Viruses Slideshow

Virus definition: An infectious particle consisting of genes packaged in a protein coat

• lacks machinery and structures found in cells

• cannot reproduce outside host cell 

• cannot carry out metabolic activities on own

• contains either DNA (ss or ds) or RNA (ss or ds)

Structure:

Capsid -- protein shell

2. Genetic Material

3. Some contain an 

envelope -- surrounds capsid & comes from membrane of host cell

Bacteriophages:

• Viruses that infect bacteria

• Almost all have DNA genomes

Host range: Virus can only infect a limited number of host species

• “lock and key” between viral surface proteins and specific receptor molecules on outside of host cells

Infection: Begins when virus binds to a host cell, viral genome enters the cell 

• viral proteins take over the host

• reprograms cell to copy viral nucleic acids and proteins

  • cell provides the nucleotides, amino acids, enzymes, tRNA, etc.

  • host cell provides DNA polymerase but RNA viruses use viral RNA polymerase

Simple Replication:

• Copies nucleic acids and proteins

• Components self-assemble spontaneously

• Hundreds or thousands of assembled viruses now leave cell often damaging or destroying it.

• This cell death, as well as the body’s response, causes many of the symptoms associated with viral infections

Lytic Cycle of Bacteriophages:

• Ends in death of host cell -- lyses (breaks open) the cell

• Virulent phages can only reproduce using this method

• Bacterial survival?

  • mutant receptors are favored

  • recognizes foreign DNA and restriction enzymes cut it up

  • coexist in lysogeny

    Lysogenic cycle of Bacteriophages:

  • Allows replication of the phage genome without destroying the cell -- lysogenic = generation of phages that will lyse their host cells

  • Only Temperate Phages can use this method

  • Viral DNA integrates with bacterial DNA 

  • As the bacterium replicates, so does the prophage = large number of viral copies within the large number of bacteria (without killing the host cells)

  • Environmental signal = trigger switch to lytic cycle

Animal virus classes:

• Determined by the genome

• Almost all that have RNA 

     genomes have an envelope

• Envelope has viral glycoproteins 

as well as host plasma membrane 

that binds to specific receptor

molecules

Animal Viruses:

• dsDNA viruses replicate within the host, use viral & cellular enzymes to replicate

  • Some viral DNA can stay behind and remain latent stress new round of production flare up.

• ssRNA can either

  • serve as mRNA and be translated immediately

  • serve as a template for mRNA, which uses viral enzymes only (no proofreading capabilities = high mutation rate)

Retroviruses:

• Two identical ssRNA enters and is used as a template to form DNA using reverse transcriptase (no proofreading capability)

• ssRNA ssDNA dsDNA (reverse transcriptase creates complement to ssDNA)

• viral dsDNA enters and is integrated in host DNA now called provirus

never leaves host DNA

Viroids and prions:

• Viroids = circular RNA molecules, only a few hundred nucleotides long, that infect plants

  • do not encode proteins but can replicate and cause errors in plant growth

• Prions = infectious proteins; converts the normally folded protein into misfolded protein aggregates interferes with normal protein act.

  • can cause degenerative brain diseases (mad cow, Creutzfelt-Jakob disease)

  • act slowly

  • indestructible with no known cures