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What Is A Virus? What Is A Viral
A virus (from the Latin virus meaning
toxin or poison) is a microscopic
organism consisting of genetic material
(RNA or DNA) surrounded by a protein,
lipid (fat), or glycoprotein coat.
Some microbiologists classify viruses as
microorganisms, while others don't
because they are "nonliving" and describe
viruses as microscopic infective agents.
Viruses are unique microorganisms
because they cannot reproduce without a
host cell. After contacting a host cell, a
virus will insert genetic material into the
host and take over that host's functions.
The cell, now infected, continues to
reproduce, but it reproduces more viral
protein and genetic material instead of its
usual products. It is this process that
earns viruses the classification of
What are "friendly" viruses?
Most of us know about friendly bacteria
that exists in our intestines and help us
digest food. Scientists from San Diego
State University reported in the
Proceedings of the National Academy of
Sciences (May 2013 issue) that humans
also carry friendly viruses that help
protect us from dangerous bacteria,
including E. coli.
Jeremy Barr and team believe their
discovery may change how several
diseases are treated. They discovered that
mucus contains bacteriophages, viruses
that actively protect their hosts form
harmful bacteria by destroying them.
Barr wrote "Taking previous research
into consideration, we are able to propose
the Bacteriophage Adherence to Mucus -or BAM -- is a new model of immunity,
which emphasizes the important role
bacteriophage play in protecting the body
from invading pathogens . . . . . We
envision BAM influencing the prevention
and treatment of mucosal infections seen
in the gut and lungs, having applications
for phage therapy and even directly
interacting with the human immune
How are viruses spread?
Viruses may spread vertically (from
mother to child) or horizontally (from
person to person). A virus's ability to
spread depends on the makeup of the
Some viruses can spread by simple
contact, exchanges of saliva, coughing, or
sneezing. Some require sexual contact,
while others go through the fecal-oral
route via contaminated food or water.
Still other viruses require an insect like a
mosquito to carry the virus from person
to person.
What diseases are caused by viruses?
Several human diseases are caused by
viruses. These include:
The common cold
Human papilloma virus
Hanta fever
HIV (the virus that causes AIDS)
Cold sores
SARS (Severe respiratory syndrome)
Epstein-Barr virus
What are the essential characteristics
of viruses?
Viruses are the most primitive cellular
and non-cytoplasmic infectious agents.
Russian botanist D.J. Iwanowski (1892)
first discovered virus in an infected
tobacco plant. However, M.W. Beijerink
(1898) coined the term virus. Then
American chemist W.M. Stanley (1935)
isolated pure crystal of Tobacco Mosaic
Viruses (TMV) and concluded that viruses
are made of nucleoproteins.
infectious property of a virus is due to its
nucleic acid.
General Characteristics of Viruses:
- Viruses are a cellular, non-cytoplasmic
infectious agents.
- They are smaller than bacteria, and this
can pass through bacteriological filter.
- Viruses are transmissible from disease
to healthy organisms.
- All viruses are obligate parasites and
can multiply only within the living host
- Viruses contain only a single type of
nucleic acid either DNA or RNA.
- Viruses are host specific that they infect
only a single species and definite cells of
the host organisms.
- Viruses are effective in very small doses.
They are highly resistant to germicides
and extremes of physical conditions.
Biological position of viruses:
Viruses lack a cytoplasmic membrane and
they do not have the basic component of a
cell. They can only replicate inside the
host cell. Outside the host cell, they are
non-living. Thus, viruses show characters
of both living and non-living.
- Non-living Characters of Viruses:
Generalized Structure of Viruses:
- Shape and size: The shape varies
considerable. They may be spherical or
golf ball-like, rod-shaped, tadpole-like,
helical or polyhedral. Plant viruses are
smaller than bacteria.
- Chemical structure and function:
Viruses have a very simple structure. The
core of the viruses is made upon of
nucleic acid, which is surrounded by a
protein coat called capsid. The nucleic
acid always contains only a single kind of
nucleic acid i.e. either DNA or RNA. The
Capsid or the protein coats: It is made up
of many identical protein sub-units called
capsomeres. The capsomeres are
composed of either one or several type of
proteins. Capsomeres are arranged in a
very symmetrical manner and give a
specific shape to a particular virus. The
host specificity of virus is due to proteins
of the capsid.
Following characters of viruses assign
them as non-living:
(a) They can be crystallized.
(b) Outside the cell, they behave like inert
(c) They do not show growth,
development, nutrition, reproduction, etc.
(d) They can be precipitated.
Living characters of viruses:
(a) They multiply within host cells.
(b) They possess genetic material, either
(c) There are definite races or strains.
(d) They exhibit mutations.
Because of the above reasons, viruses
form unique bridge between living and
non-living things.
I. The Basics of a Virus – “What is a
Virus? What is a Viral Infection?”
a) What is the definition of a virus?
b) Where does this term come from?
c) How can and do viruses spread (list
four ways)?
II. “What are the Essential
Characteristics of Viruses?”
a) List three non-living characteristics of a
virus. 6pts.
b) List four living characteristics of a
virus. 8pts.
c) Based on the above characteristics, do
you think viruses are living or non-living
Explain. 10 pts.
d) True or false
1. Virus has 2 nucleoids DNA and RNA.
2. The cell wall of the virus is very thick.
3. Virus constantly mutates.
4. Virus is contagious.
5. Beijerink discover virus in tobacco.
6. Virus is small than bacteria.
7. Virus cytoplasm is yellow.
8. Virus is parasites.
9. All viruses are round.
10. Virus reproduce using a host cell.
III. Classify in virus or bacteria (COPY
___ flu
___ bad breathe
___ yogurt
___ common cold
___ AIDS
___ Strep throat
___ E. coli
___ Digest oil
___ Warts
___ Anthrax
___ Rabies
___ Small pox
___ Non cellular
___ unicellular
___ parasites
___ Ebola
___ Has cell wall
___ Polio
___ HIV
IV. Match the letter on the diagram
with the best statement
____ Virus nucleic acid is injected into host
____ The virus nucleoid acid is copied.
____ The host cell explodes, releasing more
____ The virus attaches itself to the host cell.
____ cell reproduce more viruses.
V. Multiple choice
1. The study of viruses is a part of biology
a. they belong to the kingdom Monera.
c. they are living organisms.
b. they are about to become extinct.
d. they are active inside living cells.
2. A typical virus consists of
a. a protein coat and a cytoplasm core.
b. a carbohydrate coat and a nucleic acid
c. a protein coat and a nucleic acid core.
d. a polysaccharide coat and a nucleic acid
3. All viruses have
a. cytoplasm.
b. mitochondria.
c. ribosomes. d.DNA or RNA, but not both.
4. HIV can be transmitted
a. Through sexual contact.
b. Through the sharing of no sterile needles. infants during pregnancy or through
breast milk.
d. All of the above
VI. Draw and label
1. Draw a “robot” virus. Label its protein coat
and its genetic material.
2. Draw a rod-shaped bacterium. Label its
cytoplasm, genetic material, cell
membrane, cell wall, and flagellum (or
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