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.
###########################################################################################
