VIROLOGY

BY:GROUP EIGHT
KAKOOZA JAMES:PHD/836/2019/2020
KOMUGISHA BRENDA:839/2019/2020
NABANGI JAMES :DDT/653/2019/2020
AMPIIRE JORAM:PHD/819/2019/2020
AWUBWA ERIAS:PHD/835/2019/2020
MUHEREZA GERALD:PHD/840/2019/2020
NAMBAZIIRA KAMIAT :PHD/820/2019/2020
BIGALUKA WASSWA AMROSE:PHD/844/2019/2020
VIROLOGY
This refers to study of all kinds of viruses.
Viruses are the smallest known infective agents
and are perhaps the simplest form of life known.
Viruses are particles composed of an internal core
containing of either DNA or RNA covered by a
protective protein coat.
Viruses are obligate intracellular parasites.
They require having a host cell for them to
multiply.
Replication is by assembly of the individual
components rather than by binary fission.
CHARACTERISTIC
Viruses do not have a FEATURES OF VIRUSES
cellular organization.
They are filterable agents.
They are obligate intracellular parasites.
They contain only one type of nucleic acid i.e. DNA or
RNA but not both.
They lack enzymes necessary for protein and nucleic
acid synthesis and are dependant on the replication
and synthetic machinery of the host cell.
The virion of the virus particle consists of the
genome packed into the protein coat (capsid) which
is sometimes enclosed by an envelope of lipids,
proteins and carbohydrates.
They are unaffected by bacterial antibiotics
GENERAL STRUCTURE OF THE VIRUS
NATURE OF ANIMAL VIRUSES
The nature of animal viruses can be described in two forms i.e. morphology
and composition.
1. Morphology
Animal viruses morphology can be divided into two important types
basing on how the nucleic acid is packed i.e. rod-like/helical and
spherical/isomeric.
 Rod-like/helical form.
The nucleic acid of the virus is surrounded by a specific geometric array of a
protein molecule that form a coat called a capsid.
The capsid is made up identical subunits called capsomeres. When the
protein subunits bind in a periodic way along the nucleic acid, a helical
shape is produced.
It is always observed as a rod containing flexible cells.
 Spherical/isomeric form
In spherical viruses, the nucleic acid which is condensed within the virus is
dependant on the organization of the protein subunits s surrounding it.
The capsid surrounding the nucleic acid of a spherical virus has icosahedral
(20-sided object) symmetry.
Most animal viruses such as adenovirus exhibit a combination of both helical
2. COMPOSITION
The composition of the animal viruses include: nucleic acids,
lipids, and proteins.
 Nucleic acids
The nucleic acid of the animal virus is either DNA or RNA
and may be single or double stranded . Most viral nucleic
acids are linear molecules except papovirus whose DNA is
circular.
The molecular weight of animal virus nucleic acids range
from 1.5-3x10*6nm for picornaviruses to 2x10*8nm for the
largest viruses, the poxviruses.
Some viruses have segmented genomes.
A genome is a complete set of hereditary factors. The
reoviruses and rotaviruses consist of 10 to 11 segments
respectively.
The influenza and the renaviruses are also segmented. One of
the segmentations is for efficient genetic recombination
than unsegmented viral genomes
 Lipids
Many nucleocapsids are surrounded by an
envelope of lipids that mimics the composition of
the cytoplasmic membrane of infected host.
The lipid envelope is a bilayered structure that acts
as a protective device for the nucleocapsid.
If the envelope is removed, the virus loses its
infectivity.
In addition to the structural proteins that make up the capsid, there are other
 proteins
Proteinsthat have enzymatic as well as structural functions.
Some enveloped viruses have nucleocapsids that are tightly bounded/enveloped
within the assembly of proteins.
The nucleocapsid is surrounded by the layer of proteins called protein shell.
Complex proteins such as glycoprotein are found on the surface of viral
envelopes and are in form of spikes or peplomers which act as ant genetic
determinants and are involved in binding a virus to host tissue. One of the
types of glycoprotein spikes is found on the influenza virus which gives it
ability to agglutinate with red blood cells and is called a heamagglutinin spike
Enzymatic proteins are also found in or on viruses. some viruses like poxviruses
contain DNA dependant RNA polymerase while tumor viruses possess an
RNA-dependant DNA polymerase
These enzymes and others are found in core of the virus and become active once
the virus invade the cell and capsid is partially degraded .
