Viruses are the smallest infectious agents that are obligate intracellular parasites. They contain either DNA or RNA, but not both, and are surrounded by a protein coat. Viruses replicate inside host cells using the cell's machinery. The viral replication cycle involves adsorption, penetration, uncoating, biosynthesis of viral components, assembly, maturation, and release. Viruses do not undergo binary fission like bacteria but instead use a complex replication process within the host cell.

1. DR. SHAHRIAR HABIB
GeneralVirology
Virology

2. History
The
fi
rst human virus to be identi
fi
ed was
the yellow fever virus.
First animal virus discovered- foot-and-mouth disease.
First virus discovered is the tobacco mosaic virus.
contagium vivum
fl
uidum (soluble
living germ) and re-introduced the
word virus (Toxin)
Carlos
Finlay
Dmitri Ivanovsky
Friedrich Loef
fl
er
Adolf Eduard Mayer
Charles Edouard Chamberland
Paul Frosch

4. A virus is an acellular infectious agent that is minimally constructed of two
components: (1) a genome consisting of either ribonucleic acid (RNA) or
deoxyribonucleic acid (DNA), but not both, and (2) a protein-containing
structure (capsid) designed to protect the genome.
Viruses are the infectious units (obligate intracellular parasites)
consisting of either RNA or DNA enclosed in a protective protein
coat.
Viruses are the smallest unicellular organisms that are obligate
intracellular.

5. Viruses are not organisms, and contain no functional ribosomes or other
cellular organelles and no energy producing enzyme systems, although
many viruses contain enzymes involved in nucleic acid transcription.
They can not grow in size but their nucleic acid contains the necessary
information for their replication in a susceptible host cell.
This cell may provide some of the enzymes necessary for viral replication
but its main function is to provide the energy-producing systems.
The host cell may or may not be destroyed in the process of viral
replication and release.

6. They differ from bacteria and other prokaryotes, as:
• They are obligate intracellular
• They possess either DNA or RNA, but never both.
• Filterable: They are smaller than bacteria, can be passed through the
bacterial
fi
lters.
• They cannot be grown on arti
fi
cial cell free media (However, grow in
animals, eggs or tissue culture).
• They multiply by a complex method, but not by binary
fi
ssion as seen in
bacteria.
• Viruses do not have a proper cellular organization.
• They do not have cell wall or cell membrane or cellular organelles
including ribosomes.
• They lack the enzymes necessary for protein and nucleic acid synthesis.
• They are not susceptible to antibacterial antibiotics.

7. CLASSIFICATION AND MORPHOLOGY OF VIRUSES
Nomenclature

8. DNA Viruses
DNA Virus
Family
DNA Viruses
DNA Virus
Family
DNA Viruses
Poxviridae
(largest virus
in size)
Variola virus,
Vaccinia virus,
Molluscum
contageosum
virus
Herpesviridae
Herpes simplex virus
1 and 2, VZV, CMV,
EBV,
Human herpesvirus-6,
7 and 8
Papovaviridae
Human Papilloma
virus, Polyoma
virus, BK and JC
virus
Hepadnaviridae Hepatitis B virus
Adenoviridae Adenovirus
Parvoviridae
(smallest virus
in size)
Parvovirus
(only DNA virus
to have ss DNA)

9. RNA Viruses
Family RNA Viruses Family RNA Viruses
Picornaviridae
Poliovirus, Coxsackievirus
Echovirus, Enterovirus
Rhinovirus, Hepatitis A
virus
Paramyxoviridae
Parainfluenza virus , Mumps virus
Measles virus, Respiratory
syncytial virus Newcastle disease
virus, Metapneumovirus
Caliciviridae
Hepatitis E virus, Norwalk
agent
Orthomyxoviridae Influenza viruses: A, B, and C
Togaviridae
Rubella virus
Eastern equine encephalitis
virus Western equine
encephalitis virus
Bunyaviridae
Hantavirus
California encephalitis virus
Sandfly fever virus
Flaviviridae
Yellow fever virus, Dengue
virus St. Louis encephalitis
virus, West Nile virus,
Hepatitis C virus
Arenaviridae
Lymphocytic choriomeningitis virus
Lassa fever virus, South American
hemorrhagic fever virus
Coronaviridae Coronaviruses Reoviridae
Rotavirus, Reovirus, Colorado tick
fever (only RNA virus family to
have ds DNA)
Rhabdoviridae
Rabies virus
Vesicular stomatitis virus
Retroviridae
HTLV (Human T Lymphotropic
virus) HIV (Human
immunodeficiency virus)
Filoviridae
Marburg virus and Ebola
virus

10. Size of Viruses
• Size of the viruses is determined by
○ Ultra
fi
ltration: Passage through membrane
fi
lters membrane of graded porosity
○ Ultracentrifugation
○ Electron microscopy
• Largest virus: Pox virus (300 nm)
• Smallest virus: Parvovirus (20 nm)
GENOME SIZE
LARGEST GENOME-RETROVIRIDAE (7–115 KB)
SMALLEST GENOME-HEPATITIS D (1.6 KB) < HEPADNAVIRIDAE (3.2 KB) < PARVOVIRIDAE (5.6 KB)

11. Shape of Viruses
Most of the viruses are roughly spherical except:
Rabies: Bullet shaped
Pox virus: Brick shaped
Ebola virus: Filamentous shaped
Tobacco mosaic virus: Rod shaped
Adenovirus: Space vehicle shaped
Corona virus: Petal shaped peplomers
Astrovirus: Star shaped peplomers
Rotavirus: Wheel shaped

12. Viral Structure
Viruses consist of nucleocapsid (nucleic acid and
capsid), which is further surrounded by envelope (in
some viruses). Capsid is a protein layer; is made up
capsomer units.
Nucleic Acid: Viruses possess either DNA or RNA,
but never both.

