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24.1 Viral Structure and Replication
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Are Viruses Living or Non-living?
They have some properties of life but not others
For example, viruses can be killed, even crystallized but they can’t maintain a constant internal state (homeostasis).
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What are Viruses?A virus is a non-cellular
particle made up of a nucleic acid (DNA or RNA) and protein
A) range in size from 20 nm to 250 nm (1 nm = 0.00000004 in)
B) Viral Structure 1. capsid – protein coat surrounding the
nucleic acid2. envelope – membrane that allows new
viruses to infect host cellsC) Can only reproduce inside a HOST cell
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Others may have spikes to help attach to the host cell
Most viruses infect only SPECIFIC host cells
CAPSID
ENVELOPE
DNA
SPIKES
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Some viruses cause diseases like smallpox, influenza, colds, AIDS.
MEASLES
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Influenza Virus
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Chickenpox Virus Small Pox Virus
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Phages
Viruses that attack bacteria are called bacteriophage
III) Viral ReplicationA) Retrovirus – is an RNA virus that contains Reverse Transcriptase(uses RNA as a template to make DNA)
IV) The Lytic Cycle –A) Virulent – are the viruses that undergo the lytic cycle
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5 Steps of Lytic Cycle
1. Attachment to the cell2. Penetration (injection) of viral
DNA or RNA3. Replication (Biosynthesis) of new
viral proteins and nucleic acids4. Assembly (Maturation) of the
new viruses5. Release of the new viruses into
the environment (cell lyses)
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Bacteriophage Replication
V) The Lysogenic Cycle – viruses stay dormant in the host cell for days, months, or years (HIV or Herpes)
A) Temperate viruses – replicates through the lysogenic cycle and does not kill the host cell immediately
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Lysogenic Cycle
Phage DNA injected into host cell
Viral DNA joins host DNA forming a prophage
When an activation signal occurs, the phage DNA starts replicating
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Lysogenic Cycle
Host cell makes copies of the virus indefinitely