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binary fission



A method of
asexual reproduction

that is employed by most
prokaryotes
. In binary fission, the
living cell divides into two
equal, or nearly equal, parts. It begins when the
DNA

of the cell is
replicated. Each circular strand of DNA then attaches to the
plasma membrane. The cell el
ongates, causing the two
chromosomes

to separate. The plasma membrane then invaginates
(grows inward) and splits the cell into two daughter cells through
a process called
cytokinesis
.


Binary fission theoretically results in two identical cells.
However, the DNA of
bacteria

ha
s a relatively high
mutation

rate.
This rapid rate of genetic change is what makes bacteria capable
of developing resistance to
antibiotics

and helps them exploit
invasion into a wide range of environments.


Similar to more complex organisms, bacteria also have
mechanisms for exchanging genetic material. Although not equivalent to sexual reproduction,
the end

result is that a bacterium contains a combination of traits from two different parental
cells. Three different modes of exchange have thus far been identified in bacteria. (See
gene
transfer
.)


Conjunction involves the direct joining of two bacteria, which allows their circular DNAs to
undergo recombination. Bacteria can also undergo transformation by absorbing remnants of
DNA from dead bacteria and integrating these fragments
into their own DNA. Lastly, bacteria
can exchange genetic material through a process called transduction, in which genes are
transported into and out of the cell by bacterial viruses, called
bacteriophages
, or by
plasmids
, an
autonomous self
-
replicating extrachromosomal circular DNA.


Related categories




MICROBIOLOGY





CELL BIOLOGY




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Encyclopedia of

Alternative Energy & Sustainable Living

Encyclopedia of History




A bacterial cell reproducing by
binary fission. The two resultant
daughter cells are genetically
identical. (Photo: Dr. Vincent A.
Fischetti, Laboratory of Bacterial
Pathogenesis and Immunology,
Rockefeller University)



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http://www.daviddarling.info/encyclopedia/B/binary_fission.html



CELL DIVISION: BINARY FISSION AND MITOSIS

Table of Contents

The Cell Cycle

|
Prokaryotic Cell Division

|
Eukaryotic Cell Division

|
Mitosis

Prophase

|
Metaphase

|
Anaphase

|
Telophase

|
Cytokinesis

|
Links

The Cell Cycle |
B
ack to Top

Despite differences between prokaryotes and eukaryotes, there are several common
features in their cell division processes. Replication of the DNA must occur.
Segregation of the "original" and its "replica" follow. Cytokinesis ends the cell
divi
sion process. Whether the cell was eukaryotic or prokaryotic, these basic events
must occur.

Cytokinesis

is the process where one cell splits off from its

sister cell. It usually
occurs after cell division. The Cell Cycle is the sequence of growth, DNA replication,
growth and cell division that all cells go through. Beginning after cytokinesis, the
daughter cells are quite small and low on ATP. They acquire

ATP and increase in size
during the G1 phase of Interphase. Most cells are observed in Interphase, the longest
part of the cell cycle. After acquiring sufficient size and ATP, the cells then undergo
DNA Synthesis (replication of the original DNA molecules
, making identical copies,
one "new molecule" eventually destined for each new cell) which occurs during the S
phase. Since the formation of new DNA is an energy draining process, the cell
undergoes a second growth and energy acquisition stage, the G2 phas
e. The energy
acquired during G2 is used in cell division (in this case mitosis).


The cell cycle.

Image from Purves et al.,
Life: The Science of Biology
, 4th Edition, by Sinauer
Associates (
www.sinauer.com
) and WH F
reeman (
www.whfreeman.com
), used with permission.

Regulation of the cell cycle is accomplished in several ways. Some cells divide
rapidly (beans, for example take 19 hours for the complete cycle; red blood cells mus
t
divide at a rate of 2.5 million per second). Others, such as nerve cells, lose their
capability to divide once they reach maturity. Some cells, such as liver cells, retain but
do not normally utilize their capacity for division. Liver cells will divide i
f part of the
liver is removed. The division continues until the liver reaches its former size.

