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