A2.3 Viruses

• viruses are considered to be non-living, non-cellular organisms
• viruses can carry out some functions of life but not all of them
• viruses are not made out of cells
• viruses evolved spontaneously multiple times and unlike living organisms they have multiple origins (too many differences to have a single common universal ancestor)
• types of viruses include:

  1. helical viruses i.e. tobacco or mosaic viruses
  2. polyhedral viruses i.e. adenovirus
  3. spherical viruses i.e. influenza
  4. complex viruses i.e. bacteriophage
• common features of viruses are; small with an average size of 20-300nm, they don't grow and once they're assembled they have a fixed size, can use either DNA or RNA as its' nucleic acid with protein capsids and contain no cytoplasm and have very few enzymes
• viruses rely on host cells to do everything for them
• all viruses are composed of two basic parts:

  1. A nucleic acid (DNA or RNA)
  2. A protein capsid - symmetrical repeating protein units outside the virus
• protein capsids is essentially the protein coat for viruses and every virus has a different protein capsid (but they all have one)
• since viruses have many different origins, lots of diversities in their genetic makeup can be observed
• All viruses contain a genetic material, however, whether that material is DNA or RNA and how that material is used can vary
• In addition to a protein capsid, some viruses also have an envelope but not all
• an envelope is the cell membrane that a virus has burst through
• viruses infect a host by injecting its' genetic material (DNA or RNA)
• After viruses inject a host cell with their genetic material they can undergo either:

  • lysogenic cycle
  • lytic cycle
• The lytic cycle of a virus is as follows: (part 1)

  • A virus (with a protein capsid and genetic material) attaches to a cell. This attachment is dependent on protein compatibility, etc.,
  • The virus then inserts its' genetic material into the cell (the protein capsid remains outside the cell)
  • The genetic material injected will replicate and many copies will be made
  • And then the genetic material undergoes transcription to produce mRNA
• The lytic cycle of a virus: (part 2)

  • One mRNA is produced its' translated into proteins and the viruses can start to assemble (genetic material inside the protein)
  • the viruses then burst out of the cell - lysis - and the host cell is now killed/eliminated
  • the cycle repeats
• Lytic cycle - the virus reproduces and bursts through the host cell and it must now find a new cell because it just killed the host cell via lysis
• the lytic cycle is very common in plants and animals because there are many nearby cells to infect after the virus kills the host
• When something is too virulent that means it runs out of host cells due to rapid infection and lysis i.e. the cells are being killed too quickly
• When something is not virulent enough that means the immune system can detect and destroy it because the viruses are not undergoing lysis rapidly enough
• The lysogenic cycle of a virus is as follows:

  • The virus will attach to a cell and the genetic material is injected
  • instead of being transcribed and translated like the lytic cycle the viral genetic material will become part of the host genetic material
  • the end result is viral genetic material becoming integrated with the host genetic material
  • when the host cell divides, the viral genetic material remains intact and is transported to the new cells being made
  • the cycle repeats and all new cells will have the viral genetic material
• In the lysogenic cycle the genes of the virus become integrated into the host and it doesn't kill the host cell and because of that its' undetectable by the immune system
• Sometimes, certain stimuli can cause the virus cycle to switch from lysogenic into lytic and once that happens the cells will burst
• Common ancestry is an example of divergent evolution
• divergent evolution is when a common ancestor diverges into different species
• convergent evolution is when species evolved from different ancestors but possess similar traits
• viruses are an example of convergent evolution and one of the similar traits they all possess is the fact that they had to evolve to use genetic material
• viruses had to have had evolved in order to mimic the genetic material of cells which explains how different species from different ancestors could have evolved to have the same genetic code
• there are two hypotheses for the evolvement of viruses:

  1. Progressive
  2. Regressive
• If viruses evolved progressively that means they evolved from modified cell components (where different pieces of cells could have come together and formed a virus)
• If viruses evolved regressively that means they evolved from the loss of cell components (where they evolved from cells that have lost some of its' components)
• Viruses evolve very rapidly
• Since viruses evolve rapidly, sometimes immune systems or vaccines can be ineffective against new variations of viruses
• The reasons why viruses evolve rapidly are:

  1. their very short generation time (the shorter the generation time, the more generation you can have at any given time)
  2. lots of mutations (which leads to variation especially in RNA based viruses because they don't have complementary base pairings and cannot proofread instructions)
  3. Natural selection will favor viruses with genetic traits that help it evade detection (viruses that carry genes that can be easily detected by immune systems are generally less successful and don't continue on)
• Mutations are the source of variation
• The influenza viruses use RNA as its' genetic material. This RNA virus has 8 separate RNA molecules which means high mutation and variation.
• Influenza viruses can cause transmission between species and can produce new strains. This is done by antigens (proteins outside the virus used for attachment to other cells) and they can change and form new combinations resulting in these new strains.
• viruses can't connect to every cell as their antigens aren't the right shape but if there is a mutation in the RNA that can affect the antigens protein shape which may result in a new strain that might be transmitted between species
• HIV (human immunodeficiency) viruses mutate very quickly and its' difficult to find vaccinations for them