Cancer cells are those which undergo a series of rapid divisions such that the daughter
cells divide before they have reached "functional maturity". Environment
al factors
such as changes in temperature and pH, and declining nutrient levels lead to declining
cell division rates. When cells stop dividing, they stop usually at a point late in the G1
phase, the R point (for restriction).

Prokaryotic Cell Division |
Back to Top

Prokaryotes

are much simpler in their organization than are eukaryotes. There are a
great many more organelles in eukaryotes, also more chromosomes. The usual method
of prokaryote cell division is termed
binary fission
. The prokaryotic chromosome is a
single DNA molecule that first replicates, then attaches each copy to a different part
of the cell membrane. When the cell begins to pull apart, the replicate and o
riginal
chromosomes are separated. Following cell splitting (
cytokinesis
), there are then two
cells of identical genetic composition (except for the rare
chance of a spontaneous
mutation).


This animated GIF of binary fission is from:
http://www.slic2.wsu.edu:82/hurlbert/micro101/pages/Chap2.html#two_bact_gr
oups

The prokaryote chromosome is much easier to manipulate than the eukaryotic one.
We thus know much more about the location of genes and their control in
prokaryotes.

One consequence of this asexual method of reproduction is that all organisms in a
colo
ny are genetic equals. When treating a bacterial disease, a drug that kills one
bacteria (of a specific type) will also kill all other members of that clone (colony) it
comes in contact with.


Rod
-
Shaped Bacterium,
E. coli
, dividing by binary fission (TEM

x92,750).

This image
is copyright Dennis Kunkel at
www.DennisKunkel.com
, used with permission.


Rod
-
Shaped Bacterium, hemorrhagic
E. coli
, strain 0157:H7 (division)

(SEM x22,810).
This image is copyright Dennis

Kunkel at
www.DennisKunkel.com
, used with permission.

Eukaryotic Cell Division |
Back to Top

Due to their increased numbers of chromosomes, organelles and complexity,
eukaryote cell division is more complicated, although the same processes of
replication, segregation, and cytokinesis still occur.

Mitosis |
Back to Top

Mitosis

is the process of forming (generally) identical daughter cells by rep
licating
and dividing the original chromosomes, in effect making a cellular xerox. Commonly
the two processes of cell division are confused. Mitosis deals only with the
segregation of the chromosomes and organelles into daughter cells.

Click here to view an animated GIF of mitosis from
http://www.biology.uc.edu/vgenetic/mitosis/mitosis.htm
.

Eukaryotic chromosomes occur in the cell in greater numbers than prokaryotic
c
hromosomes. The condensed replicated chromosomes have several points of interest.
The
kinetochore

is the point where microtubules of the spindle apparat
us attach.
Replicated chromosomes consist of two molecules of DNA (along with their
associated
histone proteins
) known as
chromatids
. The area where both chromatids are
in contact with each other is known as the
centromere

the kinetochores are on the
outer sides of the centromere. Remember that chromosomes are condensed
chromatin

(DNA plus histone proteins
).


Structure of a eukaryotic chromosome.

Image from Purves et al.,
Life: The Science of
Biology
, 4th Edition, by Sinauer Associates (
www.sinauer.com
) and WH Freeman
(
www.whfr
eeman.com
), used with permission.

During mitosis replicated chromosomes are positioned near the middle of the
cytoplasm and then segregated so that each daughter cell receives a copy of the
original DNA (if you start with 46 in the parent cell, you should
end up with 46
chromosomes in each daughter cell). To do this cells utilize microtubules (referred to
as the
spindle apparatus
) to "pull" chromoso
mes into each "cell". The microtubules
have the 9+2 arrangement discussed earlier. Animal cells (except for a group of
worms known as nematodes) have a
cent
riole
. Plants and most other eukaryotic
organisms lack centrioles. Prokaryotes, of course, lack spindles and centrioles; the cell
membrane assumes this function when it pulls the by
-
then replicated chromosomes
apart during binary fission. Cells that contai
n centrioles also have a series of smaller
microtubules, the
aster
, that extend from the centrioles to the cell membrane. The aster
is thought to serve as a bra
ce for the functioning of the spindle fibers.


Structure and main features of a spindle apparatus.

