Viruses

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Viruses

• Latin Poison slippery toxin

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Size comparisons

• Submicroscopic
• Cannot be seen using a light microscope
• We use Electron Microscopes to look at viruses

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VirusesNon living, infectious particles!

• Much smaller and simpler in organization not
  cells!
• Lack most of the structures and metabolic
  machinery found in living organisms
• Must hijack another cell to reproduce non
  living

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Structure

• 2 parts capsid and nucleic acid
• Capsid protein shell that encloses the nucleic
  acid
• Shapes helical, polyhedral, very complex
  geometric
• Some have additional accessory structures that
  aid in host infection.
• Envelope extra layer outside of the capsid,
  derived from hosts membrane,
• These have glycoproteins that can act as
  anchoring spikes
• Tail act as hypodermic syringe

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Figure 18.2 Viral structure
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Figure 18.02x2 Phages
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Structure

• Viral Genomes are the basis for classification
• 4 Types
• 1. Double Strand DNA
• 2. Single Strand DNA
• 3. Double Strand RNA
• 4. Single Strand RNA
• Classified DNA-like or RNA-like/Retrovirus

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Virus Replication

• Obligate intracellular parasites
• Viruses lack the enzymes for metabolism, have no
  ribosomes
• Cannot make their own proteins
• Each type of virus can only infect within a HOST
  RANGE this specificity depends on the
  lock-and-key fit between capsid of the virus
  and specific receptor molecules on cell surface

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Figure 18.3 A simplified viral reproductive cycle
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5 basic steps of virus replication

• 1. Attachment lock and key
• 2. Penetration genome injection
• 3. Replication and Synthesis - hijack host
  machinery
• 4. Assembly - hijack host energy
• 5. Release burst free

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Tale of 2 Cycles the speed of takeover

• Lytic vs. Lysogenic cycles
• Lytic cycle -virus takes over the host cell
  immediately and reproduces quickly steps 1-5 in
  succession
• Ex. Cold virus, influenza
• virulent viruses quick!

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• Lysogenic cycle - virus hides in the original
  host cells DNA until optimal conditions for
  viral survival are present
• host cell will reproduce (mitosis) many times
• the virus will reproduce and emerge from MULTIPLE
  cells at once, causing much more severe cellular
  damage
• Steps 1-5 NOT in success delayed!
• Ex. AIDS, herpes

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5 Steps in the Lytic Cycle

• Attachment
• Entry
• Replication
• Assembly of new viruses
• LYSIS cell breaks open and releases new viruses
  into the organism or environment

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Figure 18.4 The lytic cycle of phage T4
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Lytic Virus
Lytic Bacteriophage
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8 steps of the Lysogenic Cycle

• Attachment
• Entry
• Integration into the host cells DNA (prophage or
  provirus)
• Replication of cells (including virus DNA)
  through mitosis
• Trigger
• Replication of Virus
• Assembly
• Lysis

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Lysogenic Virus
Prophage infection
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Ex Lysogenic CycleHerpesvirus in Humans

• Herpesviruses genomes of double-stranded DNA
• Reproduce within the cell nucleus
• Hides in cell DNA replicated during mitosis
• Infections tend to recur throughout life in times
  of stress ? viruses will emerge from thousands of
  cells then

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Figure 18.5 The lysogenic and lytic reproductive
cycles of phage ?, a temperate phage
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Retroviruses

• Transcription (normally) making RNA from DNA
• RNA virus, ex. HIV
• Reverse Transcriptase
• enzyme that the virus
• carries
• Make DNA using its
• RNA