ANA SAYFA


THE VIRUSES
 
 
*The viruses:
 
→Viruses do not have all of the characteristic of living things.
 
→They only have four characteristic of living organisms.
 
 
 
→Therefore, they do not considered as living organisms.
 
→They are a transitional form between living and non-living things.
 
 
The virus structure:
 
 
 
→The viruses do not have a cell structure, cytoplasm and organelles.
 
→They only have a nucleic acid and a protein coat which called capsid.
 
→Their nucleic acid either made of DNA or RNA, but never both.
 
 
→They do not have enzyme system for metabolic reactions.
 
→Therefore, the antibiotics do not effect to viruses.
 
 
→They cannot reproduce outside a living cell.
 
→They are obligate intracellular parasites that can reproduce only within a specific host cell.
 
Examples: 
 
→Rabies viruses reproduce within the brain cells.
 
→Measles viruses reproduce within the skin cells.
 
→HIV viruses (AIDS) reproduce within white blood cells.
 
 
 
→They invade living cells and force them to make new copies of viruses.
 
→The human immune system, produces interferon to inactivate them.
 
 
The types of viruses:
 
 
 
Plant viruses: Tobacco mosaic virus...
 
Animal viruses: HIV, Influenza, Polio, Hepatitis B...
 
Bacterial viruses: Bacteriophages...
 
 
The plant viruses mostly have RNA.
 
→The animal and bacterial viruses mostly have DNA.
 
Note: Some of the animal viruses have RNA.
 
Examples: HIV, Influenza, Polio...
 
 
 
*Virus replication:
 
→A virus attaches to cell membrane of a host cell.
 
 
→Then, it digests to cell membrane's the host cell.
 
 
→After that, it injects its nucleic acid, into the host cell.
 
→The protein coat, does not enter into the host cell.
 
 
→The viral nucleic acid, forces to the host cell to copy new of the viral nucleic acid.
 
 
→Then, it forces to the host cell to produce viral protein coat.
 
 
 
→After that, the nucleic acid assembles with the protein coat and thus, new viruses form.
 
 
Finally the host cell explodes and the new viruses release.
 
 
 
 
 
 
 
 
 
 
 
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