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More
of the Same
The earliest
cells on earth reproduced by
simply dividing into
two cells. This is termed asexual
reproduction because the cell just divides
itself into two identical
cells and does not
need a partner to
contribute genetic
material.
This is called binary
fission.
Some bacteria can
reproduce very
quickly, and may even
divide every twenty
minutes under ideal
conditions.
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Prokaryotes are much
smaller and more
simple than the
Eukaryotes that
eventually evolved
from them.
The genetic material (the DNA)
holds the directions about how to
make the cell and what the cell
should do.
Remember, life processes are
chemical processes. Everything
that the cell does uses
energy. The important molecule
that fuels living organisms is
called ATP. Organisms produce
ATP by using photosynthesis or
taking in (eating) organic
molecules. Processing
ATP also takes energy, but energy is
left over for respiration and
reproduction.
This
is how Binary Fission
happens. Note
that the cells use energy when they
divide.
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Interphase (just
living) |
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The
cell has reached close to
maximum size.
The cell has a good supply
of ATP (fuel).
Temperature
favors cell chemistry.
Nutrients are available in
the environment |
| DNA
Synthesis |
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The
cell makes a copy of its
loop of DNA.
There are now two DNA loops
in the cell.
This
took energy and used up some
of the ATP.
The cell rests for a little
while.
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| Separation
of genetic loops |
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DNA
strands separate and attach
themselves to the cell wall. |
| Physical
division of cell begins |
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The
cell starts to divide into
two cells.
Each
one will have its own loop
of DNA.
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| Two
identical cells have formed. |
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One
cell becomes two identical
cells.
Each cell has a loop of DNA
and some
ribosomes.
Dividing
the cell took energy.
The new cells rest and
produce more ATP,
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Conjugation
In
addition to binary
fission, bacteria
sometimes share
genetic material by a
process called
conjugation. There
is a special set of
genes that governs
this.
Plasmids:
small circles of DNA
floating in the
cytoplasm
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Bacteria
swimming freely in water.
Note
the loop of genetic material.
Note
the blue ribosomes
Note
the round orange plasmids.
Plasmids
are free floating little
loops of DNA.
They can reproduce themselves
independently.
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Gotcha!
One
of the bacteria sends
out a "grappling
hook" and catches the
other one!
This
little hook is called a pilus.
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The
pilus draws the bacteria
close to each other.,
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The
pilus becomes a tube and some
of the little plasmids (small
independent loops of 'DNA)
cross over from one cell to
the other cell.
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The
tube closes and the bacteria
go on their way with some
genetic material that they
did not have before.
These
genes may find their way into
the main DNA loop.
This
really happens! You
have heard of infections that
become resistant to penicillin
and that then pass their
resistance to other
infections and
diseases. This is how
they do it!
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Advantages:
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Disadvantages:
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Cells are identical to parents and
so are vulnerable to the same environmental stresses.
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The characteristics of the cells change
very slowly
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there is very little innovation in
survival strategies.
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Unchanging cells may be slow to take
advantage of new energy sources.
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