Viruses vs. Unicellular Organisms

• Molecular Virology is the BSc III course MVI3MV1 for The Department of Virology in 2025. Study material will be posted on Learn2025.

Course Handout

• Read the course handout posted on Learn2025™ for detailed information.

Compulsory Material

• The following course materials will be posted on Learn2025:
  • Course Handout.
  • Updated Timetable.
  • Lectures (PowerPoint Presentations).
  • Mini-quizzes.
  • Announcements.

Optional Reading Material

• Easy to Read Textbooks:
  • "Virology: an illustrated colour text" by Stephen NJ Korsman
  • "Principles of molecular virology" by Alan Cann
• Reference Books:
  • "Principles of virology" by S Jane Flint
  • "Fields Virology" by Bernard N Fields and David M Knipe
• Reputable Websites:
  • http://www.virology.ws/course/
  • https://hivbook.com/

What is a Virus?

• It is an infectious, obligate intracellular parasite; it comprises genetic material (DNA or RNA) surrounded by a protein coat and/or a membrane.
• A virus is "a piece of bad news wrapped in protein" according to Peter Medawar.
• Viruses are organisms with two phases: virion and infected cell.
• A complete, mature, infectious viral particle is called a virion.
• When a virion infects a host cell, it disassembles.
• Building blocks for new virions are then produced by the infected host cell.
• The virus then exits the cell and enters a new cell to start another round of replication.
• Viruses infect all living things.
• Viruses are smaller than plant cells, animal cells, and bacteria, and, in fact, the head of a pin could hold 500 million rhinoviruses.

Properties of Viruses

• Smaller than bacteria; almost all viruses are submicroscopic and can only be visualized by electron microscopy
• Diameters range from 10-1000 nm (most are 20-300 nm)
• Contains only one type of nucleic acid; either DNA or RNA, but never both.
• Nucleic acid is surrounded by a protein coat.
• Some have an additional lipoprotein envelope; viruses replicate by "tricking" cells into making multiple copies of viral nucleic acid and protein, which then self-assemble to form new virus particles.
• They do not have organelles, such as mitochondria and ribosomes.
• They cannot replicate outside living cells.
• They do not undergo binary fission.
• They do not grow.
• They cannot generate their own energy/metabolism, and do not move on their own i.e. movement is passive.

Viruses vs. Unicellular Microorganisms

• Size:
  • Viruses are submicroscopic and need an electron microscope to be seen.
  • Unicellular microorganisms are microscopic and can be seen with a light microscope.
• Binary Fission:
  • Viruses do not undergo binary fission.
  • Unicellular microorganisms do.
• Nucleic Acid:
  • Viruses only contain one of RNA or DNA.
  • Unicellular microorganisms have both RNA and DNA.
• Infectious Nucleic Acid:
  • Viruses have infectious nucleic acid.
  • Unicellular microorganisms do not.
• Ribosomes:
  • Viruses do not have ribosomes.
  • Unicellular microorganisms do.
• Generate Energy:
  • Viruses cannot generate energy.
  • Unicellular microorganisms can.
• Growth in Media:
  • Viruses cannot grow in non-living media.
  • Unicellular microorganisms can.

Why Study Viruses?

• Almost everyone has had a viral infection, and most of the population are chronically infected.

Consequences resulting from Viruses

• Severe infections +/- hospitalization.
• Chronic infections +/- sequelae such as cancer.
• Requirement for long term therapy.
• Death.
• Viruses burden patients, healthcare systems, and society.
• Viruses have been a threat to mankind throughout history and will continue to be, despite advances in medicine.

Ancient Evidence of Viral Infections

• 3700 BC - Ruma showed evidence of Polio.
• 1196 BC - Ramses V died.
• 1193 BC - Pharaoh Siptah had foot deformation.

Examples of Important Viruses

• Polio
• Influenza killed 6,000,000 people during a 12 week period.
• HIV (2017):
  • 36.9 million people worldwide were living with HIV/AIDS.
  • 940,000 people died of HIV-related illnesses.
• SARS-CoV-2
  • There was a new coronavirus (2019-nCoV) outbreak.

Learning Goals

• Define a virus.
• Define virion.
• List the properties of a virus.
• Differentiate viruses from other microorganisms.

Explore the key differences between viruses and unicellular microorganisms. Understand viral replication mechanisms and the unique properties of viruses, including their reliance on host cells. Test your knowledge of virion structure and viral components.