Influenza virus also possess a surface spike embedded in the lipid envelope and
has an enzymatic activity .
This spike exhibits neuramidase activity. The neuramidase spike is used by virus
to dissolve neuramic acid and a component of cytoplasmic membrane of
mammalian cell
BY: MUHEREZA GERALD PHD/840/2019/2020S
VIRAL PARTICLES
These are particles or components that make up the virus. These
include;

1. A genome of RNA or DNA

This can be double or single stranded, linear or circular and in
some cases segmented.
The viral genome is either RNA or DNA and viruses are hence
categorized into DNA or RNA viruses
The nucleic acid of DNA viruses is usually double stranded and
linear or circular depending on the family.
The nucleic of RNA viruses is usually single stranded with
exception of the reoviruses. Viruses with single stranded RNA
are divided into two;
a) Plus strand/positive strand /sense RNA
This is when the RNA of the genome has the same polarity as the
viral mRNA and can function directly as mRNA.
2) Capsid
This is the shell of virus-coded proteins that encloses
the nucleic acid.
A combination of the nucleic acid and the capsid is
known as the nucleocapsid especially when they are
in close association e.g. myxoviruses.
The capsid is made up of identical subunits called
capsomeres.
The number of capsomeres vary but specific and
constant for each viral species.
They are made up of several polypeptides.
The capsid protects the nucleic acids from
degradation.
3) Envelope
The envelope surrounds the capsid and is
dependant on the cell membrane o the host
cell.
Both the cell-coded and viral proteins are
integrated in the membrane when these
elements are transformed into the envelopes
frequently in form of spikes.
Enveloped viruses do not adsorb to the host
cell with the capsid rather with the envelope.
Removing the envelope with organic solvents
or detergents reduces the infectivity of viruses.
Other components of the viral particles
Enzymes
Viruses require different types of enzymes depending
on the genome type and mode of infection.
In several virus species are enzymes e.g.
neuraminidase required for invasion and release of
myxoviruses
Nucleic acid polymerases such as RNA dependent
DNA polymerases in antisense viruses
DNA polymerase in small pox viruses
Reverse transcriptase in Hepatitis B viruses and
retroviruses.
Haemagglutinin
 Some viruses are capable of agglutinating
various human or animal erythrocytes
 These viruses bear a certain surface protein
in their envelope that enables them to do so.
 The haemaglutination phenomenon can be
made use of for quantitative viral testing or in
virus identification and anti-body identification.
 Haemagglutination plays a decisive role in
adsorption and penetration of the virus into the
host cell.
BY NABANGI JAMES DDT/653/2019/2020
VIRAL REPLICATION
The genetic information necessary for viral replication
is contained in the viral nucleic acid but lacking
biosynthetic enzymes, which means that viruses
depend on the synthetic machinery of the host cell for
replication.
Replication takes place in six sequential phases;
 Adsorption
 Penetration
 Uncoating
 Biosynthesis
 Maturation
 Release
1. Adsorption or absorption
Virions come in contact with cells by random
collision but adsorption/attachment is specific
and is mediated/helped by the binding of the
virion surface structures known as ligands to
receptors on the cell surface.
In case of the influenza virus, a surface
glycoprotein (haemaglutinin) binds
specifically to the sialic acid residue of
glycoprotein receptor sites on the surface of
the respiratory epithelium.
2) Penetration
After binding, the virus particle is taken up
inside the cell. This is accomplished by the
receptor mediated endocytosis (viropexis)
with up take of ingested virus particles with
in endosome.
Enveloped viruses fuse their membranes with
cellular membranes to deliver the
nucleocapsid or genome directly into the
cytoplasm.
3) uncoating
This is the process of stripping the virus of its
outer layer and capsid so that the nucleic acid
is released into the cell.
With most viruses, uncoating is affected by
action of lysosomes/enzymes of the host cell
4) Biosynthesis
Here there is synthesis of viral nucleic acid, capsid
protein, and also enzymes necessary in the various
stages of viral synthesis, assembly and release.
Certain irregular proteins are also synthesized
which serve to shutdown the normal cellular
metabolism and direct the sequential production
of viral components.
Most DNA viruses synthesize their nucleic acid in
the host cell nucleus with exception of poxviruses.
Most RNA viruses synthesize all their components
in the cytoplasm with exception of
orthmyxoviruses.
STEPS OF BIOSYNTHESIS
1) Transcription of mRNA from the viral
nucleic acid
2) Translation of the mRNA into early proteins
3) Replication of viral nucleic acid
4) Synthesis of late or structural proteins
which are the components of daughter
virion capsid.
