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Introduction to
Virology
WHAT ARE
VIRUSES
Acellular organisms whose genomes consist of nucleic acid, and
which obligately replicate inside host cells using host metabolic
machinery and ribosomes to form a pool of components which
assemble into particles called virions, which serve to protect the
genome and to transfer it to other cells.
CHARACTERIZATION OF
VIRUS (1)
•
A cellular forms, most range in size from 5 to 300nm in diameter.
• Obligate intracellular parasitic microorganisms which are smaller
than bacteria and so could pass through bacterial filters.
• Highly selective and specific to their hosts.
• Responsible for several diseases of human, animal, plant, etc.
• Lack of nuclear membrane and external cell wall
• Two types recognized to belong to this forms :
1. Viruses.
2. Bacteriophages.
CHARACTERIZATION OF
VIRUS (2)
Viruses are infectious agents with both living and
nonliving characteristics.
1. Living characteristics of viruses
a. They reproduce at a high rate, but only in living host
cells.
b. They can mutate.
CHARACTERIZATION OF
VIRUS (3)
2- Nonliving characteristics of viruses
a. They are acellular, that is, they contain no
cytoplasm or cellular organelles.
b. No metabolic enzymes but must replicate using the
host cell's metabolic machinery. In other words,
viruses don't grow and divide. Instead, new viral
components are synthesized and assembled within
the infected host cell.
c. The vast majority of viruses possess either DNA or
RNA but not both.
NATURE OF VIRUSES
• Viruses cannot be grown on artificial media.
• Grow in living organisms or tissue cells which are
kept alive in suitable medium.
COMPOSITION OF
VIRUSES (1)
• Viruses have the same general chemical
characteristics, being composed of
molecules of nucleic acid (either RNA
or DNA) and protein.
• Virus unit or particle is called a virion.
• Virion is composed of folded strands
of nucleic acid inside a protein coat
made of regularly arranged protein
subunits called capsomere.
COMPOSITION OF
VIRUSES (2)
• The simple types of viruses are only nucleoproteins, while the most
complex types (as cowpox) contain in addition other compounds such as
lipids, carbohydrates and sometimes traces of metals and vitamin-like
substances.
• Viruses contain either DNA or RNA , but both never occur together in
one virus.
Plant viruses contain only RNA while animal viruses may contain either RNA
or DNA .
• Bacterial viruses (bacteriophage) usually contain DNA.
VIRUSES HAVE THREE BASIC FORMS
VIRUSES HAVE THREE BASIC FORMS
1. Complex :
• Poxviruses :No capsid DNA surrounded by core membrane
• Bacteriophages :Complex capsid head and tail structures
2. Naked or non-enveloped:
• Capsid which contains DNA or RNA
3. Enveloped:
• Outer membrane
• Capsid which contains DNA or RNA
VIRAL TAXONOMY
Viral classification is based on:
• Shape
• Type and form of nucleic acid
• Enveloped or naked
• Mode of replication
• Organization of the genome and antigenic
differences
GENOMIC ORGANIZATION OF
VIRAL NUCLEIC ACIDS
I.
RNA viruses
1.
2.
RNA single stranded
RNA double stranded: one piece
or segmented
II. DNA viruses
1.
2.
Single stranded
Double stranded
VIRUS INFECTION AND
REPLICATION
STEPS OF REPLICATION
1. Adsorption/attachment: The infective virus has to be
bound at first to certain receptors on the outer surface of
the host cell.
2. Penetration: There are two views concerning this phase :
The whole virus consisting of nucleic acid and protein
penetrates into the cell , although the protein part has no role
on the further steps of infection.
 Only the nucleic acid part penetrates into the cell, i.e. the
virus has to get rid of its protein coat before penetration
3. Uncoting: Enzymes in lysosome digest capsid, envelope (if
present) and release viral genome
STEPS OF REPLICATION
4. Replication:
Copies of viral genome
Copies of viral proteins
5.
Assembly
Viral capsid reforms
Packaging of genome
6. Release:
• Budding taking pieces of host cell membrane
• Lytic host cell burst open releasing many virus particles