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Human Biology (Year 12) - Pathogens

Ben Whitten

What is a virus?

Viruses are classed as non-cellular pathogens. They consist of one or more strands of nucleic acid, either RNA or DNA, which are contained within a protein coat. Viruses are not made out of cells and are therefore non-living. They do not contain any metabolic machinery for processes such as cellular respiration and can't be classed as prokaryotic or eukaryotic.

A virus is often called an obligate parasite as it cannot function outside the host cell. Unlike bacteria, viruses cannot be grown and studied outside of live cells.

All type of organism on Earth are susceptible to viral infection. Viruses are very significant pathogens of many animals, plants and even bacteria have their own group of viral pathogens called bacteriophages.

Each virus is usually limited to infecting a specific host cell or organism. For example, an adenovirus infects epithelial cells in the upper respiratory tract, causing the common cold; this is as the virus is able to recognise and bind to receptors that are only expressed on respiratory tract epithelial cell surfaces.

What are the structural features of viruses?

Viruses are a non-cellular agent composed of a protein coat, otherwise known as a capsid, and nucleic acid which is either DNA or RNA; it cannot be both. Viruses are microscopic and very small in comparison to all other pathogens.

What are the steps in viral replication for a eukaryotic host?
  1. The virus will attach itself to the cell on the cell membrane.

  2. The virus will enter into the host cell, and the virus contains nucleic acids.

  3. Viral DNA or RNA enters the nucleus of the eukaryotic host cell.

  4. The viral DNA or RNA directs the host cell to replicate it and make many copies of viral proteins through means of translation.

  5. New viral DNA or RNA and proteins assemble at the host's cell membrane and ext the cell.

What are the steps in viral replication for a bacterial host?
  1. The virus particle binds to the wall of the host cell, and viral DNA enters the cell's cytoplasm.

  2. Viral DNA directs host cell machinery to produce viral proteins and copies of viral DNA.

  3. Viral proteins are assembled into coats; DNA is packaged inside.

  4. Tail fibres and other components are added to coats.

  5. The host cell undergoes lysis and dies, releasing the new infectious virus particles.

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