Cell Theory, Form, and Function: Viruses
Viruses are small nucleic acid units, either DNA or RNA, surrounded by a protective protein coat, or capsid, making them little more than packaged genes. Some viruses, such as influenza (flu), have a cloaking protein envelope, making it easier to penetrate a host cell. Other viruses such as HIV also have an unusual complement of enzymes that create interesting products. In general, their overall size ranges from 20 nm to 250 nm (one nanometer, nm = 0.00000004 inches), making them much smaller than any single-celled organism and only visible through an electron microscope. Viruses have plagued man for millennia, causing such human maladies as chickenpox, warts, hepatitis, smallpox, polio, mononucleosis, colds, herpes, and rabies, just to mention a few. Although viruses contain either DNA or RNA, they are actually considered nonliving because they do not grow, reproduce on their own, maintain homeostasis, nor metabolize. Their “life cycle” is an interesting study of deception, pillage, and piracy.
A bacteriophage is a virus that infects bacteria. They are useful because they are easy to study and have added greatly to our knowledge of viruses and how they work.
A virus, also known as a phage, can only survive by infecting a living host cell and turning that cell into a factory to manufacture more viruses. Research in molecular biology often studies bacteriophages because they are common and easier to culture and maintain than more pathogenic types. There are two known methods which explain how viruses are spread: lytic cycle and lysogenic cycle.
In the lytic cycle, the phage always destroys the host cell as the final act of the following five-part event:
- The phage attaches to the cell membrane and injects viral DNA or RNA into the living host cell.
- Injected phage nucleic acids contort into a circle inside the cell.
- The infected cell mistakenly copies the phage DNA or RNA (whichever nucleic acid the phage possesses).
- The copied nucleic acids organize as phages.
- When the number of completely assembled phages becomes too large for the host cell to contain, the cell membrane breaks, releasing numerous phages to infect neighboring cells.
The lysogenic cycle also has five stages, but the host cell is not destroyed, but is used to continually reproduce more phages:
- The phage attaches to the cell membrane of a living cell and inserts its DNA or RNA.
- Phage DNA or RNA reforms as a circle inside the host cell.
- Phage DNA becomes incorporated into the host cell DNA, called a prophage.
- Host cell reproduces normally and mistakenly makes new phage nucleic acids at the same time as normal nucleic acids. The phages are released into the environment to infect other cells.
- Under certain conditions, a prophage may switch to a lytic pathway. Otherwise the host cell continues to generate more phages.
Certain viruses have a cloaking cover made of a protein-lipid combination with glycoprotein projections from the surface. These viruses, such as mumps, use their glycoprotein spikes to simulate a normal protein and thereby mask their identity so they can attach to receptor sites on the cell membrane of the host. The envelope then fuses with the cell membrane and allows the viral nucleic acids to spill into the host. Vaccines have been developed and are effective in combating certain viral diseases such as smallpox, mumps, and polio. A vaccine is a harmless variation of the microbe that is designed to stimulate the immune system of the individual.
Excerpted from The Complete Idiot's Guide to Biology © 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.