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Notes About Viruses

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Copyright © 2007, 2009 by Zack Smith.
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Viruses

Small: measured in nm.

Viruses are not really living. They are simply pieces of genetic code in a container that are sent into a living cell.

That container is typically made of protein.

Viruses require cells as hosts. They have no metabolism on their own.

Viruses infect every form of life. A virus that targets bacteria is called a bacteriophage.

Virus parts

  • Genetic material, either DNA or RNA
  • A "capsid" = a container made of protein
  • An optional phospholipid envelope with glycoproteins on the outside surface

Virus shapes

There are three:
  • Helical i.e. a spiral like shape
  • Polyhedral up to 20 sides
  • Complex = with head & tail elements

Viruses have 2 kinds of metabolism

  • Lytic, where they take over a cell
  • Lysogenic, where their DNA/RNA is integrated into the host's

Viruses types by envelope

  • Naked i.e. no envelope, easy for the immune system to kill.
  • Enveloped

The lytic life cycle is AESAR

  • Attachment
  • Entry & degradation of host's DNA
  • Synthesis of new viruses
    • duplication of virus' genetic material
    • creation of new virus parts
  • Assembly of parts into new viruses
  • Release of new viruses

Burst size = the number of viruses that a virus can produce before the host cell bursts. The T4 phage makes about 200.

The lysogenic life cycle is AEIM

  • Attachment
  • Entry (no degradation of host DNA)
  • Integration using integrase enzyme
  • Mitosis
    • Host+virus DNA duplicated
    • cytokinesis

A virus that uses the lysogenic life cycle is termed either a provirus or prophage. What causes various proviruses to shift to the lytic cycle is not completely understood, but in some cases stress is the stimulus.

Well known lytic viruses

  • SARS
  • Common cold
  • Influenza
  • Rabies
  • AIDS mode of HIV
  • Tobacco mosaic virus

Well known lysogenic viruses (proviruses)

  • HIV+
  • Herpes
  • Hepatitis B
  • Varicella-Zoster (chicken pox)

Herpes

There are kinds of Herpes
  • Herpes simplex = highly contagious, you can get it from sharing a bottle, utensil, chapstick; touching a doorknob. Effects are sores at point of contact e.g. mouth.
  • Genital Herpes = you get it from sexual intercourse. Effects are sores on genitals: painful.

Chicken Pox

  • If you get it as a child, you become immune.
  • If you get it as an adult, you will endure painful shingles.

HIV

There are 250 variants of HIV, therefore a vaccine is hard to create. Some variants are strong, others weak.

HIV is an enveloped RNA virus that is unusual in that it also contains two useful enzymes inside the capsid:

  • HIV reverse transcriptase: converts RNA from HIV to cDNA
  • HIV integrase: inserts the cDNA into the hosts DNA.
HIV's surface glycoprotein gp120 latches onto the CD4 receptor of the human immune system's T4 cells. When HIV becomes lytic, it destroys the T cells and renders the immune system unable to defend a person against even the common cold.

HIV viruses are created by budding, in which the cell membrane of the host is used to create the envelope of the new viruses.

HIV-resistant people have a defective T-cell receptor.

Prions

Prions are proteins that attack other proteins, converting them into something like itself. These then combine to form structures.

Examples of prion diseases include:

  • Mad Cow disease (BSE)
  • Creutzfeld-Jacob disease
  • Scrapie

Viroids

These are newly discovered particles of RNA that are known to attack plants, at this point. They lack both capsid and envelope, so they are bare RNA.

Oncogene Theory

This theory states that 15% of cancers are caused by viruses. One example is HPV, which causes cervical cancer i.e. cancer of the cervix in women. The virus is lysogenic and it inserts a destructive gene into the tumor suppressor T53 gene. This allows a mutated oncogene to cause a cell to become cancerous.

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