College of Veterinary Medicine
Virology- Lecture 2
Virus Morphology
Virology: Is the science that deals with discovery, isolation, identification, characterization,
pathogenecity, pathogenesis and classification of viruses.
Virus:Is an infectious agent which is obligate intracellular parasite require a hosts to cause damage, they
contain only one type of functional nucleic acid either RNA or DNA.
Viruses are not organisms, and contain no functional ribosomes, mitochondria or other cellular
organelles. The virus was capable to replication only within the living cells such as bacteria,
animals or plants by using the synthesizing machinery of the cell cause the synthesis of
specialized structures that can transfer viral nucleic acid to other cells.
Viruses are generally made up of two parts, the outer protein shell (called a capsid) and the
genetic information inside. Generally the morphology of a virus can be one of two structures,
that of a sphere or that of a tube.
Viruses can be found either inside a cell (intracellular) or outside of a cell (extracellular).
If it is found extracellular, the virus is called a Virion.
Virions: are complete virus particle and is made up of from ten to fifteen percent nucleic acid
and from fifty to ninety percent protein. The general purpose of the proteins is to protect the
genetic information.
Viruses can be found either inside a cell (intracellular) or outside of a cell (extracellular). If it is
found extracellular, the virus is called a Virion. A virion contains a protein coating called a
capsid, which surrounds the core of the virus containing the nucleic acid (either DNA or RNA).
Some virions also contain an envelope which is made up of a phospholipids membrane. Both the
capsid and the envelope are important in protection and providing shape to the virus.
Viriod:Viroids contain RNA only. They are small (less than 400 nucleotides), single
stranded, circular RNAs. The RNAs are not packaged, do not appear to code for any
proteins, and so far have only been shown to be associated with plant disease such as
Bean mosaic virus, Wound tumor virus, Potato yellow dwarf virus.
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Prions:Are proteinaceous infectious particles and do not contain nucleic acid but contain a
single protein called PrP, prion PrP leads to prion disease such as the mad cow disease or bovine
spongiform encephalitis(BSE), Scrapie in sheep, Prions present in nervous tissue, and
transmitted by Surgical instruments and Consumption of infected meat, nervous tissue.
Capsid: The protein shell, or coat that encloses the nucleic acid genome.
Capsomeres: Morphological units seen in the electron microscope on the surface of icosahedral
virus particles.
General characteristic of virus:
1. Very small agent can not seen ordinary (ranging from about 20 nm to about 300 nm in
diameter),whereas a bacterial cell like a staphylococcus might be 1000nm in diameter, the largest
of the human pathogenic viruses, the poxviruses, measure only 300nm and the smallest, the
poliovirus, is only 20nm in diameter. Are mostly, therefore, beyond the limit of resolution of the
light microscope and have to be visualized with the electron microscope.
2. They contain one type of nucleic acid either RNA or DNA.
3. They multiply by replication of their nucleic acid comparing with other microorganisin which
replicated by binary fission.
4. May be surrounded by a lipid-containing membrane called envelope.
5. They don’t carry enzyme.
6. They don’t growth on synthetic media but grow in living cell.
7. They can pass through the filters that don’t allow bacteria to do so.
8. There are no ribosomes and organelles inside the virus.
9. Antibiotic have not affected on viruses, whereas viruses sensitive to interferon.
10. Some viruses can caused latent infection.
11. The nucleic acid of some viruses can caused infection
The Differences between Viruses and Unicellular microorganisms
Property
Bacteria Rickettsiae Mycoplasmas Chlamydiae Viruses
1- >300nm diameter
+
+
±
±
2- Growth on nonliving
+
+
media
3- Binary fission
+
+
+
+
4- DNA and RNA
+
+
+
+
5- NA infectious
+a
6- Ribosomes
+
+
+
+
7- Metabolism
+
+
+
+
8- Sensitivity to antibodies
+
+
+
+
-b
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9- Sensitive to Interferon
Legends
-
-
-
-
+
+ Positive, - negative, NA=Nucleic Acid,
a-Some among both DNA and RNA viruses
b-The antibiotic rifampicin inhibits poxvirus replication
Viral Morphology
In 1959 the knowledge of viral ultrastructure was trasnsformed when negative staining was
applied to the electron microscopy of viruses. A solution of potassium phosphotungstate (PTA),
when used to stain virus particles, fills the interstices of the viral surface, giving the resulting
electron micrograph.
