General Virology

1
General Virology
By
Dr. Emad AbdElhameed Morad
Lecturer of Medical Microbiology and Immunology
2

• General properties of viruses

3

• Very small in size (30-200 nm).
• So, they are seen only electron microscope
  EXCEPT pox viruses as they are large in size.
• Contain one kind of nucleic acid (RNA or DNA).
• Metabolically inert, they do not possess
  ribosomes or other organelles.
• Obligate intracellular (replicate only inside
  the host cell).
• Can not be cultivated on artificial media but
  grown in
• Tissue culture
• Embryonated eggs
• Laboratory animals

4

• Size of viruses

5

• Due to the small size of viruses
• Viruses can pass through the bacterial filters.
• Seen only by the electron microscope.
• Require high speed for sedimentation (10.000
  30.000 rpm). This is called ultracentrifugation.

6

• Structure of viruses

7

• Each virus particle (virion) is composed of
• Protein coat called capsid.
• Nucleic acid (RNA or DNA).
• Many viruses are naked (only nucleocapsid).
• But some viruses are enveloped.
• In enveloped viruses, matrix proteins are
  present and mediate the interaction between the
  capsid and envelope.
• Some viruses contain internal proteins
  (enzymes).

8
Enveloped virus
9
Viral capsid

• Composed of small protein subunits called
  capsomeres.
• Functions of the capsid
• Protects the viral genome.
• Determines the virus symmetry (cubical or
  helical).
• Attachment of the virus to the target cells.
• Capsid proteins carry the viral antigens that
  induce production of antibodies which neutralize
  the virus infectivity.

10
Viral envelope

• Some viruses are surrounded by lipid envelope.
• This envelope is derived from the host cell
  membrane during release of virus by budding.
• Enveloped viruses are sensitive to ether due to
  their lipid content.
• In some viruses, the lipid envelope is covered
  by glycoprotein spikes (Example influenza virus
  carries two types of spikes called hemagglutinin
  HA and neuraminidase NA).

11
Enveloped virus with glycoprotein spikes
12
Viral nucleic acid

• Viruses contain either DNA or RNA but not both.
• Most DNA viruses are double stranded while most
  RNA viruses are single stranded.
• The nucleic acid may be linear or circular.
• Some RNA viruses have segmented genome like
  influenza virus and rota virus.
• All viruses have one copy of their genome
  (haploid) except retroviruses which have two
  copies (diploid).
• Functions
• It carries the genetic information for virus
  replication.
• It is the essential infectious part of the
  virion.

13
Virus symmetry

• Viruses have 3 types of symmetry
• Icosahedral (cubical) the virus resembles a
  crystal.
• Helical symmetry the capsomeres are arranged in
  a ribbon which is wound in the form of a helix
  around the nucleic acid. Note that all human
  viruses with helical symmetry are enveloped.
• Complex symmetry such as pox viruses which are
  brick shaped with ridges on the external surface.
  

14
Virus symmetry
Icosahedral (cubical)
Helical
15

• Atypical virus like agents

16

• Defective viruses
• Composed of nucleic acid and capsid.
• But, can not replicate without a helper virus
  which provides the missing function.
• Pseudovirions
• Contains host cell DNA instead of viral DNA
  within the capsid.
• Prions
• Composed only of proteins no nucleic acid.
• Cause Creutzfeldt-Jakob disease.
• Viroids
• Composed of single circular RNA molecule no
  capsid or envelope.
• Cause several plant diseases.

17

• Virus replication

18

• Replication cycle of the virus consists of the
  following steps
• Attachment (adsorption)
• Interaction of the virus with specific receptors
  on the host cells (viral tropism).
• Example Hepatitis viruses infect the liver.
• Penetration
• Non-enveloped viruses penetrate the host cell by
  phagocytosis.
• Enveloped viruses penetrate the host cell by
  fusion of the envelope with the host cell
  membrane.

19

• Uncoating
• Separation of the nucleic acid from the capsid.
• Eclipse
• During this phase, the host cell is directed by
  the viral genome toward synthesizing viral
  components.
• Intracellular viral synthesis
• Synthesis of the viral nucleic acid.
• Synthesis of viral proteins.
• Assembly
• The viral nucleic acid becomes surrounded by
  capsid to form the virion.
• Areas of viral assembly are called inclusion
  bodies.
• Release
• In non-enveloped viruses release occurs by
  rupture of the host cell.
• In enveloped viruses release occurs by budding.

20

• How do viruses enter the body?

21

22

• Types of viral infections

23

• Viral infection may be
• Superficial infection
• Infection remains local at the portal of entry.
• Short incubation period
• No viremia
• Example influenza
• Systemic infection
• After viral entry, the virus spreads to local
  lymphoid tissues then causes viremia then spreads
  to target organs.
• Example Measles and mumps.

24

• Persistent viral infections
• The virus persists in the body for a long time.
• Three types are present
• Chronic infection
• Chronic disease with mild symptoms
• Example hepatitis viruses
• Latent infection
• The virus persists hidden most of the time with
  periodic reactivation.
• Example herpes viruses
• Slow virus infection
• Very long incubation period of months to years
  with no clinical symptoms
• Example AIDS

25
BEST WISHES