5) Maturation
Newly synthesized viral genomes and capsid
polypeptides assemble together to form progeny
viruses thus assembly comes shortly after the
replication of the viral nucleic acid and takes
place either in the nucleus (herpes and
adenoviruses) or cytoplasm (picorna and
poxviruses).
In case of enveloped viruses the envelope is derived
from the host cell nuclear membrane and from
plasma membrane when the assembly occurs in
the cytoplasm of the host cell.
6) Release
Viruses can be released from cells after lysis or
from the cell by exocytosis or by budding
from the plasma membrane. Viruses that exist
as naked nucleocapsid may be released by
the lysis of the host cell(polio viruses) or they
may be extruded by the process called
reverse phagocytosis.
Release of many enveloped viruses occur after
budding from the plasma membrane.
VIRAL GENETICS
Viruses obey the law of genetics like all other living beings.
Main mechanism for genetic modification in viruses are
A)Mutation.
It is a random undirected and heritable variation. Mutations
occur during every viral infection. many mutation are
lethal, because the mutated virus is unable to replicate
 A mutant becomes evident only if the mutation confers
some readily observable properties or affords the mutant
virus, some selection or survival advantage.
 Mutation may occur spontaneously or may be induced by
mutagens, physical agents such as UV light or radiations.
 Conditioned lethal mutation i.e. permissive condition and
non permissive or restrictive condition.
B) Recombination
When two different but related viruses infect a cell simultaneously, genetic
recombination may take place.
The two viruses exchange segments of the nucleic acid between them so that a
hybrid is formed possessing genes from both parents so recombination may
occur between;
 Two active(infectious viruses)
 One active and one inactive virus
 Two inactive viruses
Recombination between one active and one inactive virus (cross reactivation
or marker resue).
When a cell is infected by active virus and a different but related inactivated
virus ,progeny processing one or more genetic traits of inactivated virus
may be produced.
Recombination between two inactive viruses (multiplicity reactivation).
The different virions that cause multiple infection of the cell may have suffered
damage to different gene, thus form the total genetic pool it may be possible
to obtain a full complement undamaged genes.
c) Non genetic mixing
 Phenotypic mixing
When two different virions infect the same cell, some mix up
may take place during assembly so that progeny genome of
one virus may be surrounded by the capsid belonging
entirely or partly to the other virus known as phenotypic
mixing which is not a stable variation.
If the genome is eneased in a completely heterologous protein
coat, this extreme example is called phenotypic masking or
transcapsidation
 Genotypic mixing or heterozygosis
This results from the incorporation of more than one complete
genome into a single virus particle. There is no
recombination between the different genomes so that the
two kinds of progeny are formed on passage.
3) Complementation
The basis for complementation is that one virus produces a
gene product in which the second is defective, allowing the
second virus to grow. The genotype of the two genes
remain unchanged.
Interference
Infection of either cell cultures or whole animals with two
viruses often leads to an inhibition of multiplication of one
of the viruses, an effect called interference.
Viral interference has been applied in the field in controlling
poliomyelitis outbreak by introducing into the population,
the live attenuated poliovirus vaccine
BY AWUBWA ERIAS: PHD/835/2019/2020
NAMBAZIIRA KAMIAT: PHD/820/2019/2020
Classification of Viruses
Viruses are classified in different ways as follows
1. Type of nucleic acid
This is divided into two;
 Riboviruses containing RNA as their nucleic acid
 Deoxyriboviruses containing DNA
2. According to the presence or absence of a lipid envelope
 Enveloped
 Non enveloped or naked viruses
3. According to number of strands of nucleic acid
 Double stranded
 Single stranded
4. According to polarity of the genome
RNA viruses in which the viral genome can be used directly as mRNA are by convention
termed as positive –ve stranded
5. According to symmetry of the nucleocapsid
 Spherical or circular
 Rhod shaped or cylindrical shaped
 Complex
BY KOMUGISHA BRENDA PHD/839/2019/2020
Type of virus Examples
DNA non enveloped Adenovirus
DNA Enveloped Herpes simplex, Hepatitis B,
Poxvirus, Cytomegalovirus
RNA enveloped Measles virus, Rabies virus,
Influenza and Parainfluenza
RNA non enveloped Poliovirus, Hepatitis A virus,
Rhinovirus
Viral Pathogenesis
Viruses cause infection of the host by breaking the natural protective mechanism of the body,
then evading the immune system of the host and then killing of the host cell, thereby
triggering the immune response and inflammatory responses.