The virion of the simplest viruses consists of a single molecule of nucleic acid surrounded by a
protein coat, called the capsid. In some of the more complex viruses the capsid surrounds a
core, which consists of one or more proteins surrounding the viral nucleic acid; the capsid and
associated nucleic acid then constitute the nucleocapsid. For some viruses the capsid is
surrounded by a lipoprotein envelope.
The capsid is composed of a defined number of morphological units called capsomers, which
are held together by noncovalent bonds. Within an infected cell, the capsomers self-assemble to
form the capsid. The manner of assembly is strictly defined by the nature of the bonds formed
between individual capsomers, which give the symmetry to the capsid.
Contain a single type of nucleic acid
DNA
Or, Or, Or, Or, Or, Or
RNA
The virus contain a protein coat (sometimes envelope of lipids, proteins.
The virus multiply inside living cells by using the synthesizing machinery of the cell
cause the synthesis of specialized structures that can transfer viral nucleic acid to other cells.
All viruses contain the following two components:
1- Nucleic acid genome
2-Capsid: That covers the genome. Together this is called the nucleocapsid
In addition, many animal viruses contain
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3- Envelope: Envelope: A lipid- containing membrane that surrounds some virus particles. It
is acquired during viral mutation by budding process through a cellular membrane. Virusencoded glycoproteins are exposed on the surface of the envelope. These projections are called
peplomers.
1- Nucleic acid genome (Viral Genomes): While the genomes of all known cells are
comprised of double stranded DNA, the genomes of viruses can be comprised of
single or double stranded DNA or RNA. They can vary greatly in size, from
approximately 5-10 kb (Papovaviridae, Parvoviridae, etc.) to greater than 100-200 kb
(Herpesviridae, Poxviridae). The virus have few thousand to 250,000 nucleotides, Bacteria
have 4 million.
The known structures of viral genomes are summarized below.
DNA: Double Stranded - linear or circular, or Single Stranded - linear or circular.
One family DNA ss (Parvoviridae)
RNA: Single Stranded - linear: and one family RNA double strand (such as,
Reovirus, rotavirus). These single stranded genomes can be either( + )sense,( -) sense. The
sense strand is the one that can serve directly as mRNA and code for protein, so for these
viruses, the viral RNA is infectious.
The viral mRNA from( - )strand viruses is not infectious, since it needs to be copied or
converted into the( +) strand before it can be translated. The genome of some RNA viruses
is segmented, meaning that a virus particle contains several different molecules of RNA,
like different chromosomes.
Double-stranded RNA viruses
Further information: The double-stranded (ds)RNA viruses represent a diverse group of
viruses that vary widely in host range (humans, animals, plants, fungi, and bacteria).
Members of this group include the rotaviruses, renowned globally as the most common
cause of gastroenteritis in young children,
And Reovirus, that contain double-stranded RNA and are associated with various diseases
in animals, including human respiratory and gastrointestinal infections.
The differences between DNA and RNA viruses
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2- Capsid: Viral genomes are surrounded by protein shells known as capsid.Capsid is the
storage site for genome;a capsid is made up of protein and almost always made up of repeating
structural subunits that are arranged in one of two symmetrical structures, a helix or
icosahedrons. Both helical and icosahedral structures are described in more detail below.
The function of the capsid
1-To protect viral nucleic acid.
2- Interactspecifically with the viral nucleic acid for packaging.
3-Interact with vector for specific transmission.
4-Interact with host receptors for entry to cell.
5- Allow for release of nucleic acid upon entry into new cell and assist in processes of viral
and/or host gene regulation.
3- Envelope: A lipid- containing membrane that surrounds some virus particles. It is acquired during
viral mutation by budding process through a cellular membrane. Virus-encoded glycoproteins are exposed
on the surface of the envelope. These projections are called peplomers.
Viral Symmetry: Is the arrangement of the capsomers to give the final morphology of the
viruses.
Types of Viral Symmetry:
Electron microscope and X-ray diffraction have made it possible to resolve fine differences in
the basic morphology of viruses. Viral architecture can be grouped into three types based on the
arrangement of morphologic subunits.
