Viruses have a basic structure consisting of nucleic acid surrounded by a protein coat called a capsid. Some viruses have an additional phospholipid envelope. Viruses come in a few different symmetries for their capsids including icosahedral, helical, binal, and complex. Icosahedral capsids can be enveloped or non-enveloped while helical capsids are long and spiral-shaped. Binal symmetry combines icosahedral and helical features with a head and tail. Complex symmetry refers to asymmetrical virus structures.

1. WHAT IS A VIRUS
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2. VIRUS
 INTRODUCTION
 BASIC STRUCTURE
 SYMMETRY
• Icosahedral
• Helical
• Binal
• Complex
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3. INTRODUCTION TO VIRUSES
 Latin: vīrus = poison, term coined by Martinus
Beijerinck
 Ultramicroscopic
 Acellular
 Intracellular obligate parasite
 Lack lipmann system (enzyme system)
 Specific to host
 Viruses are active inside host and inactive outside
the host
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4. INTRODUCTION TO VIRUSES
 Viruses either has DNA or RNA, not both (exception)
 Depending upon host interaction viruses are of 3 types:
1. Animal viruses
2. Plant viruses
3. Bacterial viruses
 Viruses are nucleo-proteins
 Viruses are uniform in size and shape
 Viruses are sensitive to interferon
 Viruses ranges from 20 to 14,000 nm in length, 20-300
nm in diameter.
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5. BASIC STRUCTURE OF VIRUSES
 Basic rules of virus architecture, structure, and
assembly are the same for all families
 Some structures are much more complex than
others, and require complex assembly and
dissassembly
 The capsid (coat) protein is the basic unit of
structure; functions that may be fulfilled by the capsid
protein are to:
• Protect viral nucleic acid
• Interact specifically with the viral nucleic acid for
packaging
• Interact with vector for specific transmission
• Interact with host receptors for entry to cell
• Allow for release of nucleic acid upon entry into
new cell
• Assist in processes of viral and/or host gene
regulation
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6. BASIC STRUCTURE OF VIRUSES
 Virions are complete, fully developed viral particles
composed of nucleic acid surrounded by a
protein coat.
 Some viruses have an envelope composed of a
phospholipid bilayer with viral glycoproteins
(spikes).
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7. BASIC STRUCTURE OF VIRUSES
1. Nucleic acid or Genome
 Viral genomes are either DNA or RNA (not both).
 Nucleic acid may be single- or double-stranded
 Nucleic acid may be circular or linear or separate
molecules.
 Nucleic acid : protein ranges from about 1% - 50%.
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8. BASIC STRUCTURE OF VIRUSES
2. Capsid
 Capsid - protein coat
 Capsomeres are subunits
of the capsid
 Protomeres are capsomere
subunits
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9. BASIC STRUCTURE OF VIRUSES
3. Envelope
 The outer covering of some viruses, the envelope is
derived from the host cell plasma membrane when the
virus comes out. Some enveloped viruses have spikes.
 Influenza virus has two kinds of spikes, H
(Hemagglutinin) and N (Neuraminidase):
 The H spike allows the virus to attach to host.
 The N spike allows the mature viral particles to
escape from the host cell.
 Non-enveloped or naked viruses are protected by their
capsid alone.
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11. 1) Icosahedral or Polyhedral (nature’s favourite
shape)
• In this nuclic acid is compact and folded.
• There are two types of capsid:
a) Enveloped
Eg. Herpes Virus
b) Non-Enveloped
Eg. Polio Virus,
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12. 2) Helical or Spiral
Virus contain genome
enclosed in Helical
capsid.
About 12 to 812
Capsomeres are arranged
on the surface of Nuclic
acid.
Eg. TMV
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13. 3) Binal Symmetry
(known as Best structure)
• Combination of Icosahedral
and Helical.
• Head and tail are joined by
collar.
• Tail is hollow tube, base plate
contain tail fibers.
Eg. Bacteriophage
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14. 4) Complex Symmetry
• Viruses with asymmetrical
structures are referred to as
"complex"
• These viruses possess
a capsid that is neither purely
helical nor purely icosahedral,
and may possess extra structures
such as protein tails or a complex
outer walls.
Eg. Pox virus family
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