WHAT IS A VIRUS?

Virus - Latin word meaning “poisonous fluid”.

Viruses are referred to as subcellular organisms, which means they are smaller than most cells (including human cells and bacteria). Viruses are so small that we need to measure them in nanometers, which in comparison is a thousand times smaller than what we use to measure bacteria with (micrometers). The only way so see a virus would be to use the most powerful of electron microscopes. Viruses were thought as to be the smallest infectious agent, but recently that position has been taken over by two smaller pathogens known as prions and viroids.

Viruses can enter the body silently via various routes such as ingestion, inhalation and direct contact. When they gain entrance, they use human cells to manufacture substances needed for their own replication and life cycle. Viruses are only active when inside a host because they have no metabolic life of their own outside a host cell. Viruses cannot make their energy or proteins and cannot reproduce without a host (human, animal, plant, or bacteria). Outside host cells, viruses are in dormant stages where they exist in a world between the living and nonliving.

The basic viral particle or single virus is called a virion. This virion consists of a nucleic acid genome (where the virus’s heredity information is stored), surrounded by a protective shell of protein called a capsid (or lipid envelope).

Fig 1 - Structure of a Virus

Unlike most cells which have a cell wall membrane, a virus has a capsid made of proteins that may contain protein spikes which help penetrate host cells. It protects the nucleic acid genomes containing the virus’s genetic material, which directs the activity and function of the virus. Viruses contain either DNA or RNA, but not both. Because both DNA and RNA are required for cell to reproduce, viruses invade host cells and take over their genetic equipment.

All viruses seek to penetrate a host cell and "trick" it into replicating more of the virus until ultimately, it bursts.

There are five steps in the viral life cycle:

• Attachment – A virus comes into contact with a host cell and attaches specific spots on the cell called receptors.
• Entry – The virus secretes chemicals that weaken the host cell wall. Once a weak spot on the cell wall is made, DNA or RNA is injected through.
• Replication – When inside a cell, the virus takes over and instructs the host cell to make more viral DNA and proteins
• Assembly – Cellular activity of the host cell help assemble new viruses until the host cell is filled with new viruses.
• Release – When assembling is complete, the virus particles release enzymes that digest the host cell wall. The new virus particles released go to infect new cells where the cycle is repeated.

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Project created by James Li. Sir Winston Churchill High School Grade 11. 2004.