Lesson: Chapter - 8

Living or Not? Viruses

Viruses are extremely small infectious agents that invade cells of all types. Once inside another cell, viruses become hijackers, using the cells’ machinery to produce more viruses. Whether viruses constitute living organisms or not—they can only reproduce by means of using another cell’s machinery—has been a source of debate for many years. Because of their in-between status, viruses do not fit into the taxonomic system; neither do they commonly appear on the SAT II. All you need to know about viruses appears below.

Structure

All viruses have a protein capsid or head region that contains genetic material. The genetic material can be either DNA, RNA, or even in some cases a limited number of enzymes. Some viruses also have an elaborate protein tail region. The tail aids in binding to the surface of the host cell and penetrating the surface of the host so that the virus’s genetic material can be introduced.

Virus “Life Cycle”

Though the details of virus infection and replication vary greatly with the type of host a particular virus attacks, all viruses share four basic steps in their replication cycles:

1. Attachment:Using specialized protein structures located on the exterior of the capsid or tail, the virus latches onto the cell it will attack and hijack. The protein structures are specific to specific cells. A virus that can attach to a bacterium is unlikely to be able to attack animal cells.
2. Penetration: The virus breaks through the cell wall and cell membrane, releasing its genetic material into the host cell.
3. eplication and assembly:R The viral genetic material hijacks the cell machinery. Host ribosomes begin to produce viral proteins and nucleic acids. The virus uses the host cell to assemble many new viruses.
4. Release:Viruses are bad guests. In addition to the production of new viruses, the viral genetic material usually forces the host cell to produce an enzyme that kills, or lyses, the host and breaks it open, freeing the many new viruses to go and hunt new host cells to attack. Almost always, the host cell is killed when it is invaded by a virus.

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