Viruses – Cellular Pirates
Adenovirus showing
icosahedral shape
Varicella zoster virus
causes chickenpox
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Characteristics of Viruses
 Must reproduce(replicate) inside a host
- The cell in which the virus replicates is called the host cell
 Structure
- Nucleic Acid – DNA or RNA
- Protein coat – called a capsid
- Envelope – some contain a layer of membrane taken
from a host cell
 Mutate Rapidly
 Host Specific
- can only infect one type of cell
 Small – from 20nm to 250nm
- most are too small to be seen with a light microscope
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Why are viruses considered non-living?
Viruses don’t exhibit all the criteria for life:
- they don’t grow or develop
- they do not respire
- they require a host for reproduction
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Virus Structure
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Bacteriophage
 Bacteriophages are viruses that infect bacteria
- they are responsible for transduction in bacteria
 The virus has an inner core of nucleic acid and an
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outer coat made of proteins
Viruses have two life cycles
 Lytic Cycle-Viruses-immediately infect the host cell
 Lysogenic Cycle - lay dormant in the cell
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Lytic Cycle
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Steps of Infection in the Lytic Cycle
1. Attachment
2. Entry/Infection
3. Replication
4. Assembly
5. Lysis
(cell ruptures)
and releases
new viruses
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Small Pox- Lytic
Jenner developed first vaccine 1798
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Lysogenic Cycle
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Herpes Simplex 1/Lysogenic
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Shingles-Lysogenic/ Chicken pox virus (herpes
varicella-zoster)
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Lytic and Lysogenic Cycles
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Retroviruses
 Contain RNA and not DNA
- to be able to take over a host cell, the retrovirus
must be able to convert the RNA into DNA
- to accomplish the conversion, retroviruses insert an
enzyme called Reverse Transcriptase into the host
cell
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Retroviruses
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Vocabulary
 Obligate intercellular parasite
 Capsid
 Envelope
 bacteriophage
 Lytic Cycle
 Lysogenic Cycle
 Phage
 prophage
 Retrovirus
 Reverse Transcriptase
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