chapter 15

Bio1100 Chapter 15

Evolution and diversity among the microbes

Microbes are tiny organisms that include viruses .
- Microbes are tiny life too small to see without magnification.
  These are unicellular organisms found in all 3 domains of life: Bacteria, Archaea and Eukarya (in the Protist kingdoms).
  Viruses do not share many of the properties of life, and are classified outside of the 3 domains.
  - Are viruses alive?
    - If viruses are not alive, why are we trying to kill them?
- A large diversity of microbes, mostly bacteria, live in an on a typical human.
  Hundreds of species live on our skin, in the mouth, and in our intestinal tract.
  Out of about 100 trillion cells in a human, only 10 trillion are human cells, vastly outnumbered by microbial cells.
- Kingdom Protista is an eukaryote group that includes unicellular predators as well as multicellular organisms such as fungus-like slime molds or plant-like kelp.
  Many scientists are beginning to separate these diverse protists into separate clades.
- Microbes include unicellular protists such as Amoeba and Euglena which are slightly below visibility to the human eye, down to about 10 µm.
  Unicellular bacteria are an order of magnitude smaller, below 10 µm in size.
  Viruses are even smaller, less than 1 µm in size.
- Basic properties of life
  - A complex, ordered organization consisting of one or more cells
  - Use and transformation of energy
  - Sensitivity and responsiveness to the environment
  - Regulation and homeostasis
  - Growth, development, and reproduction
  - Evolution: species change over time
Viruses are intracellular parasites and can reproduce only within host cells.
- Viruses cannot reproduce on their own, and need to infect a host cell.
  A virus is made up of nucleic acid genome (DNA or RNA) enclosed in a protein coat called a capsid.
  Some animal viruses are surrounded by an envelope derived from the plasma membrane of a host cell.
- Viruses cannot reproduce on their own, and need to infect a host cell.
  The protein container of a virus is called the capsid; the virus that causes the common cold is enclosed only by the capsid.
  Some viruses, such as the flu virus, are enveloped viruses that wrap a bit of the plasma membrane of the host cell around the capsid.
The human immunodeficiency virus (HIV) is a retrovirus that infects human white blood cells.
- HIV is a retrovirus with two single strands of RNA genome.
  - A virus infects white blood cells by binding to receptor proteins in the plasma membrane of the host.
  - Its RNA and enzymes enter the host by endocytosis.
  - The viral enzyme reverse transcriptase copies the RNA genome into DNA.
  - The viral DNA is then inserted into the host DNA using the viral enzyme integrase.
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  Review:
- The HIV DNA resides permanently within the host DNA, but its genes may remain un-expressed for a long time.
  - Eventually the viral DNA is transcribed into RNA molecules, which serve both as mRNA for synthesis of viral proteins and as new viral genomes.
  - The proteins and RNA assemble into new virus particles and exit the host cell, enveloping the capsid with the host's plasma membrane.
  The exocytosis may kill white blood cells and reduce the body's immune response, causing Acquired Immune Deficiency Syndrome (AIDS).
The influenza virus is another RNA virus noted for its mutability.
- Influenza viruses are RNA viruses with two kinds of surface proteins (H and N) embedded in its envelope.
  Animal antibodies are specific to variations (subtypes) of H and N proteins.
  Different combinations of H and N subtypes are the basis for flu strains.
- Genetic recombination in the flu
  Bird flu viruses usually do not infect humans, but can do so via a pig intermediate host.
  Some flu strains may originate in birds, then undergo genetic recombination in a pig before propagation among humans.
  The Hong Kong flu of 1968, for example, arose from recombination between duck H3N8 and human H2N2.
  The recombinant H3N2 was a new strain never experienced in human populations before.