The Cell Cycle
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 division 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 phase. 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 Freeman (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 must
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 if 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".
Environmental 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).
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
original 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_groups
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 colony 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.
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 is the process of forming (generally)
identical daughter cells by replicating 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 chromosomes.
The condensed replicated chromosomes have several points of interest. The kinetochore is the point where microtubules of the spindle
apparatus 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.whfreeman.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" chromosomes 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 centriole. 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 contain 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 brace 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 Freeman (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 is the first stage 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 follows Prophase. The chromosomes (which at
this point consist of chromatids held together by a centromere) migrate to the
equator of the spindle, where the spindles attach to the kinetochore fibers.
Anaphase begins with the separation of the centromeres,
and the pulling of chromosomes (we call them chromosomes after the centromeres
are separated) to opposite 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 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.
The 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 with permission.
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.
