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structure & classification of virus copy.pptx

INTRODUCTION
 Viruses are the smallest infectious agents (ranging
from about 20 nm to 300 nm in diameter) and contain
only one kind of nucleic acid (RNA or DNA) as their
genome.
 The nucleic acid is encased in a protein shell, which
may be surrounded by a lipid-containing membrane.
 They replicate only in living cells, being parasites at
the genetic level.

SIZE & SHAPE OF VIRUSES
 Viruses range from 20 to 300 nm in diameter.
 Their shapes are frequently referred to in colloquial
terms (e.g., spheres, rods, bullets, or bricks), but in
reality they are complex structures of precise
geometric symmetry .
 The shape of virus particles is determined by the
arrangement of the repeating subunits that form
the protein coat (capsid) of the virus.

SHAPES AND SIZES OF MEDICALLY IMPORTANT VIRUSES

VIRAL NUCLEIC ACIDS
 Viruses contain a single kind of nucleic acid—either
DNA or RNA—that encodes the genetic information
necessary for replication of the virus.
 The genome may be single or double stranded,
circular or linear, and segmented or non
segmented.
 The type of nucleic acid, its strandedness, and its
size are major characteristics used for classifying
viruses into families.

Capsid:
 The protein shell, or coat, that encloses the nucleic acid
genome.
Capsomeres:
 Morphologic units seen in the electron microscope on the
surface of icosahedral virus particles. Capsomeres represent
clusters of polypeptides.

VIRAL CAPSID & SYMMETRY
 The nucleic acid is surrounded by a protein coat called
a capsid made up of subunits called capsomers.
 Each capsomer, consisting of one or several proteins, can
be seen in the electron microscope as a spherical particle.
 The structure composed of the nucleic acid genome and
the capsid proteins is called the nucleocapsid.
 The arrangement of capsomers gives the virus structure its
geometric symmetry.

TYPES OF SYMMETRY OF VIRUS PARTICLES
 Viral architecture can be grouped into three types
based on the arrangement of morphologic subunits:
(1) cubic / Icosahedral symmetry(eg, adenoviruses),
(2) helical symmetry (eg orthomyxoviruses), and
(3) complex structures (eg, poxviruses).

Nucleocapsid:
 A structural unit consisting of the genetic material of
a virus and the protein that surrounds it.

Envelope:
 A lipid-containing membrane that surrounds some virus
particles.Virus encoded glycoproteins are exposed on the
surface of the envelope.These projections are called
peplomers.

ATYPICAL VIRUS LIKE AGENTS
 DEFECTIVE VIRUS: are composed of viral nucleic
acid and proteins but cannot replicate without a
helper virus, which provides the missing function.
 PSEUDOVIRONS: contains host cell DNA instead of
viral DNA within the capsid. They are formed during
infection with certain viruses.
 VIROIDS: consists of single molecule of circular RNA
without protein coat or envelop.

 PRIONS:
 Prions are infectious particle that are composed
solely of protein ( i.e they contain no nucleic acid).
 They are implicated as the cause of certain slow
disease called Transmissible Spongiform
encephalopathies which include disease as
Creutzfeidt-Jakob disease in humans and Scrapie
in sheep.

BACTERIOPHAGE
 A virus which parasitizes a bacterium by infecting it
and reproducing inside it.
 A virus that lives within a bacterium replicating itself
and eventually destroying the bacterial cell.
 The length of a bacteriophage is between 24 and
200 nm.

 The structure of a bacteriophage consists of
a head and
a tail.
• The genetic material of the bacteriophage is stored in the
head or capsid region.
 From the capsid, a long string of proteins make up the tail
structure of a bacteriophage.
 The tail is a hollow tube that serves as a passageway for the
genetic material to pass from the capsid to the host bacteria.

CLASSIFICATION OF VIRUSES
Virus classification is the process of
naming viruses and placing them into
a taxonomic system.

BASIS OF CLASSIFICATON
1.Virus capsid, its size and type of
symmetry & enveloped or not
2. Virus nucleic acid ,its type (DNA or
RNA), strandedness (single or double),
whether linear or circular.

MEDICALLY IMPORTANT DNA VIRUSES

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structure & classification of virus copy.pptx

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