5. Basic properties of virus

Date:4/17/2021 www. qadricohs.edu.pk FB Page: Qadri College of Health Sciences, Karachi, Sindh, Pakistan. 1
QADRI COLLEGE OF HEALTH SCIENCES, KARACHI
5.
SOME BASIC PROPERTIES
OF THE VIRUS
Faculty: Aftab H. Abbasi
RN, DCHN, BSN, MA, LL.B
Lecturer Nursing
Qadri College of Health Sciences Karachi
QADRI COLLEGE OF HEALTH SCIENCES, KARACHI

BASIC PROPERTIES OF THE VIRUS
Definition of Virus:
A virus is a submicroscopic
infectious agent that
replicates(repeat of something) only
inside the living cells of
an organism, is a small
parasite that cannot
reproduce by itself.
Viruses infect all types of
life forms, from animals
and plants to
microorganisms,
including bacteria and
archaea.

-OR-
A virus is an infectious
agent of small size
and simple
composition that can
multiply only in living
cells of animals,
plants, or bacteria.

Virology:
Is the scientific study of
viruses –
submicroscopic (too small to be
seen by an ordinary light microscope),
parasitic particles of
genetic material
contained in a protein
coat – and virus-like
agents.
Virology is a subfield
of microbiology.

-Viruses are inert (lacking the ability or strength to move) (nucleoprotein ) filterable Agents.
-Viruses are obligate(restricted to a particular function or mode of life) intracellular parasites.
-Viruses cannot make energy or proteins independent of a host cell.
-Viral genome(complete set of genetic information in an organism) are RNA or DNA but not both
-Viruses lack the enzymes necessary for protein and nucleic acid
synthesis.
-Viruses do not have the genetic capability to multiply by division.
-Viruses occupy the twilight(conceptual area/mysterious) zone that separates the
‘living’ from the ‘nonliving’.

Medical importance of viruses
-Viral diseases range from minor ailments such as the
common cold to terrifying diseases such as rabies or
AIDS.
-They may be sporadic(occurring at irregular intervals or only in a few places) like mumps,
endemic(regularly found among particular people or in a certain area) like infectious hepatitis,
epidemic(that affects a large number of people within a community, population, or region) like dengue
fever or pandemic like influenza, COVID-19.
-They may be localized to circumscribed(within limits)areas (as
some arbovirus(viruses which are transmitted by mosquitoes, ticks) diseases) or
worldwide (as Herpes simplex).
-Viruses can cause cancer in animals and birds, as well
as in humans.

Size of Viruses:
A small virus has a
diameter of about
20 nm (nanometer).
Example:
Parvovirus
A large virus have a
diameter of up to
400 nm.
Example:Poxvirus

Structure and shape:
1-Virion:
The complete infectious unit of virus particle.
2-Capsid:
The protein shell, or coat, that encloses the nucleic acid
genome.
Functions:
a. Protect the viral nucleic acid.
b. Participate in the viral infection.
c. Antigenic and specific for each virus type.
d. Provides structural symmetry to the virus particle.
3-Nucleocapsid: The capsid (protein shell of a virus) with the enclosed
nucleic acid.

• Replication of viruses:
The viral multiplication cycle can be divided into six
sequential phases, though the phases may sometimes
be overlapping.
1. Adsorption or attachment
2. Penetration
3. Uncoating
4. Biosynthesis
5. Maturation
6. Release

1- Adsorption: (the adhesion)
- Virions may come into contact with cells by random
collision but adsorption takes place only if there is an
affinity between the two.
- Differences in susceptibility to viral infection are to a
large extent based on the presence or absence of
receptors on cells.
2- Penetration:
- Virus particles may be engulfed by a mechanism
resembling phagocytosis, a process known as
‘viropexis’
- In the case of enveloped viruses, the viral envelope
may fuse (join or blend) with the plasma membrane of the host
cell and release the nucleocapsid into the cytoplasm.

3- Uncoating:
- Process of stripping(physical separation process) the the virus of its outer
layers and capsid so that the nucleic acid is released into the
cell.
- With most viruses, uncoating is effected by the action of
lysosomal enzymes of the host cell.
4- Biosynthesis:
- During this phase, viral nucleic acid, capsid protein, enzymes
necessary in the various stages of viral synthesis, assembly
and release will be synthesised.
- Certain ‘regulatory proteins’ which serve to shut down the
normal cellular metabolism and direct the sequential
production of viral components are also synthesised.
- The site of viral synthesis depends on the type of virus.

5- Maturation: (process of reaching a stage of full or advanced development)
- Assembly of daughter virions follows the synthesis of nucleic
acid & proteins.
- Virion assembly may take place in the host cell nucleus.
- Naked viruses are present intracellularly as fully developed
virions but in the case of enveloped viruses, only the
nucleocapsid is complete.
6- Release:
- Progeny (offspring of animals or plants) of bacterial viruses release by the lysis
(breaking down) of the infected bacterium. In the case of animal
viruses, release usually occurs without cell lysis.
- Myxoviruses are released by budding from the cell membrane
over a period of time.
- In the case of some viruses (varicella), transmission occurs
directly from cell to cell and poliovirus causes profound
damage to the host cell and may be released by cell lysis.

Classification and nomenclature of viruses:
- Viruses are classified on the basis of
biological, physical and chemical
properties.
- Viruses are broadly classified into DNA
and RNA viruses and then further divided
into families, subfamilies, genera and
species.

• DNA viruses:
1. Poxviridae
2. Herpesviridae
3. Adenoviridae
4. Papovaviridae
5. Parvoviridae
6. Hepadnaviridae

• RNA viruses:
1. Picornaviridae
2. Orthomyxoviridae
3. Paramyxoviridae
4. Togaviridae
5. Flaviviridae
6. Bunyaviridae
7. Arenaviridae
8. Rhabdoviridae
9. Reoviridae
10. Coronaviridae
11. Retroviridae
12. Caliciviridae
13. Filoviridae

Viroids: (small infectious pathogens)
Viroids are single-stranded circular RNA
molecules that lacks a protein coat and they
are mainly plant pathogens.
Prions: (are misfolded proteins with the ability to transmit their misfolded shape onto normal variants of the same
protein. They characterize several fatal and transmissible neurodegenerative diseases in humans and many other animals)
- Prions are infectious proteins without any
detectable nucleic acid.
- They are highly resistant to physical and
chemical agents.
- They produce slow infections with very long
incubation period.

- Diseases caused by prions in animals
include, scrapie (a disease of sheep involving the central nervous system) of
sheep and goats, mink encephalopathy,
bovine spongiform encephalopathy.
- Responsible for Kuru (Kuru is a very rare disease. It is caused by an infectious
protein (prion) found in contaminated human brain tissue. Kuru is found among people from New Guinea who practiced
a form of cannibalism in which they ate the brains of dead people as part of a funeral ritual),
Creutzfeldt-Jakob disease ((CJD), also known as subacute
spongiform encephalopathy or neurocognitive disorder due to prion disease, is a fatal degenerative brain
disorder. Early symptoms include memory problems, behavioral changes, poor coordination, and visual
disturbances) and some other chronic
neurological degenerative diseases of
humans.

5. Basic properties of virus

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