Study of viruses

Viruses, Characteristics,structure, classification, reproduction, impacts

1. Bahauddin Zakariya University Lahore
Campus

All about viruses
General Microbiology
BS. Biotechnology

Instructor:
Sir Hafiz Muzammel Rehman
Submitted By:
Umair Tariq
Ali Nawaz
Muhammad Ahmad
Muhammad Anas
Rizwan Abbas
Muhammad Hamza

2. Viruses
Contents and References


S.no

Tittle

reference

page


A

Introduction
i: definition
ii: description.
iii.: discovery

A textbook of biology chapter 5
Author
Doctor Abdul Qayoom



01


B

The Nature of Viruses
iv. Viral morphology
v. genetic make up
vi. habitat
vii. Size Range

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biology 1lectures videos part 1

A textbook of biology chapter 5
Author Doctor Abdul Qayoom

02



C

Structure
viii. A comparison with cell
ix. The Position of Viruses
x. characteristics

A textbook of biology chapter 5
Author Doctor Abdul Qayoom
Microbiology book chaptr 13 by Tortoa
Biodiversity/Thallophytes by Haq Nawaz

05




D
Viral growth and reproduction
xi .Growth?
xii. Reproduction?
xiii. Multiplication cycle
xiv. Life cycle of bacteriophage

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Microbiology book chaptr 13 by Tortoa

08



E

Diversity of viruses
xv. classification
xvi. DNA *RNA viruses


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biology 1lectures part 3

11



F

Importance of viruses
xvii. impacts of viruses
xviii, Viral biotechnology


Microbiology and immunology
Iv edition by
Levin Anderson

14

3. A. Introduction
i:Define virus?
Viruses are genetic elements closed in protein and are not considered
as organisms, as they cannot reproduce independently. Because of
their disease producing ability, viruses are important biological
particles.
ii:Brief description
 Living organisms are cellular and able to grow and reproduce
independently, guided by information encoded within genetic
material (DNA).
 The simplest creatures living on earth today that satisfy these
criteria are bacteria. Even simpler than bacteria are viruses.
 Viruses are so simple that they do not satisfy the criteria for
“living.”
 Viruses possess only a portion of the properties of organisms.
Viruses are literally “parasitic” chemicals, segments of DNA or
RNA wrapped in a protein coat.
 Earlier theories that viruses represent a kind of halfway point
between life and nonlife have largely been abandoned. Instead,
viruses are now viewed as detached fragments of the genomes
of organisms due to the high degree of similarity found among
some viral and eukaryotic genes.
iii:Discovery of viruses
 Biologists first began to suspect the existence of viruses near the
end of the nineteenth century.
 European scientists attempting to isolate the infectious agent
responsible for hoof-and-mouth disease in cattle concluded that
it was smaller than a bacterium.
 Investigating the agent further, the scientists found that it could
only reproduce itself within living host cells that it infected. The
infecting agents were called viruses.

4.  The true nature of viruses was discovered in 1933, when the
biologist Wendell Stanley prepared an extract of a plant virus
called tobacco mosaic virus (TMV) and attempted to purify it.
 TMV virus was acting like a chemical off the shelf rather than
an organism. Stanley concluded that TMV is best regarded as
just that—chemical matter rather than a living organism.
B: The Nature of Viruses

Molecularly a virus is composed of regular, repeating subunits that
give rise to their crystalline appearance with external coating known
as Capsid formed by combination of identical subunits called
capsomers
Made up of protein molecules
Envelope- is external protective layer in 13 of the 20 families of
animal viruses and naked viruses are those which lack envelop.
The central genetic Core is consisting of DNA or RNA. The capsid
and the nucleic acid together are called the Nucleocapsid and a full
formed virus that is able to establish an infection in a host cell is
called as virion
The viral capsid and envelop play important role in infection of host
and stimulation.

5. iv. Virus Genetic make up
 All viruses have the same basic structure: a core of nucleic acid
enclosed in protein.
 Individual viruses contain only a single type of nucleic acid,
either DNA or RNA.
 The DNA or RNA genome may be linear or circular, and single-
stranded or double-stranded.
 Viruses are frequently classified by the nature of their genomes.
RNA-based viruses are known as retroviruses.
 Nearly all viruses form a protein sheath, or capsid, around their
nucleic acid core.
 The capsid is composed of one to a few different protein
molecules repeated many times.
 In some viruses, specialized enzymes are stored within the
capsid. Many animal viruses form an envelope around the
capsid rich in proteins, lipids, and glycoprotein molecules.
 Some of the material of the envelope is derived from the host
cell’s membrane; the envelope does contain proteins derived
from viral genes as well.

