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Section 18.1 Summary – pages Viruses are considered nonliving because: They are NOT cells. They don’t carry out respiration, grow, or develop. They do not reproduce like living cells. What is a Virus?
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Virus NotesTEK 4C
• Viruses are composed of nucleic acids enclosed in a protein coat and are smaller than the smallest
bacterium.
What is a Virus?
Viruses are considered nonliving because:
• They are NOT cells.• They don’t carry out respiration, grow, or develop. • They do not reproduce like living cells.
What is a Virus?
What is a Virus?
•Viruses cannot reproduce without the help of a living cell.
•Viruses infect a host cell and inject their DNA/RNA.
• A cell in which a virus replicates inside of is called the host cell.
What is a Host Cell?
Before a virus can replicate, it must enter a host cell.
• A virus recognizes and attaches to a host cell when one of its proteins attaches with a molecular shape that is the receptor site on the host cell’s plasma membrane.
Virus
• Each virus has a specifically shaped attachment protein. Therefore, each virus can usually attach to only a few kinds of cells.
• In general, viruses are species specific, and some also are cell-type specific. For example, polio viruses normally infect only intestinal and nerve cells.
Viral Attachment to Host
• A virus has an inner core of nucleic acid, either RNA or DNA
and an outer protein coat called a capsid.
Capsid
Nucleic acid
Viral Structure
• Some larger viruses, such as human flu viruses, may have an additional layer, called an envelope, surrounding their capsids.
Capsid
Nucleic acid
Envelope
Viral Structure
• Envelopes are composed primarily of the same materials found in the plasma membranes of all cells.
Polyhedral virus shape
Human Papilloma Virus
Envelope studded with projections
(flu and AIDS)
VIRUS SHAPES
Long narrow helical shape (Tobacco Mosaic
Virus)
Polyhedral-shaped head attached to a cylindrical tail
with leg-like fibers
VIRUS SHAPES
Viruses and Disease
• A vaccine is an injection of particles of viruses or bacteria that provide the human body with a defense against disease.
• Antibiotics are ineffective against viral infections.
• For bacterial infections, antibiotics are a good option.
Common Viral DiseasesInfluenza
Common Cold
Chicken Pox
Common Viral Diseases
Small Pox
HIV / AIDSHIV attacking T-Cell
West Nile VirusTransmitted by mosquito