C. Envelope
- • Outer covering of some viruses
• Envelope is derived from the host cell plasma membrane when the virus buds out
• Some enveloped viruses have spikes, which are viral glycoproteins that project from the envelope
• Naked (non-enveloped) viruses are protected by their capsid alone
Fig. 5. Enveloped helical virus
2. Size of viruses:
- • Determined by electron microscopy
• Ranges from 20 to 14000 nm in length
Fig. 6. Size of different viruses
3. Shape of viruses:
Four basic morphologies
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• Icosahedral - efficient means to conserve and enclose space; form capsomers (planar faces formed by association of proteins)
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• Helical - capsid is shaped like a hollow protein tube
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• Enveloped - outer covering derived from the host cell's nuclear or plasma membrane and often possessing spikes or peplomer projections involved in attachment and entry into a host cell sometimes via their enzymatic activity
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• Complex symmetry - viruses that fit neither of the above categories or which may employ portions in combination, e.g., bacteriophage
Fig. 7. Types of viral symmetry