6. v.Habitate
 Viruses occur in virtually every kind of organism that has been
investigated for their presence. However, each type of virus can
replicate in only a very limited number of cell types.
 The suitable cells for a particular virus are collectively referred
to as its host range.
 The size of the host range reflects the evolved histories of the
virus and its hosts.
vi. Size Range
 Their size varies as follows
Smallest infectious agents
 Most are so small; they can only be seen with an electron
microscope
 Proviruses- around 20 nm in diameter
 Mimi viruses- up to 450 nm in length
vii.Examining viruses
Viruses are too small to be seen with the first microscopes and
examined by:
 Special stains and an electron microscope
 Negative staining outlines the shape
 Positive staining shows internal details C.

7. Structure

viii .Structure of virion and a living cell
 Viruses vary greatly in appearance and size.
 The smallest are only about 17 nanometers in diameter, and the
largest are up to 1000 nanometers (1 micrometer) in their
greatest dimension .
 The largest viruses are barely visible with a light microscope,
but viral morphology is best revealed using the electron
microscope.
 Viruses are so small that they are comparable to molecules in
size; a hydrogen atom is about 0.1 nanometer in diameter, and a
large protein molecule is several hundred nanometers in its
greatest dimension.

a: virion b: Eukaryotic cell

8. ix. A comparison with living cell

Structure RNA or DNA core
(center), protein coat
(capsid
Cell membrane,
cytoplasm, genetic
material, organelles
Genetic Material DNA or RNA DNA or RNA
Reproduction Replication
inside host cell
Asexual or Sexual
Growth and
Development
NO YES—Multicellular
Organisms
Obtain and Use
Energy
NO YES
Response to
Environment
NO YES

x. A summary of virus characteristics
Virus properties include
 Obligate intracellular parasites of bacteria, protozoa, fungi,
algae, plants, and animals.
 Ultramicroscopic size, ranging from 20 nm up to 450 nm
(diameter).
 Not cellular in nature; structure is very compact and
economical.
 Do not independently fulfill the characteristics of life.
 Inactive macromolecules outside the host cell and active only
inside host cells.
 Basic structure consists of protein shell (capsid) surrounding
nucleic acid core.
 Nucleic acid can be either DNA or RNA but not both.
 Nucleic acid can be double-stranded DNA, single-stranded
DNA single-stranded RNA, or double-stranded RNA.

9.  Molecules on virus surface impart high specificity for
attachment to host cell.
 Multiply by taking control of host cell’s genetic material and
regulating the synthesis and assembly of new viruses.
 Lack enzymes for most metabolic processes.
 Lack machinery for synthesizing proteins.
 Most RNA viruses multiply in & are released from the
cytoplasm.
 l Viral infections range from very mild to life threatening.
 l Many viruses are strictly human in origin, others are zoo noses
transmitted by vectors.



D. Viral growth and reproduction
xi . Can a virus grow?
As viruses are infectious agent made up of nucleic acid (DNA or
RNA) wrapped in a protein coat called a capsid and have no
nucleus, no organelles, no cytoplasm or cell membrane and are
Non-cellular particles due to the facts,
 That they do not breathe.
 Viruses do not metabolize.
 Viruses do not grow.
However, they increase their population in some conditions so
we can say they can reproduce.
xii.Viral reproduction
 Viruses can reproduce only when they enter cells and utilize the
cellular machinery of their hosts.
 Viruses’ code their genes on a single type of nucleic acid, either
DNA or RNA, but viruses lack ribosomes and the enzymes
necessary for protein synthesis.

10.  Viruses are able to reproduce because their genes are translated
into proteins by the cell’s genetic machinery.
 These proteins lead to the production of more viruses.
 In general, the details of a viral replication cycle are dictated by
the type of nucleic acid carried into the host cell by the infecting
virion..
.Transduction:
Transduction occurs when DNA from one bacterial cell is transferred
to another bacterial cell through phage. When assembly of new virus
occurs in the host cell, bacterial DNA occasionally is packaged into
the viral protein coat.
When the bacterium lyses the viral particles that are released can
adsorb to a new bacterial cell. If the virus that adsorbs happens to be
one that packaged bacterial DNA, then the newly infected bacterium
will receive bacterial DNA and not viral DNA.

xiii.Replication /Multiplication cycle
Viral multiplication proceeds as following manner.
Adsorption,
Penetration,
Uncoating,
Synthesis,
Assembly and Release
Adsorption.
Virus encounters susceptible host cells and adsorbs specifically to
receptor sites on the cell membrane because of the exact fit required,
viruses have a limited host range.
Penetration.

