Transcribes mRNA from the dsRNA genome without prior protein
synthesis using a virion associated RNA-polymerase
 Early phase of mRNA synthesis is monocistronic mRNA molecules.
3 dsRNA dsRNA mRNA Reovirus

25. Virus Groups
 “Positive” RNA viruses - Genome RNA is of the same sense as mRNA and
can be infectious.
 First stage in replication is the translation of the genome RNA with the
production of the virus polymerase.
 “Negative” RNA viruses – Genome RNA is complementary to mRNA.
 Virion-associated RNA-polymerase and first stage in replication is
mRNA transcription.
4 +ve ssRNA dsRNA +ve ssRNA [Acts as mRNA] Enterovirus
5 -ve ssRNA Influenza A
virus
dsRNA -ve ssRNA mRNA

26. Virus Groups
 Unique among RNA viruses in that they induce tumours.
 Characteristic feature is their ability to produce a DNA copy of the
genome RNA using a virion associated ReverseTranscriptase.
 DNA copy integrates into the cellular genome.
 Circular DNA genome - double stranded with a nick in one strand.
 The nick is repaired at an early stage in the virus replication cycle.
 The virus encodes RNA polymerase with a reverse transcriptase
activity which produces a RNA intermediate from which the genome
DNA can be copied.
6 ssRNA mRNA
dsDNA
ssRNA Retrovirus
(e.g. HIV)
7 Nicked dsDNA
dsDNA Sobek
Hepatitis B
virus
nicked dsDNA intact dsDNA
dsDNA Utuh
mRNA
RNA

27. UNINFECTED CELLS
INFECTED
CELLS
Rate
of
Protein
Synthesis
2 4
Hours after Infection
7MeG
p220
IRES
AUG
U
5’
A. Cellular mRNA
B. Picornavirus mRNA
Poliovirus protein synthesis

28. The (dsDNA) Virus Life
Cycle
1. Virus enters host cell (method
is variable, involves host
receptor molecule on cell
surface)
2. Viral DNA replicated using the
host's DNA polymerase,
nucleotides, etc.
3. DNA transcribed into mRNA
using host's RNA polymerase,
nucleotides
4. mRNA translated using host's
ribosomes, tRNAs, amino
acids, GTP, etc.
DNA
Protein
capsid
1
3
2
mRNA
DNA
capsid
proteins
4

29. The dsDNA Virus Life Cycle
5. New DNA and capsid proteins
assemble into new virus
particles, exit the cell (in
various ways)
DNA
Protein
capsid
1
3
2
mRNA
DNA
capsid
proteins
4
5

30. The ssRNA (type V) Virus Life
Cycle
1. Virus enters host cell
2. Capsid removed, RNA released
3. complementary RNA made from
genomic RNA by enzyme encoded
in viral genome
4. new genomic RNA made from
complementary strand
5. complementary strand is mRNA,
transcribed into viral proteins
6. Virus assembled, exits cell (by
various means)
1
2
5
4
3
6
RNA
cRNA

31. The Retrovirus Life
Cycle
1. Virus enters host cell
2. Reverse transcriptase (encoded in
viral genome) catalyzes synthesis
of DNA complementary to the
viral RNA (cDNA)
3. RTase catalyzes synthesis of 2nd
strand of DNA complementary to
the first
4. dsDNA incorporated into host
genome ("provirus")
 provirus may remain unexpressed
for a period of latency
1
4
3
2
5
6
Host's DNA
RNA
cDNA
RTase

32. The Retrovirus Life
Cycle
5. Proviral genes are transcribed by
host's transcriptional machinery
into RNA
• RNA serves as mRNA for
translation into viral proteins and
as genomic RNA
6. New viruses are assembled
containing genomic RNA and
ReverseTranscriptase
7. Virus exits cell
1
4
3
2
5
6
Host's DNA
RNA
cDNA
RTase

33. Bacteriophages
 Viruses that infect bacterial cells
 Two types of infections:
1. Lytic infection: phage replicates its DNA and
lyses the host cell
2. Lysogenic infection: phage DNA is maintained
by the host cell, which is only rarely lysed

34. Virulent phages only
undergo a lytic cycle
Temperate phages can
follow both cycles
Prophage can
exist in a dormant
state for a long
time
It will undergo
the lytic cycle
Bacteriophage

35. Life Cycle of a Lytic Phage

36. Temperate phages can
follow both cycles
Prophage can
exist in a dormant
state for a long
time
It will undergo
the lytic cycle
Bacteriophage

37. Viruses are usually very host-specific:
one virus infects only one strain,
maybe not even other members of the
same species
Why?
Viruses enter cells via specific proteins in
the membrane
Phage’s host specificity

38. Lipid bilayer
(same in all
cells) cannot
be penetrated
Proteins differ,
even within a
species

39. Consequences of viruses attacking
specific proteins
1. A cell cannot be totally immune to all
viruses because it needs the membrane
proteins to communicate with outside
environment
Best example: lambda phage attacks
E.coli via the maltose transporter. No
transporter, no phage problem—but no
maltose (a sugar) also.
So, viruses can affect uptake, etc.

40. Bacteriophages:
Quantification
 There are three methods :
 Electron Microscopy
 Epifluorescence microscopy
 Plaque Assay

41. Electron microscopy:
Difficult, expensive
More definitive—you’re sure it’s a virus
More information from morphology
Epifluorescence microscopy
Easy, less expensive
Less definitive: “viral-like particles”
More quantitative

42. Phage
One of many
phages
27

43. Virus counts with epifluorescence are higher
than with electron microscopy (TEM). Why?
1.Epifluorescence counts things that are
viruses.
2.TEM misses things that are viruses
3.Loss of viruses during preparation of
samples for TEM.
24

44. Quantification of bacteriophages by plaque assay:
host bacterial cells
plaques
“lawn” of host bacteria
Ph2

45. l forms plaques on a lawn of
bacteria

46. Uses for Bacteriophages
 Phages as vectors in genetic engineering and
biotechnology designs
 Phage lytic enzymes to control infections
 Phage therapy in animals and other uses of
phage in agriculture
 Bacteriophage therapy
 Phages for detection of pathogenic bacteria

47. TERIMA KASIH