Transcript Viruses Versus Living Organisms

Viruses Versus
Living Organisms
Biology
Virus
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Consists of genetic material surrounded by
a protein coat.
Viruses are much smaller than bacteria.
Can only be seen through an electron
microscope.
Viruses
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Exhibit a variety of shapes and sizes.
They cannot survive alone.
They can reproduce only when they are
inside living cells.
Once inside the cell, a virus uses the host
cell to produce many copies of itself.
Viruses cannot reproduce on their own,
they are not considered living organisms.
Capsid
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The protein coat of a virus is called the
capsid.
The proteins of the capsid bind to receptors
on the surface of a cell.
This is the first step in the infection of a cell
by a virus.
Bacteriophage
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Most viruses are specific to the cells they
infect.
Plant viruses only infect plant cells.
A virus that infects bacteria is called a
bacteriophage.
Bacteriophages are among the most
common viruses.
Bacteriophage
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They use bacterial cells to reproduce by attaching
their legs to a bacterium and then injecting their
genetic material into it.
The virus takes control on the bacterial cell and
directs it to produce more viruses.
A single infected cell may make thousands of
viruses.
These new viruses go on to infect other cells.
Viruses are responsible for colds, measles,
rabies, and chicken pox.
Two different processes
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Can occur once the genetic material of a
virus in inside its host cell.
They are the lytic infection and the
lysogenic infection.
Lytic infection
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Injects its DNA directly into the cell.
The host cell is unable to tell the difference between the viral
DNA and its own.
The host cell begins to make mRNA from the genes of the
virus.
The mRNA is then translated into thousands of proteins that
destroy the DNA of the cell.
The cell then makes thousands of copies of the virus’ DNA
which gets assembled into new virus particles.
The cell bursts and releases the virus particles.
Lysogenic viruses
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Do not destroy the host cell right away.
These viruses insert their DNA into the host’s DNA.
The inserted DNA is called a prophage.
The prophage can exist in the DNA of the host cell for many
generations without becoming active.
At some point, the DNA of the prophage becomes active and
removes itself from the DNA of the host.
It then directs the production of new viruses by entering the
lytic cycle.
Are viruses alive?
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NO.
Living things must be able to live
independently of other organisms.
However, viruses do share many
characteristics with living things.
Differences Between Cells and
Viruses
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Living Cells
Reproduce through cell
division
Genetic code is DNA
Use energy
Respond to environment
Change with time
Develop and grow
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Viruses
Reproduce inside host
Genetic code can be DNA
or RNA
Do not use energy
Do not respond to
environment
Change with time
Do not develop and grow
Vaccines
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We have vaccines for measles, mumps, and
polio.
A piece of the antigen ( the part that the body that
reacts to) is injected into the body.
The immune system then produces antibodies
against it.
If you later come into contact with the pathogen,
your body will have a defense against it.