11. Flexible cell membrane of the host is penetrated by the whole virus or
its nucleic acid
Endocytosis: entire virus engulfed by the cell and enclosed in a
vacuole or vesicle. The viral envelope can also directly fuse with the
host cell membrane
Uncoating.
Enzymes in the vacuole dissolve the envelope and capsid.
Synthesis.
Free viral nucleic acid exerts control over the host’s synthetic and
metabolic machinery. DNA viruses- enter host cell’s nucleus where
they are replicated and assembled
DNA enters the nucleus and is transcribed into RNA. The RNA
becomes a message for synthesizing viral proteins (translation)
New DNA is synthesized using host nucleotides
Assembly
Mature virus particles are constructed from the growing pool of parts
Nonenveloped and complex viruses are released when the cell lyses
or ruptures while Enveloped viruses are liberated by budding or
exocytosis. Anywhere from 3,000 to 100,000 virions may be released,
depending on the virus. Entire length of cycle- anywhere from 8 to 36
hours
xiv.Life cycle of bacteriophage
Bacteriophages are viruses that infect bacteria, Bacterial viruses
exhibit two sorts of reproductive cycles.
The Lytic Cycle
During the process of bacterial infection by T4 phage get attached to
the bacterium cell surface. The tail contracts and the tail tube passes
through an opening that appears in the base plate, piercing the

12. bacterial cell wall. The contents of the head, mostly DNA, are then
injected into the host cytoplasm.
When a virus kills the infected host cell in which it is replicating, the
reproductive cycle is referred to as a lytic cycle.
The Lysogenic Cycle
Many bacteriophages do not immediately kill the cells they infect,
instead integrating their nucleic acid into the genome of the infected
host cell. While residing there, it is called a prophage.
At least 23 proteins are associated with the development and
maturation of phage, and many other enzymes are involved in the
integration of these viruses into the host genome.
The integration of a virus into a cellular genome is called lysogeny.
At a later time, the prophage may exit the genome and initiate virus
replication. This sort of reproductive cycle, involving a period of
genome integration, is called a lysogenic cycle.
Viruses that become stably integrated within the genome of their host
cells are called lysogenic viruses r temperate viruses.

13. Cultivation of viruses
We can grow viruses artificially for beneficial purposes by different
techniques.
Primary purposes of viral cultivation
 To isolate and identify viruses in clinical specimen.
 To prepare viruses for formation of vaccines we need
 To do detailed research on viral structure, multiplication cycles,
genetics, and effects on host cells.
Using Live Animal Inoculation
 Specially bred strains of white mice, rats, and rabbits
 Occasionally invertebrates or nonhuman primates are used
 Animal is exposed to the virus by injection

14. Using Bird Embryos
 Enclosed in an egg- nearly perfect conditions for viral
propagation
 Chicken, duck, and turkey are most common
 Egg is injected through the shell using sterile techniques and we
can achieve result useable for different putposes
Tissue culture technique
 Most viruses are propagated in some sort of cell culture
 The cultures must be developed and maintained
 Animal cell cultures are grown in sterile chambers with
special media
 Cultured cells grow in the form of a monolayer which may be
Primary or continuous

E. Diversity of viruses
xv.classification
Viruses are classified on the basis of habitat (host).which is trivial
system beside this Viruses are classified on following criteria.
 Structure
 Chemical composition
 Similarities in genetic makeup
However International Committee on the Taxonomy of Viruses,
which includes
3 orders
63 famillies “-viridae”
263 genera “-virus”
Some virologists use a species naming system, but it is not an
official designation

15. Classification on basis of host
According to the habitat viruses are classified as
Animal viruses:
Viruses that lives or attack animals
Rabies, Polio, Mumps, Chicken pox, Small pox, and Influenza.
Plant Viruses:
Are viruses which show their live characteristics when attached
to plants.
Tobacco mosaic virus (TMV), Banana streak virus, Carrot thin
leaf virus
Bacterial Virus: Viruses which complete its life cycle in
bacteria
Bacteriophages.
Viruses which life activities are depend on bacterial host.
(T1, T2, T3, and T4.)
Other viruses include protozoan algal and fungal parasitic
viruses depending upon them.
Another criterion
With relevant to morphology of viral structure viruses are
organized as Enveloped and Nonenveloped viruses.
However they are also arranges subclasses of DNA and RNA
viruses
According to genetic consequences viruses are classified as.
They are called either a DNA or RNA virus depending on the
type of nucleotide in the make-up.