The pathogenesis of viruses can be divided into the following stages;
1. Entry into the body
Viruses enter the body majorly through the following routes.
a. Respiratory tract
Viruses may enter the respiratory tract through droplets containing viruses that can be
inhaled
b. Skin. Viruses may enter the skin through abrasion or skin wounds
c. Conjunctiva. Viruses may also enter the body through the conjunctiva e.g. adenoviruses
d. Alimentary canal. Viruses such as rotaviruses enter the body through the alimentary
canal by ingestion with food
e. Genital tract. Viruses that cause STDs enter the body through the genital tract e.g. HIV
virus
2. Initiation of infection at primary infection
Viruses on entry into the human host may remain at the primary site of infection, replicate
and cause infection of the target tissue
Viruses cause infection basing on the fact that they have specific attachment proteins and
tissue specific expression of receptors during replication.
3) Replication of viruses after entry into the
cell
Viruses replicate using the host cell machinery
and enzymes to produce more virions.
4) Spread to secondary site
Viruses are spread in the body mainly by the blood
stream and the lymphatic system, this is known
as viremia.
In the blood stream, viruses may either be free or
ingested by microphages or lymphocytes.
5) Manifestation of a disease
This is the stage where the viruses cause the
intended disease to the host
Types of viral infections
At the cellular level, depending on the nature of the virus, and the cell infected,
the virus infection in a cell can produce any of the three infections;
1. Failed/abortive infection
Here the virus do not multiply therefore disappear from the cell.
2. Lytic infection
Some viruses cause cell death or lytic infection.
This occurs due to replication of viruses in infected cells which kills the target
cells.
The virus may lead to cell death in the following ways;
a. Inhibition of cellular protein synthesis
b. Cell fusion (viruses cause cells to fuse)
c. Transformation (integration of viral DNA and host DNA )
3. Infection without cell death ( persistent viral infection)
The persistent viral infection is characterized by viral infection without cell death.
The cell does not die because of slow release of viruses by exocytosis or budding
through plasma membrane.
BY AMPIIRE JORAM PHD/819/2019/2020
HOST RESPONSE TO VIRAL INFECTION
The outcome of a virus infection is influenced by the virulence of the infecting strain and the
resistance offered by the host.
The mechanism of host resistance may be immunological or non specific
1. Immunological response
Viruses in general are good antigens and thus induce both and cell mediated immunity (CMI)
 Antibody mediated immunity
Antibodies may affect viruses by causing neutralization of viruses through the following ways
 They may prevent adsorption of viruses to cell receptors, cause enhanced cell degradation or
prevent release of progeny virus from infected cell
 Opsonisation of virions for phagocytosis and killing macrophages
 Complement acts in conjunction with antibodies causing surface damage to enveloped
virions and in producing cytolysis of virus infected cells.
 Cell mediated immunity
This prevents infection of target organs and promotes recovery from disease by destroying virus
and virus infected cells through any of the following four different processes.
 Cytolysis by cytotoxic T cells (Tc) cells
 Cytolysis by natural killer (NK) cells
 Antibody- complement-mediated cytotoxicity
 Antibody-dependent- mediated cytotoxicity.
2. Non immunological responses
a. Phagocytosis by macrophage phagocytose viruses
b. Interferon
This is a good of proteins produced by human cells after viral infections or after
exposure to other inducers
They are grouped into three based on their cell of origin; namely leukocytes, fibroblast
and lymphocyte
These three groups are
 Alpha
 Beta
 Gamma interferon
Alfa interferons are produced by leukocytes, beta by fibrocytes ad gamma by T
lymphocytes
Interferon act by blocking the synthesis of viral proteins thereby preventing replication
c. Fever
Rise in body temperature above 39c affects viral replication
d. Natural killer cells
These are a type of T lymphocytes. They recognize infected cells and kill them by
secreting perfoctins and granzymes which cause apoptosis (programmed cell death)
e. Defensins
This is a family of positively charged peptides
with anti-viral activity; it also has
antibacterial activity.
They interfere with the (HIV) by binding to the
CD4 receptor sites and prevent entry of the
virus into the cell.

BY: KAKOOZA JAMES PHD/836/2019/2020

BIGALUKA WASSWA AMBROSE
PHD/844/2019/2020