1- Helical symmetry
In case of helical symmetry, it has helix protein subunits are bound in a periodic way to the viral
nucleic acid, winding. The filamentous viral nucleic acid-protein complex (nucleocapsid) is then
coiled inside a lipid containing envelope. In all such animal viruses the helical nucleocapsid is
wound into a secondary coil and enclosed within a lipoprotein envelope
The first and best studied example is the plant tobacco mosaic virus (TMV), which contains a SS
RNA genome and a protein coat made up of a single, 17.5 kd protein. This protein is arranged in
a helix around the viral RNA, with 3 nt of RNA fitting into a groove in each subunit.In tobacco
mosaic virus, a single polypeptide forms a capsomer and 2130 capsomers assemble in ahelix .
Helical capsids can also be more complex, and involve more than one protein subunit.
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Several
families
of
animal
virus contain helical
nucleocapsids, including
the Orthomyxoviridae (influenza), the Paramyxoviridae (bovine respiratory syncytial virus), and
the Rhabdoviridae (rabies). All of these are enveloped viruses.
2-Icosahedral or Cubic Symmetry:
In these structures, the subunits are arranged in the form of a hollow, spherical structure, with
amount of nucleic acid that can be packaged by the particles. An icosahedron is defined as being
made up of 12 vertices (corners), 30 edges and 20 faces, each an equilateral triangle. It has axes
of two, three and fivefold rotational symmetry passing through its edges, faces and vertices
respectively.
Since proteins are not equilateral triangles, each face of an icosahedron contains more than one
protein subunit. The simplest icosahedron is made by using 3 identical subunits to form each
face.
Many viruses have too large a genome to be packaged inside an icosahedron made up of only 60
polypeptides (or even 60 subunits), so many are more complicated. In these cases, each of the 20
triangular faces is divided into smaller triangles; and each of these smaller triangles is defined by
3 subunits.
The icosahedral symmetry virus including envelope virus such as (Herpesvirus and togavirus, yellow
fever
virus, rubella
virus),
whereas
the
non-envelope(naked)
virus
included
(poliovirus, adenovirus, hepatitis A virus).
The number of capsomers can be calculated by the following equation:
M=10(N-1)2+2
If N=5
M=10(N-1)2+2
M=10(5-1)2+2
M=162 this is for herpes virus
M/ number of capsomer, N /number of capsomer in edges.
3- Complex Symmetry:
Some large viruses like poxviruses have a multilayer complex structure, details of which have
not been clarified and capsomers have not yet identified like (Pox virus) have a complex
symmetry, it means not helical and not Icosahedral and have shape look like a brick.
4-Binary Symmetry:
Some viruses like Bacteriophages have both cubical and complex or helical symmetry.
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They have a complex structure consisting of icosaheadral head carry the genetic
material, bound to helical tail. The genetic material can be ssRNA, dsDNA, or dsDNA
along with either circular or liner arrangement.
Viral Envelope
In some animal viruses, the nucleocapsid is surrounded by a membrane, also called
an envelope. This envelope is made up of a lipid bilayer, and is comprised of hostcell lipids. It also contains virally encoded proteins, often glycoproteins which are
trans-membrane proteins. These viral proteins serve many purposes, such as binding
to receptors on the host cell, playing a role in membrane fusion and cell entry, etc.
They can also form channels in the viral membrane.
The envelope of influenza virus consists of a lipid bilayer in which several hundred glycoprotein
peplomers or spikes are inserted; beneath the lipid bilayer there is a virus-specified matrix
protein. The glycoprotein peplomers of influenza virus comprise two different proteins,
hemagglutinin (a rod-shaped trimer) and neuraminidase (a mushroom-shaped tetramer), each of
which consists of a hydrophobic internal domain, a transmembrane domain, and a hydrophilic
external domain.
Many enveloped viruses also contain matrix proteins, which are internal proteins that
link the nucleocapsid to the envelope. They are very abundant (ie, many copies per
virion), and are usually not glycosylated. Some virions also contain other, nonstructural proteins that are used in the viral life cycle. Examples of this are
replicases, transcription factors, etc. These non-structural proteins are present in low
amounts in the virion.
Enveloped viruses are formed by budding through cellular membranes, usually the
plasma membrane but sometimes an internal membrane such as the ER, golgi, or
nucleus. In these cases, the assembly of viral components (genome, capsid, matrix)
occurs on the inside face of the membrane, the envelope glycoproteins cluster in that
region of the membrane, and the virus buds out. This ability to bud allows the virus to
exit the host cell without lysing, or killing the host. In contrast, non-enveloped
viruses, and some enveloped viruses, kill the host cell in order to escape.
The presence of the envelop on the virus were divided the viruses to envelope virus
and naked virus.
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