introduction to virology

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INTRODUCTION TO VIROLOGY

• WHAT IS VIROLOGY
• Virology is the study of viruses and virus-like
  agents
• their structure,
• classification and evolution,
• their ways to infect and exploit cells for virus
  reproduction,
• the diseases they cause,
• the techniques to isolate and culture them, and
  their use in research and therapy

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What is Virus

• A virus is an invasive biological agent
  that reproduces inside the cells of living hosts.
• When infected by a virus, a host cell is forced
  to produce many thousands of identical copies of
  the original virus, at an extraordinary rate.
• Unlike most living things, viruses do not have
  cells that divide new viruses are assembled in
  the infected host cell

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General Characteristics of Viruses

• Viruses are 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 cells.
• .

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General Characteristics.

• 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

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Generalised 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

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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
  infectious property of a virus is due to its
  nucleic acid.

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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

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• A simplified diagram of the structure of a virus

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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.

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Non-living Characters of Viruses

• They can be crystallized.
• Outside the cell, they behave like inert
  chemicals.
• They do not show growth, development, nutrition,
  reproduction, etc.
• They can be precipitated

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Living characters of viruses

• They multiply within host cells.
• They possess genetic material, either DNA or
  RNA.
• There are definite races or strains.
• They exhibit mutations
• Because of the above reasons, viruses form unique
  bridge between living and non-living things.

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Classification of viruses

• Viruses can be classified as the following
• According to the host cell they infect animal
  viruses, plant viruses, fungal viruses, and
  bacteriophages (viruses infecting bacteria, which
  include the most complex viruses).
• Another classification uses the geometrical shape
  of their capsid (often a helix or an icosahedron)
  or the virus's structure (e.g. presence or
  absence of a lipid envelope).

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Classification of viruses

• according to the type of nucleic acid they use as
  genetic material and the viral replication method
  they employ to coax host cells into producing
  more viruses
• -DNA viruses (divided into double-stranded DNA
  viruses and the much less common single-stranded
  DNA viruses),

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Classification.

• RNA viruses (divided into positive-sense
  single-stranded RNA viruses, negative-sense
  single-stranded RNA viruses and the much less
  common double-stranded RNA viruses), -reverse
  transcribing viruses (double-stranded
  reverse-transcribing DNA viruses and
  single-stranded reverse-transcribing
  RNA viruses including retroviruses).

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Ways of Virus attacking the human/Virus Life cycle

• Attachment binding of the virus to specific
  molecules on the surface of the cell and this is
  specific example Plant viruses can only attach to
  plant cells and cannot infect animals. This
  mechanism has evolved to favour those viruses
  that only infect cells in which they are capable
  of reproducing

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Ways of Virus

• Penetration viruses penetrate the host cell
  by endocytosis or by fusion with the cell.
• Uncoating  happens inside the cell when the
  viral capsid is removed and destroyed by viral
  enzymes or host enzymes, thereby exposing the
  viral nucleic acid

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Ways of Virus.

• Replication is the stage where a cell uses viral
  messenger RNA in its protein synthesis systems to
  produce viral proteins. The RNA or DNA synthesis
  abilities of the cell produce the virus's DNA or
  RNA.
• Assembly  takes place in the cell when the newly
  created viral proteins and nucleic acid combine
  to form hundreds of new virus particles.

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Ways of Virus

• Release occurs when the new viruses escape or are
  released from the cell. Most viruses achieve this
  by making the cells burst, a process
  called lysis. Other viruses such as HIV are
  released more gently by a process called budding

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Effect of Virus to the host cell

• Due to bursting there will be death of host cell
  (lyisis)
• Also death cell can be caused by cessation of
  normal activity of the cell due to protein
  produced by virus.
• alterations to the cell's surface membrane
  and apoptosis (cell "suicide

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Effect of Virus to the host cell

• Some viruses cause no apparent changes to the
  infected cell. Cells in which the virus
  is latent and inactive show few signs of
  infection and often function normally. This
  causes persistent infections and the virus is
  often dormant for many months or years. This is
  often the case with herpes viruses
• Some viruses, such as Epstein-Barr virus, often
  cause cells to proliferate without causing
  malignancy but some other viruses, such
  as papillomavirus, are an established cause of
  cancer 

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Effect of Virus..

• When a cell's DNA is damaged by a virus, and if
  the cell cannot repair itself, this often
  triggers apoptosis. One of the results of
  apoptosis is destruction of the damaged DNA by
  the cell itself. Some viruses have mechanisms to
  limit apoptosis so that the host cell does not
  die before progeny viruses have been
  produced HIV, for example, does this.
•  

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Spread of viruses

• Viruses cannot exist on their own and for
  survival they need to spread to another host.
  This is because the original host may either die
  or eliminate the infection

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Some important routes of viral transfer include
Route Examples
Skin contact HPV (warts)
Respiratory Cold viruses, influenza, measles, mumps, rubella
Faecal-oral Polio, echo, Coxsackie, Hepatitis A, Rotavirus
Milk HIV, HTLV-1, CMV
Transplacental Rubella, CMV, HIV
Sexually Herpes 1 and 2, HIV, HPV, Hepatitis B
Insect vector Yellow fever, Dengue fever
Animal bite Rabies
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Host resistance

• Innate immunity of animals
• Animals, including humans, have many natural
  defenses against viruses.
• Some are non-specific and protect against many
  viruses regardless of the type
• This innate immunity is not improved by repeated
  exposure to viruses and does not retain a
  "memory" of the infection

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Innate immunity

• The acidity of the contents of the stomach
  destroys many viruses that have been swallowed.
  When a virus overcomes these barriers and enters
  the host, other innate defenses prevent the
  spread of infection in the body.
• A special hormone called interferon is produced
  by the body when viruses are present, and this
  stops the viruses from reproducing by killing the
  infected cell and its close neighbors.
• Inside cells, there are enzymes that destroy the
  RNA of viruses. This is called RNA interference.
  Some blood cells engulf and destroy other virus
  infected cells

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Adaptive immunity of animalsImmune response

• Specific immunity to viruses develops over time
  and white blood cells called lymphocytes play a
  central role.
• Lymphocytes retain a "memory" of virus infections
  and produce many special molecules
  called antibodies. These antibodies attach to
  viruses and stop the virus from infecting cells.
  Antibodies are highly selective and attack only
  one type of virus.

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• Thank you for your attention