Image from Purves et al.,
Life: The
Science of Biology
, 4th Edition, by Sinauer Associates (
www.sinauer.com
) and WH F
reeman
(
www.whfreeman.com
), used with permission.

The phases of mitosis are sometimes difficult to separate. Remember that the process
is a dynamic one, not the static process displayed of necessity in a textbook.

Prophase |
Back to Top

Prophase

is the first s
tage of mitosis proper. Chromatin condenses (remember that
chromatin/DNA replicate during Interphase), the nuclear envelope dissolves,
centrioles (if present) divide and migrate, kinetochores and kinetochore fibers form,
and the spindle forms.


Pea Plant
Nuclear DNA, from
Vicea faba

(TEM x105,000).

This image is copyright
Dennis Kunkel at
www.DennisKunkel.com
, used with permission.



The events of Prophase.

Image from Purves et al.,
Life: The Science of Biology
, 4th Edition,
by Sinauer Associates (
www.sinauer.com
) and WH Freeman (
www.whfreeman.com
), used with
permission.

Metaphase |
Back to Top

Metaphase

follows Prophase. The chromosomes (which at this point consist of
chromatid
s held together by a centromere) migrate to the equator of the spindle, where
the spindles attach to the kinetochore fibers.

Anaphase |
Back to To
p

Anaphase

begins with the separation of the centromeres, and the pulling of
chromosomes (we call them chromosomes after the centromeres are separated) to
op
posite poles of the spindle.


The events of Metaphase and Anaphase.

Image from Purves et al.,
Life: The Science of
Biology
, 4th Edition, by Sinauer Associates (
www.sinauer.com
) and WH Freeman
(
www.whfreeman.com
), used with permission.

Telophase |
Back to Top

Telophase

is when the chromosomes reach the poles of their respective spindles, the
nuclear envelope reforms, chromosomes uncoil into chromatin form, and the
nucleolus (which had disappeared during Prophase) reform. Where there was one cell
there are now two smaller cells each with exactly the same genetic information. These
cells may then develop into different adult forms via the processes of development.


T
he events of Telophase.

Image from Purves et al.,
Life: The Science of Biology
, 4th Edition,
by Sinauer Associates (
www.sinauer.com
) and WH Freeman (
www.whfreeman.com
), used wi
th
permission.

Cytokinesis |
Back to Top

Cytokinesis is the process of splitting the daughter cells apart. Whereas mitosis is the
division of the
nucleus, cytokinesis is the splitting of the cytoplasm and allocation of
the golgi, plastids and cytoplasm into each new cell.

Links |
Back to Top



Access Excellence page on Mitosis



Cell Division and the Cell Cycle

(University of Alberta): Similar to this page, but with i
ts own
glossary and questions.



Amoeba Proteus

Mitosis

Small photomicrographs of protistan mitosis.



Cell Reproduction

Notes from University of Georgia, plus some cool graphics of mitosis.



Phases of Mitosis

U Texas QuickTime® movies of mitosis.



Animated Mitosis

Yale University, a simplified series of cartoons about mitosis.



Mitosis

U Southern Mississippi, Grayscale d
rawings and photomicrographs of mitosis stages.



Mitosis

San Diego State U, shocked animation of the process. You will need the Shockwave
plugin to view. If you don't have it, you can download it f
rom them.



McGill University Mitosis Page

Quality site, with photos and downloadable animation and video.



Comparison of Mitosis and Meiosis

Whitman College, table summarizing each process.



Whitefish Mitosis Review

Cornell, photomicrographs of mitosis in whi
tefish. A nice review after
lab! Part of a more extensive page of
Cell Division Tutorials
.



V
irtual Mitosis

University of Cincinnati, Animated GIF and text about the stages of mitosis.


Text ©1992, 1994, 1997, 1998, 2000, 2001, by M.J. Farabee, all rights reserved. Use for educational
purposes is encouraged.

Back to Table of Contents

|
MEIOSIS AND SEXUAL REPRODUCTION

Email
:
mj.f
arabee@emcmail.maricopa.edu

Last modified: Tuesday May 18 2010

The URL of this page is: www.emc.maricopa.edu/faculty/farabee/biobk/biobookmito.html

JavaScript is required to properly view this page. Please enable JavaScript.

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