Viruses are extremely small infectious agents that invade cells of alltypes. Once inside another cell, viruses become hijackers, using thecells’ machinery to produce more viruses. Whether viruses constituteliving organisms or not—they can only reproduce by means of usinganother cell’s machinery—has been a source of debate for many years.Because of their in-between status, viruses do not fit into thetaxonomic system; neither do they commonly appear on the
SAT II. Allyou need to know about viruses appears below.
Structure
All viruses have a protein capsid or headregion that contains genetic material. The genetic material can beeither DNA, RNA, or even in some cases a limited number of enzymes.Some viruses also have an elaborate protein tail region. The tail aidsin binding to the surface of the host cell and penetrating the surfaceof the host so that the virus’s genetic material can be introduced.
Virus “Life Cycle”
Though the details of virus infection andreplication vary greatly with the type of host a particular virusattacks, all viruses share four basic steps in their replicationcycles:
- Attachment: Using specialized protein structures located onthe exterior of the capsid or tail, the virus latches onto the cell itwill attack and hijack. The protein structures are specific to specificcells. A virus that can attach to a bacterium is unlikely to be able toattack animal cells.
- Penetration: The virus breaks through the cell wall and cell membrane, releasing its genetic material into the host cell.
- Replication and assembly: The viral genetic materialhijacks the cell machinery. Host ribosomes begin to produce viralproteins and nucleic acids. The virus uses the host cell to assemblemany new viruses.
- Release: Viruses are bad guests. In addition to theproduction of new viruses, the viral genetic material usually forcesthe host cell to produce an enzyme that kills, or lyses, the host andbreaks it open, freeing the many new viruses to go and hunt new hostcells to attack. Almost always, the host cell is killed when it isinvaded by a virus.
