Acellular Microbes

  1. What microbes are acellular?
    Acellular microbes include viruses, viroids and prions.
  2. What is the range of sizes of acellular microbes?
    Acellular are usually found to be between 10-300 nm in length, but have been found up to 1um in length.
  3. What organisms can be infected by viruses?
    Any kind of organism can be infected by viruses; plants, animals, fungi, bacteria, yeasts and so on.
  4. What properties distinguish viruses from living cells?
    • - Living cells contain both DNA and RNA, viruses contain one or the other.
    • - Viruses cannot produce energy or proteins.
    • - Viruses cannot reproduce on their own, they need a host cell's infrastructure.
    • - The replicate, rather than reproduce through binary fission, mitosis or meiosis.
  5. What is the general structure of viruses?
    Viruses contain a (relatively) small genome of either DNA or RNA, surround by structure called a capsid which is itself constructed from capsomeres. Some viruses will also have an envelope composed of lipids and sugars, phospholipids from host membranes or viral glycoproteins.
  6. What is a capsid?
    A capsid is a protein structure that contains the viral genome. It isconstructed from units called capsomeres and is usually helical or icosohedral, but can form more complex shapes.
  7. What are capsomeres?
    Capsomeres are the sub units that combine to make up a viral capside. Viruses are genetically economical and so often a single or only a few proteins will make up a capsomere.
  8. What do enveloped viruses possess in particular?
    Enveloped viruses possess a second structural layer other than the capsid. It is often made of phospholipids and proteins from the membrane of a previous host cell, sugar or lipid structures and sometimes viral glycoproteins. Envelopes are not protective and can actually be quite particular to their environment, meaning enveloped viruses are often easier to sterilise. Envelopes also allow viruses to bind to receptor proteins on the outside of potential hosts.
  9. How are viruses classified?
    • Viruses can be classified by:
    • - Having DNA or RNA
    • - Their capsid shape
    • - Number of capsomeres
    • - The type of host it infects
    • - Its target cell
    • - The disease it causes
    • - Its antigenic properties
  10. What are bacteriaphages?
    Bacteriaphages are viruses that infect and replicate within bacteria.
  11. What are animal viruses?
    Animal viruses are viruses that infect and replicate within animals.
  12. What are the steps in animal virus replication?
    • 1. Attachment
    • 2. Penetration
    • 3. Uncoating
    • 4. Biosynthesis
    • 5. Assembly
    • 6. Release
  13. What determines whether viruses attach to cells?
    Viruses will only attach to cells that have specific receptors on their cell membrane. If these receptors match proteins on the virus's surface they can be thought of as fitting together like a lock and a key.
  14. What is attachment?
    Attachment is the process by which a virus attaches to the exterior of a host cell. On the exterior of a host cell are thousands of protein appendages called receptors, having a specific function for the cell. Viruses have evolved their own receptor structures so that they are able to bind onto specific receptors on a host cell. The virus receptors fit with the host receptors like a key into the lock.
  15. What is penetration?
    Penetration refers to the movement of a virus through the cell membrane to the interior of a cell. After binding to receptors on the cell, often the virus will activate mechanisms in the membrane cells which are designed to absorb particles, pulling the virus into the cell (endocytosis, translocation and fusion). If attachment is like a key fitting into a lock, then penetration is like a burglar twisting the handle and walking straight in.
  16. What is uncoating?
    Uncoating refers to the removal of the virus's capsid (and envelope if there is one) and exposing the virus's genome. This is itelf is a complicated process and does not necessarily happen at once; the uncoating process is underway as the virus is being accepted into a cell. It can include the complete digestion of the coat by the cell, or the exit of a genome from the coat.
  17. What is biosynthesis?
    Biosynthesis is the process by which a virus will utilise the machinery within a host cell to replicate parts of its structure, such as the genome and capsid.
  18. What is assembly?
    Assembly is the process by which parts of a virus created during biosynthesis are assembled into a new virus. The virus may be completed while inside the host cell, or obtain the final parts of its structure as it exits the host.
  19. What is release?
    Release refers to the release of viruses from a host cell. After replication there will be many -  often thousands or millions - viruses inhabiting the cell. They can exit via ectocytosis, using active transport to pass through the membrane and they can also simply rupture the cell membrane all together.
  20. What are latent viruses?
    Latent viruses possess the ability to make entry into a host cell and cease its activity, so as to lay dormant in the cell until a time when it is reactivated and continues to replicate.
  21. What are prions?
    Prions are infectious proteins. They are thought to be misfolded proteins and all known diseases caused by prions affect the brain or neural tissue.
  22. What are viroids?
    Viroids are incredibly small pathogens similar to viruses. They consist of a single strand of RNA and infect plants.
Card Set
Acellular Microbes
Viruses, viroids and prions.