16. DNA Viruses
DNA Viruses are the viruses which consist of DNA genome.
They complete their activities by transcription and most of them
attack on organisms of similar genome.
RNA Viruses
RNA Viruses are the viruses which consist of RNA genome.
They complete their activities by reverse transcription.

xvi. According to genetic based organization the viral genomes
(sets of genes) maybe
 Double stranded DNA: pox viruses herpes virus.
 Single stranded DNA : Parvo viruses
 Double stranded RNA: filio virus . Arena virus.
 Single stranded RNA : Reo viruses
They may be linear or circular
The smallest have only 4 genes and largest have several
hundred.
RNA Viruses
Mostly of them are single-stranded
Positive-sense RNA:
Genomes those are ready for immediate translation into proteins
Negative-sense RNA:
Genomes have to be converted into the proper form to be made
into proteins
Segmented-
Individual genes exist on separate pieces of RNA
Rhabdio virus

F: Importance of viruses
As these particles exist in nature it has great importance in the
ecosystem, which may be very fascinating or harmful,
*Harmful
 Causes disease—pathogenic
 Disease producing agent—pathogen

17.  Human Diseases: Warts, common cold, Influenza (flu),
Smallpox, Ebola, Herpes, AIDS, Chicken pox, Rabies are due to
virus actions.
 Viruses can be prevented with vaccines, but NOT treated with
antibiotics.
 Cytopathic effects- virus-induced damage to the cell that alters
its microscopic appearance
 Inclusion bodies- compacted masses of viruses or damaged cell
organelles
 Oncoviruses- mammalian viruses capable of initiating tumors
 Beneficial
 In Genetic Engineering harmless virus are used as genetic
vectors which carry good genes into cells.
 Viral envelop Stimulate the immune system to produce
antibodies that can protect the host cells against future infections
 Viral genome contain enzymes for specific operations within the
host cell,
 example : Polymerases to synthesize DNA and RNA and
Replicases to copy RNA, essential in Genetic engineering
The use of viruses biotechnology
investigation
The strategies that viruses employ to infect human and bacterial cells
can be used as tools in microbiology. Scientists have worked with
epidemiologists to use a virus to detect the slow-growing organism
Mycobacterium tuberculosis (Mtb). It can take up to a month to see a
colony of Mtb. But now it is easy to investigate by making phages
under controlled systems.
Bio control.
Viruses are also being used to prevent the spread of plant and animal
pathogens, called bio control.
 it is possible to isolate a phage that infects the plant pathogen
Erwinia carotovora. The farmer can spray a dilute solution of

18. phage on the plant, which can prevent the bacterium from
destroying a potato crop.
 The first vaccine was developed in the late 1700s by Edward
Jenner to fight smallpox
Genetic manipulation
Viruses in causing, detecting, and controlling diseases there are many
other uses. We can use viruses to help in genetic manipulation, i.e.
make a genomic library or in cloning. To treat genetic diseases (gene
therapy) viruses are used to transport replacement genes to the
problem human tissues.
xvii.Some diseases associated with viruses
In recent years, several very dangerous “emergent viruses” have risen
to prominence.
HIV, the AIDS virus, seemed to appear suddenly in the early 1980s.
Acquired Immune Deficiency Syndrome (AIDS)
AIDS is caused by a retrovirus. It attacks helper T-lymphocytes. A
retrovirus contains RNA. It also contains reverse transcriptase an
enzyme which produces viral DNA from viral RNA.The virus can
thus replicate itself the AIDS virus attaches itself by glycoprotein on
its surface to receptors on the helper T cell surface.

19. Treatment
Some recently developed drugs to combat some viruses, mostly by
interfering with viral nucleic acid synthesis.
AZT (Azido thymidine) interferes with reverse transcriptase of HIV.
Acyclovir inhibits herpes virus DNA synthesis. AIDS cocktail
• AZT, 3TC, and a protease inhibitor.
Hepatitis
Hepatitis is an inflammatory disease of liver cells that may
result from several viruses
A virus interferes with liver’s excretion of bile pigments bilirubin
accumulates in blood &tissues causing jaundice, a yellow tinge in
skin & eyes
Hepatitis is caused due to interaction of 3 principal viruses a, b and
c
Cytomegalovirus (CMV)
It is a disease caused by a type of HIV virus leading immunologically
defective. New born may exhibit enlarged liver & spleen, jaundice,
capillary bleeding microcephaly,
Treatment may be biochemical drugs which inhibit such viruses’
ganciclovir, valvcyclovir, foscarnet.



The end