Biology- Viruses

• Virus


  An infectious particle consisting of little more than genes packaged into a protein coat
• Viruses lead "a kind of borrowed life," existing in a shady area between life-forms and chemicals
• Viruses


  • Even the largest known virus is barely visible under the light microscope
  • Some viruses can be crystalized; not even the simplest cells can aggregate into regular crystals
  • Viruses are not cells but are a nucleic acid enclosed in a protein coat
• Viral genomes


  • Double- or single-stranded DNA
  • Double- or single-stranded RNA
• DNA virus


  A virus with a DNA genome
• RNA virus


  A virus with an RNA genome
• Capsid


  The protein shell that encloses the viral genome
• Capsomeres


  Protein subunits that make up the capsid
• Capsids can have various structures
• Viral envelope


  A membranous envelope that helps some viruses infect hosts, derived from the host cell's membrane and containing a combination of viral and host cell molecules
• Bacteriophages (phages)


  Viruses that infect bacteria
• Phage capsids


  • They have the most complex capsids found among viruses
  • Phages have an elongated capsid head that encloses their DNA
  • A protein tail piece attaches the phage to the host and injects the phage DNA inside
• Host range


  The limited number of host species that a virus can infect
• Some viruses have such narrow host ranges that they can only infect a single species
• Viral infections of multicellular eukaryotes are usually limited to particular tissues
• General features of viral replicative cycles


  1. Viral genome enters cell

  2. Cell manufactures viral proteins

  3. Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses

  4. New viruses exit the host cell, usually damaging or destroying it
• Lytic cycle


  A phage replicative cycle that culminates in the death of the host cell, producing new phages and lysing (breaking open) the host's cell wall
• Virulent phage


  A phage that reproduces only by the lytic cycle
• Lysogenic cycle


  A phage replicative cycle that replicates the phage genome without destroying the host
• Temperate phages


  Phages that use both the lytic and lysogenic cycles
• Prophage


  The viral DNA molecule that is incorporated into the host cell's chromosome in the lysogenic cycle
• Every time the host divides, it copies the phage DNA and passes the copies to daughter cells
• An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode
• Restriction enzymes


  Enzymes that cut up foreign DNA, like phage DNA, to defend the bacterium
• CRISPR-Cas system


  A bacterial immune system that integrates phage DNA between repeat sequences and then uses the resulting RNA to target and degrade the corresponding DNA
• Animal viruses


  • They can be classified by the nature of their genome (single- or double-stranded DNA or RNA) and the presence or absence of an envelope
• Viral envelope


  Used by animal viruses to enter the host cell, derived from the host cell's plasma membrane and containing mostly viral molecules
• Retroviruses


  Viruses that use reverse transcriptase to copy their RNA genome into DNA
• Provirus


  Viral DNA that is integrated into the host genome
• Viruses do not fit our definition of living organisms, and they probably evolved after the first cells appeared, possibly multiple times
• Candidates for the source of viral genomes are plasmids and transposons
• Vaccine


  A harmless derivative of a pathogen that stimulates the immune system to mount defenses against the harmful pathogen
• Antibiotics target enzymes specific to bacteria and are powerless against viruses
• CD4


  The main receptor used by HIV to infect cells
• CCR5


  A co-receptor, along with CD4, that is needed for HIV to infect most cells
• Emerging viruses


  Viruses that suddenly become apparent
• Three processes contribute to the emergence of viral diseases: mutation of existing viruses, spread from small isolated populations, and spread from animal populations
• Influenza A strains


  They are given standardized names identifying the forms of the viral surface proteins hemagglutinin (H) and neuraminidase (N)
• Many viruses are transmitted by mosquitoes, and climate change may be allowing mosquitoes carrying diseases like dengue fever to move to new locations
• More than 2,000 types of viral diseases of plants are known, with major impacts on agriculture and horticulture