is the study of viruses. 1.viruses are a heterogenous intracellular group of microorganisms. 2.vary...
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1.Viruses are a heterogenous intracellular group
of microorganisms.
2.Vary with respect to size, morphology, and
chemical composition that contain either
deoxyribonucleic acid (DNA) or ribonucleic
acid (RNA).
3. Acellular, and metabolically inert outside
the host cell.
4.Obligate intracellular parasites
5.No ATP generating system
6.No Ribosomes or means of Protein
Synthesis
- 1. Nucleic Acid
o DNA or RNA (But never both)
- 2. Capsid (Coat Protein)
- Some Viruses:
o A. Envelope
o B. Enzymes
• Host range is determined by Viruses ability
to interact with its host cell, Binding Sites
should match Receptor Sites
Binding Sites - on viral capsid or envelope
Receptor Sites - on host cell membrane
- Most viruses have a narrow host range:
o Polio virus - nerve cells
oAdenovirus - cells in upper Respiratory Tract
• Some viruses only infect:•plants•invertebrates•protists•fungi•bacteria (Bacteriophages)
•Viruses are unique life forms different from all other living organisms, either eukaryotes or prokaryotes, for three fundamental reasons:1. The nature of environment in
which they grow and multiply.2. The nature of their genome. 3. The mode of their
multiplication.
Viruses can function and multiply
only inside another living organism,
which may be either a prokaryotic or
eukaryotic cell or any other living
cells depending on the virus.
Whereas all other living forms can use only DNA (and not
RNA) as their genetic material (genome) for information
transmission from parent to progeny, viruses can use either
DNA or RNA as their genome, that is, some viruses can
use only RNA (and not DNA) as their genetic material.
Therefore, these classes of RNA viruses have developed
new sets of enzymes for replicating and transcribing RNA
from an RNA template.
All eukaryotic and prokaryotic cells divide and multiply as a
whole unit, that is, 1 ~ 2 -- 4 ~ 8 and so on. However, viruses
do not multiply as a unit.
In fact, they have developed a much more efficient way to
multiply just as complex machines are made in a modern
factory. Different viral components are made separately from
independent templates, and then these components are
assembled into the whole and infectious units, also called
virus particles (virions).
Is the virus particles which consist of two or
three parts: the genetic material made from
either DNA or RNA, a protein coat that
protects these genes; and in some cases an
envelope of lipids that surrounds the protein
coat when they are outside a cell.
• DNA or RNA (But never both)• The nucleic acid is the genome that contains the information necessary for viral function and multiplication.• The composition of the protein shell called the viral "nucleocapsid" surrounding the genome.
ossDNA - ds DNA - ss RNA - ds RNA
oprotects viral genome from
host endonucleases
oConsists of Capsomeres
which are structural the
units of capsid
oContains binding Sites
oDerived from the membrane of the host
cell.
oContains binding Sites
oThe absence of viral membrane gives the
naked virus.
The size of virions ranges from 20 nm
(parvovirus) to -300 nm (poxvirus) in
diameter, and therefore, too small to be seen
with the light microscope. However, it can be
studied with the electron microscope, but this
microscope is very costly and special training
facilities are required to use it.
• Meaning the capsid is spiral in shape. • It surrounds a spiral shaped core of
nucleic acid.• Helical capsids are usually flexible and
rodlike. • The length of the helical capsid is
usually determined by the length of the nucleic acids, that is, viruses having shorter nucleic acids will have a shorter helical nucleocapsid.
• Helical capsids can package only single-stranded RNA, but not double-stranded DNA or RNA, possibly because of the rigidity of the double-stranded nucleic acids.
• The term cubic is used to describe Icosahedral viruses. •Viruses with icosahedral capsids possess a closed shell enclosing the nucleic acid inside. An icosahedron has 20 triangular faces, 30 edges.
•Unlike helical nucleocapsids that package only single-stranded nucleic acid, icosahedral capsids can be used to package either single- or double-stranded RNA and DNA molecules.•An icosahedral virus can be either naked or enveloped; but, unlike the helical enveloped viruses, the icosahedron capsid structure is rather rigid.
Means the capsid symmetry is
neither icosahedral nor helical.
Only the poxviruses among the
medically important viruses show
complex symmetry.
The overall shape of poxviruses is
usually described as brick-shaped.
•Enveloped Helical•Enveloped Polyhedral
• Many of the helical
viruses and a few of
Icosahedral viruses are
surrounded by an
envelope.
Because viruses possess neither cellular structure nor
organelles, they are unable to make their own proteins
and essential enzymes. They are therefore completely
dependent on their host cells for energy and
multiplication.
Outside of living cells, viruses are metabolically inactive.
The information required for a virus to replicate is
contained in its nucleic acid. This information is used by
the host cell to produce new viruses.
• The virus leave the host cell by breaking the
cytoplasmic membrane and is released out as
viroins to attack other cells (enveloped viruses
come out of the cells like budding without
rupturing the cell.)
• As the virus comes out of the cell the host cell will
rupture i.e lyses (killed) and die.
A small group of viruses are able to change, or
transform their host cells from normal cells into
abnormal ones with properties of cancerous cells,
here the cell is not dead.
Immune system of host cell produces
antibodies to fight the virus .Antibodies will
react with virus (antigen), and Antigen-
Antibody complex is produced that deposit
in different places of body e.g. Hepatitis B
deposit in glomeruli of kidney
Some viruses after infecting cells do not
replicate, or they become active for a time and
then become inactive (latent). In response to
certain stimuli, latent viruses can be
reactivated and become active replicating
particles, for example herpesviruses.
The internationally agreed system of virus
classification is based on the structure and
composition of the virus particle (virion).
In some cases, the mode of replication is also
important in classification.
Viruses are classified into various families on
this basis.
Primary characteristics used in classification:
•Viruses are classified according to the nature of
their genome and their structure
VIRAL CLASSIFICATION
Nucleic acid
RNA or DNA
single-stranded or double-stranded
non-segmented or segmented
linear or circular
If genome is single stranded RNA, can it
function as mRNA?
whether genome is diploid (such as in
retroviruses)
Virion
structure
symmetry (icosahedral, helical, complex)
enveloped or not enveloped
number of capsomers
Adsorption
The first step in infection of a cell is attachment to the cell surface. The viral attachment protein recognizes specific receptors, which may be protein, carbohydrate or lipid, on the outside of the cell. Cells without the appropriate receptors are not susceptible to the virus.
Penetration occurs almost rapidly after attachment and is a next step for gaining entry into the cytoplasm by crossing the plasma membrane.
Thus, penetration allows the viruses to deliver their genome into the host cells to initiate replication.
The penetration process differs for different viruses:
a) Penetration by the process of endocytosis is common to both enveloped and non-enveloped viruses.
b) While some virus species can directly penetrate the plasma membrane and inject their genetic material into the cytoplasm, the majority of viruses enter cells via endocytosis.
c) Direct penetration is observed only in non-enveloped viruses, membrane fusion is observed only in enveloped viruses. Some of the non-enveloped viruses like picornaviruses and phages are capable of directly injecting their genome into the host cell.
3-Uncoating:Release of the viral genome
from its protective capsid to enable the viral nucleic acid to replicate.• 4-Transcription:Synthesis of m-RNA• 5-Translation:The viral m-RNA is translated on
cell ribosomes into structural and non-structural proteins.