Download Chpt 17 Viruses and Monerans - Virus a non

Chpt 17 – Viruses and Monerans
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Virus – a non-cellular particle made up of genetic material and protein that can invade
living cells.
Viruses are so small, they cannot be seen by a light microscope
o As a result, the first discovery of a virus did not occur until 1935 (after the
electron microscope was invented)
We now know that there are different types and sizes of viruses
Usually specific viruses will infect specific organism.
o Ex) a plant virus cannot infect an animal
o Ex) some viruses will only infect humans
There are some viruses that can infect more than one type of organism
o Ex) rabies infects mammals and some birds
Some viruses can cause cancer (oncogenic viruses)
o Ex) HPV can cause cervical cancer
Structure of a virus
Even though viruses have many different shapes (p. 357 fig. 17-3) we will be studying
one of the most studied types of viruses
Bacteriophage – a virus that invades bacteria.
The bacteriophage is composed of a core of nucleic acid surrounded by a protein coat.
Capsid – the protein coat that protects the nucleic acid
o The nucleic acid core is either DNA or RNA,
Never both
o The core may contain just a few genes, or several hundred
The capsid with its nucleic core is called the head of the virus
The tail is composed of a long tube with tail fibers that attach to the bacteria
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Virus life cycles
In order for a virus to reproduce, it must invade or infect a living host cell
o Viruses are considered parasites because they harm their host cell.
During their life cycle, the virus uses the host cell to make new viruses that can be
released to invade or infect other cells
There are 3 viral life cycles
o 1 – Life cycle of a Lytic Virus
Infection – a virus is activated when it has a chance encounter with the
appropriate host cell.
• The tail fibers attach to the host cell
• The tail injects the viral DNA into the host cell
Growth – the host cell cannot tell the difference between its own DNA
and the viral DNA so it starts to read the DNA and create m-RNA from the
viral DNA
• The viral m-RNA starts to take over the functioning of the host cell
and uses the host cells materials to create parts to make more
viruses
Replication – the host cell now uses its energy to assemble the viral
components and make new viruses
• The host cell can make thousands of new viruses
Release – once the host cell is full of new viruses, the host cell lyses
(bursts) and releases the new viruses.
• The new viruses can now infect new host cells
o 2 – Life cycle of a Lysogenic Virus
The lysogenic virus has a very similar life cycle to the lysogenic virus with
one exception
The viral DNA from the lysogenic virus is called a prophage
• The prophage enters the host cell when the virus infects the host
cell.
• The prophage is incorporated into the host DNA where it may
remain part of the host DNA for many generations
• Some prophages will assist the host cell in protecting itself from
other viral infections
At some point, the viral DNA (prophage) is activated
• Change in temperature or abundance of nutriments
The viral DNA now takes over the host cells functioning and starts the
viral life cycle
Once the host cell has made enough new viruses, the host cell lyses and
releases the new viruses
o 3 – Life cycle of a retrovirus
A retrovirus contains RNA rather than DNA
When the virus infects its host cell, it injects its RNA
• The host cell will now create a DNA copy of the viral RNA
• This new viral DNA is inserted into the host cell’s DNA (like a
prophage)
The retrovirus now goes through the lysogenic life cycle stages
The retrovirus received its name from the fact that their genetic
information is copied backwards
• From RNA to DNA
• (the prefix retro means backwards)
Some retroviruses cause cancers in humans and animals
HIV is also a retrovirus
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Assignment –
o WS – Viral Life Cycles
o Virus Flip book
o Opinion paragraph