13. Capsid
Capsid is composed of a number of repeated protein subunits
(polypeptides) called capsomeres.
Functions of capsid include:
It protects the nucleic acid core from the external environment,
e.g. nucleases
In non-enveloped viruses, it initiates the first step ofviral
replication by attaching to specific receptors on the host cells,
thus facilitating the entry of the virus
It is antigenic and specific for each virus.

14. Envelope
Certain viruses possess an envelope surrounding the nucleocapsid.
Envelope is lipoprotein in nature.
• Made up lipoprotein subunits called peplomere
○ Lipid part is host cell membrane-derived and protein part is virus-
derived,
○ Envelop provides chemical, physical and biological properties to cell.
• Enveloped Viruses are ether sensitive, heat labile, pleomorphic.
• Example – All, other than nonenveloped viruses are enveloped virus.

15. Nonenveloped Virus
• Ether resistant, heat stable and nonpleomorphic
• DNA: Parvovirus, Adenovirus, Papovavirus (PAP)
• RNA: Picorna, Astrovirus, Reovirus, Calicivirus, and Hepatitis A and E.

16. • All the DNA viruses are double stranded, except Parvoviruses (the only SS DNA virus)
• All the RNA viruses have one copy of single stranded unsegmented RNA except:
○ Reo viruses (the only double stranded RNA virus)
○ Retroviruses including HIV (possess 2 copies of SS RNA)

17. Segmented RNA Viruses
○ Bunya virus (3 segments)
○ In
fl
uenza virus (8 segments),
○ Rotavirus (11 segments),
○ Arenavirus (2 segments) e.g. LCM i.e. lymphocytic choriomeningitis virus
Most RNA viruses possess positive sense RNA except:
Myxoviruses, Rabies, Filoviruses, Bunyaviruses and Arenaviruses (bear
negative sense SS RNA).

18. Symmetry
• Icosahedron symmetry: E.g. All DNA viruses (except
Pox) and most of the RNA viruses
• Helical symmetry: Few RNA viruses (Myxo, Rhabdo,
Filoviridae, Bunya)
• Pox: Complex symmetry.

19. VIRAL REPLICATION

20. VIRAL REPLICATION

21. Viruses do not undergo binary
fi
ssion (seen in bacteria), but undergo a complex way of cell
division. Replication of viruses passes through six sequential steps:
• Adsorption/attachment is the
fi
rst and the most speci
fi
c step of viral replication. It
involves receptor interactions between virus and host.
• Penetration: After attachment, the virus particles penetrate into the host cells either by
○ Phagocytosis (or viropexis)-Through receptor mediated endocytosis ○ Membrane fusion:
seen in HIV
○ Injection of nucleic acid: seen in bacteriophages
• Uncoating: Capsid is lysed (due to host lysozymes) and the nucleic acid is released. This
step is absent for bacteriophages.
VIRAL REPLICATION

23. • Biosynthesis of various viral components: i) nucleic acid, ii) capsid protein, iii) enzymes iv)
other regulatory proteins
Site of Nucleic acid replication
• ○ In DNA viruses, the DNA replication occurs in the nucleus; except in poxviruses (cyto-
plasm).
• ○ In RNA viruses: The RNA replication occurs in cytoplasm; except in retroviruses and
ortho- myxoviruses (nucleus).
VIRAL REPLICATION

25. VIRAL REPLICATION

28. • Assembly: Viral nucleic acid and proteins are packaged together to form progeny viruses
(nucleocapsids).
○ DNA viruses are assembled in the nucleus except hepadnaviruses and poxviruses (in cytoplasm) ○
RNA viruses are assembled in the cytoplasm.
• Maturation: Take place either in the nucleus or cytoplasm or membranes
• Release of daughter virions occur either by:
○ Lysis of the host cells as shown by nonenveloped viruses and bacteriophages.
○ Budding through host cell membrane as shown by enveloped viruses.
VIRAL REPLICATION

29. Eclipse phase: It is the
interval between entry of the
virus into host cell till
appearance of
fi
rst infectious
virus particle.
○ During this period, the
virus cannot be
demonstrated inside the host
cell.
○ The duration of eclipse
phase is about 15 to 30
minutes for bacteriophages
and 15-30 hours for most of
the animal viruses.

30. Atypical virus like agents
• Viroids, Prions and Virusoids
There are some incomplete viral particles such as viroid, prion and
virusoid.
• Viroids comprise of naked, circular, small ssRNA without a
capsid. They are mostly restricted to plants.They depend on host
enzymes for replication. Hepatitis D virus in humans is similar to viroids
• The entire viral particle is called as virion; comprising of
protein capsid coat and nucleic acid.

31. Atypical virus like agents
• Prions consist of abnormal infectious protein molecules without nucleic
acid
• They are highly resistant to physical and chemical agents
• Prions produce slow infections in humans having long incubation period
(in years) called prion disease (a neurodegenerative condition of brain).
• Virusoids are plant pathogens similar to viroids, made up of circular
ssRNA without protein, but are dependent on other plant viruses for
replication and encapsidation.

32. Thank you