Viruses

• A virus is a non-cellular particle made up of genetic material and protein that can invade living cells
• All viruses have two structural characteristics:

  • Genetic material (nucleic acid): either DNA or RNA
  • Capsid (protein coat): a protective barrier around the genetic material, providing the virus with viral specificity to infect specific types of cells
• Viruses may also have additional structures like protein spikes, which are embedded in the envelope and are often important for attachment to host cells and may help viruses avoid the immune system of its host
• Methods of transfer for viruses include:

  • Vector (e.g., mosquitos)
  • Airborne transmission (viruses travel as particles in the air through coughs and sneezes)
  • Transmission through bodily fluids (e.g., Hepatitis)
• Defending against viruses involves three lines of defense:

  • Skin and outer coverings
  • White blood cells
  • Production of antibodies, interferons, interleukins
• Immunity against viruses can be achieved through:

  • Active immunity: weakened strains of pathogens are injected to activate lymphocytes into producing antibodies
  • Passive immunity: injection of antibodies to provide temporary protection
• Virus life cycles:

  • Lytic cycle: attachment, entry, replication, assembly, release
  • Lysogenic cycle: viral DNA joins onto a chromosome in the host cell, remains dormant, can be triggered to become active and enter the lytic cycle
• some viruses also contain an envelope made up of lipids from host cell membrane
• viruses have no cellular structure, they consist of nucleic acid (DNA or RNA) surrounded by protein coat called capsid
• viral diseases are caused by the introduction of virus into host cells
• viral capsid is composed of proteins that form a protective shell around the genetic material
• the nucleocapsid contains both the genomic RNA or DNA and associated proteins
• viral diseases can cause symptoms such as fever, headache, muscle pain, fatigue, joint pain, skin rashes, vomiting, diarrhea, respiratory problems, neurological disorders, and cancer
• envelope is formed by budding off from the plasma membrane of infected cells
• the most common way that viruses infect humans is via respiratory droplets when someone coughs or sneezes
• envelopes are formed when the virus buds off from the surface of infected cells
• other ways include ingestion of contaminated food/water, sexual contact, insect bites, direct contact with infected body fluids
• most viruses cannot reproduce outside living hosts
• enveloped viruses have an additional outer layer consisting of a phospholipid bilayer with embedded glycoprotein spikes
• retroviruses include HIV, HTLV-1, HBV, HPV
• some enveloped viruses contain glycoprotein spikes on their surface which allow them to attach to specific receptors on target cells
• some viral enzymes degrade antibodies and complement components
• viruses have to enter host cells to replicate themselves
• some viral diseases can be prevented through vaccination
• some viruses can survive on surfaces for hours to days
• non-enveloped viruses do not contain this extra lipid layer
• vaccines are made from weakened or dead virus particles that stimulate the immune system without causing disease
• the first step in viral entry involves attachment of the virus particle to a suitable receptor molecule on the surface of the host cell
• viral entry is the process by which a virus enters its host cell
• viral capsids protect the nucleic acid core from damage by chemicals or heat
• the envelope is not essential for infectivity but it does protect the nucleocapsid from degradation by host defenses
• HIV stands for human immunodeficiency virus
• Virus


  A tiny, nonliving particle that invades and then reproduces inside a living cell.

  
  

  Viruses are made of genetic material and a protein coat
• RNA

  ribonucleic acid
• DNA

  deoxyribonucleic acid
• Prophage


  A phage genome that has been inserted into a specific site on the bacterial chromosome.
• TMV

  tobacco mosaic virus
• Head


  The upper region of a bacteriophage containing the protein coat and DNA
• Vaccine


  substance prepared from killed or weakened pathogens and introduced into a body to produce immunity
• Lytic Cycle


  a viral reproductive cycle in which copies of a virus are made within a host cell, which then bursts open, releasing new viruses