Virology

1. Content:
Definition
Characteristic
s
Structure
Classification
Dr. Anita Tiknaik
Research Scientist
Paul Hebert Centre for DNA
Barcoding and Biodiversity
Studies
Course PGDV-001: Introductory Medical and
Molecular Virology

2. What Is a Virus ???
whether viruses are “living” entities???
Where Did Viruses Come From
Viruses cannot be grown independently in culture
viruses require a living cell for reproduction, whether animal, bacterial or plant
A virus contains genetic information—DNA or RNA—yet the statement that a
virus is alive is a controversial one
Viruses are genetic parasites—that is, they require the protein synthetic
machinery of the host to reproduce. Viruses, as visualized by the electron
microscope, are symmetrical and complicated structures.

3. Although Louis Pasteur and Edward Jenner developed the first vaccines to protect
against viral infections, they did not know that viruses existed.
The first evidence of the existence of viruses came from experiments with filters that
had pores small enough to retain bacteria.
In 1884, the French microbiologist Charles Chamberland (1851–1931) invented a filter
– known today as the Chamberland filter – that had pores smaller than bacteria. Thus,
he could pass a solution containing bacteria through the filter and completely remove
them from the solution
In 1876, Adolf Mayer, who directed the Agricultural Experimental Station
in Wageningen, was the first to show that what he called "Tobacco Mosaic Disease" was
infectious. He thought that it was caused by either a toxin or a very small bacterium.
Later, in 1892, the Russian biologist Dmitry Ivanovsky (1864–1920) used a
Chamberland filter to study what is now known as the tobacco mosaic virus.
Martinus Beijerinck called the filtered, infectious substance a "virus" and this discovery
is considered to be the beginning of virology

4. The publications of the anti-vaccine society. Print (color engraving) published June
12, 1802, by H. Humphrey, St James’s Street

5. Definition
•Virus is latin word which means poison.
•A virus is an entity whose genome is an element of nucleic acid that
replicate inside living cells, using host’s machinery leading to transfer of
genome to other cells (Laura & Darnell, 1978).
•Virion: A complete virus particle.
•Prion: Pathogen composed of proteins without any detectable nucleic
acid.
•Bacteriophage: is a virus that infects and replicates within Bacteria.

6. General characteristics
•Consist either of RNA or DNA but never both
•Obligate intracellular parasites
•Fails to grow on artificial media
•Smallest infectious agents
•Nucleic acid is encased in a protein shell
•Not inactivated by antibiotics
•Divide by replication

7. General characteristic
A basic structure of virus is
nucleic acid core (either DNA
or RNA but not both)
surrounded by protein coat.
Central core of nucleic acid of
a virus is called genome and
the protein coat surrounding
is called as capsid.
In some virus, an envelope
made up of glycoprotein and
phospholipid bilayer is
present outside the capsid.

8. The basic structural components of a virus are;
1. Genome:
•Virus contains either DNA or RNA as genetic material but not both. Virus which
contains DNA as genetic material are called DNA virus and those containing RNA are
called RNA virus.
•Unlike other living cell where ds DNA is always a genetic material, a viral genome may
consists of linear or circular ds DNA, single stranded DNA, ss linear RNA or ds linear
RNA.
•Examples; Reo virus is a RNA virus which contains ds RNA genome. Parvovirus contains
ss DNA, Papovavirus contains ds circular DNA as genetic materials.

9. 2. Capsid:
•Capsid is the outer layer
•Capsid serves as impenetrable shell around the nucleic acid core.
•Capsid also helps to introduce viral genome into host cell during infection.
•The protein coat or capsid is made up of number of morphological similar sub units
called capsomere. Each capsomere is further composed of protomere.
•Capsomere are arranged precisely and tightly together in a repetitive pattern to form
complete capsid.
•The number of capsomere in a capsid varies from virus to virus.
•The complete complex of nucleic acid and protein coat of a virus particle is called as
virus nucleo-capsid.
•Structure of capsid give the symmetry to the virus. Virus particle may be either cubical
or helical or binal or complex symmetry.

10. 3. Envelope:
•Some virus contains envelope that surrounds nucleocapsid. The virus without envelope
is called naked virus.
•The envelope is a bilayer of lipoprotein and glycoprotein.
•The envelope is acquired by the progeny virus from host cell during virus release by
budding process.
•In some virus the glycoprotein projects out in the form of spike called peplomere. Some
of the peplomers or glycoprotein spike such as Haemaglutinin and Neuraminidase which
are involved in binding of virus to host cell.

11. 4. Enzymes:
•Some virus contains enzymes which play central role during infection process.
Eg. Some bacteriophage contains an enzyme lysozyme, which makes small hole
in bacterial cell that allows viral nucleic acid to get in.
•Some virus contains their own nucleic acid polymerase which transcribe the
viral genome into mRNA during replication process. Eg. Retro virus are RNA
virus that replicates inside host cell as DNA intermediate. These virus possess
an RNA dependent DNA polymerase called reverse transcriptase.

12. •0.015 µm to 0.4 µm (µm = micrometer). One micrometer corresponds to a thousandth of
a millimeter. Viruses are therefore very small and allocated to the PM1 category. The
classification of particles into the categories PM10, PM2.5 and PM1 is based on the
"National Air Quality Standard for Particulate Matter" of the US Environmental Protection
Agency (EPA) and is accepted and used worldwide.
Size of Viruses

13. Nucleic Acid classification
1.DNA Viruses 2. RNA Viruses
Nature classification
1.Linear/Circular 2. Segmented/ Non-segmented
Strandedness classification:
1.Single-stranded (ss) 2. Double-stranded (ds)
Polarity classification:
1. Sense strand (Positive) viruses 2. Antisense strand (Negative)
viruses
Presence of an envelop classification:
• 1. Enveloped viruses 2. Non-enveloped viruses
•Criteria for viral classification

14. Classification on the basis of nucleic acid