Cells can divide, and in unicellular organisms, this makes more organisms.
In multicellular organisms, cell division is used for growth, development,
and repair of the organism. Cell division is controlled by DNA, but exact
copies of the DNA must be given to the daughter cells (note use of "mother"
and "daughter"). Bacteria reproduce by a simple process called binary
fission. They have one chromosome which is attached to the cell membrane.This chromosome replicates, then the two copies are pulled apart as the
cell grows. Eventually the cell pinches in two to make two cells.
Eukaryotes do mitosis. In mitosis, each daughter cell gets about half of
the cytoplasm from the mother cell and one set or copy of the DNA.Before cell division occurs, the cell first has to replicate the
chromosomes so each daughter cell can have a set. When the chromosomes are
replicated and getting ready to divide, they consist of two, identical
halves called sister chromatids which are joined by a central region, the
centromere.
Each chromosome is one long molecule of DNA and special
proteins. DNA makes up the genes, and we say that genes are "on"
chromosomes, or chromosomes "contain" or are made of genes. Some of the
proteins in the chromosomes "turn off" the genes that are not needed in
that cell. For example, while every cell in your body contains exactly the
same genes, you don't need your eye-color gene operational in cells in your
big toe, nor toenail-shape genes active in cells in your stomach.
Two basic types of cells occur in the bodies of eukaryotes. Somatic cells
are general body cells. These have the same number of chromosomes as each
other within the body of an organism. The number of chromosomes in somatic
cells is consistent among organisms of the same species, but varies from
species to species. These chromosomes come in pairs, where one chromosome
in each pair is from the mother and one is from the father.
Actually, since
most organisms have more than one pair of chromosomes, it would also be
correct to say that the organism received one set of chromosomes from its
mother and one matching set from its father, and that these sets match in
pairs. The other type of cells found in eukaryotes is gametes or sex cells,
consisting of eggs in females and sperm in males. These special
reproductive cells have only one set (half as many) of chromosomes
consisting of one chromosome from each pair. In humans ONLY, the somatic
cells have 46 chromosomes arranged in 23 pairs (= two sets of 23 each),
while gametes have 23 individual chromosomes (= one set). In fruit flies,
somatic cells have 8 chromosomes (= 4 pairs or 2 sets) and gametes have 4
chromosomes (= 1 set). Geneticists use the term "-ploid" to refer to one
set of chromosomes in an organism, and that term is typically combined with
another wordstem that describes the number of sets of chromosomes present.
For example, a cell with one set of chromosomes is called haploid, a cell
with two sets of chromosomes is diploid, and a cell with four sets of
chromosomes (not usually a "normal" condition, but sometimes possible) is
tetraploid.Technically, mitosis is specifically the process of division of the
chromosomes, while cytokinesis is officially the process of division of the
cytoplasm to form two cells. In most cells, cytokinesis follows or occurs
along with the last part of mitosis.Remember centrioles? They consist of nine sets of three microtubules, occur
in animal cells only, and are involved in division of the chromosomes. Each
animal cell has a pair of centrioles located just outside the nucleus. The
two centrioles in the pair are oriented at right angles to each other.
Just
before mitosis, the centrioles replicate, so the cell now has four (two
sets of two) as it starts mitosis.
The stages in mitosis include (interphase), metaphase, anaphase, and
telophase. Remembering "IPMAT" or Intelligent People Meet At Three (or is
that Twelve?) can help you remember the stages in order. Strictly speaking,
interphase is the stage in which a cell spends most of its life and is not
part of the process of mitosis, per se, but is usually discussed along with
the other stages.
Interphase may appears to be a "resting" stage, but cell growth,
replication of the chromosomes, and many other activities are taking place
during this time. Near the end of interphase just before the cell starts
into the other stages of mitosis, if the cell is an animal cell, the
centrioles replicate so there are two pairs. At this time, the strands of
DNA that make up the chromsosomes are unwound within the nucleus and do not
appear as distinct chromosomes. Thus, at this stage, the genetic material
is often referred to as chromatin. From here, the cell goes through all
other stages of mitosis.
|In prophase, the chromosomes start to coil, shorten, and become| |
|distinct. In animals, the centrioles begin to migrate to the poles| |
|of the cell. The mitotic spindle or polar fibers begin to form| |
|from the poles of the cell towards the equator. In animals, this | |
|starts as asters around the centrioles. Eventually, the spindle| |
|mechanism finishes growing toward the equator and interacts with | |
|the centromeres to line up and, later, move the chromosomes.
Also | |
|at this time, the nuclear envelope starts to disintegrate. | |
|Metaphase is characterized by the lining up of the chromosomes| |
|along the equator of the cell or what is called the metaphase| |
|plate. The nuclear envelope has totally disintegrated and the| |
|polar fibers have reached the centromeres of the chromosomes and | |
|have begun interacting with them.| |
|In anaphase, the sister chromatids separate at the centromeres,| |
|thus can now be called chromosomes. These are pulled to the poles | |
|of the cell by the mitotic spindle. | |
In telophase, the new daughter nuclei and nuclear envelopes start to reform
and the chromosomes uncoil.
Telophase frequently includes the start of
cytokinesis. In animal cells, cytokinesis starts with a cleavage furrow or
indentation around the middle that eventually pinches in, dividing the cell
in two. In plants, cytokinesis begins with a series of vesicles that form
at the equator of the cell, which subsequently join until the cell is
divided in two.|Animal |Plant |
|Cytokinesis|Cytokinesis |
| ||
| ||
One interesting offshoot of the study of mitosis is tissue culture.
In
tissue culture, the cells to be studied are removed from the organism's
body and grown on a sterile, artificial medium. When grown in this manner,
typically normal cells grow one layer thick on the surface of the sterile
medium and will undergo only 20 to 50 mitotic divisions then cease to be
able to reproduce. Also, typically, when all cells are touching neighbors
all around, they stop dividing. This phenomenon is known as contact
inhibition.
In sharp contrast, cancer cells will not stop growing with one
layer on the surface of the medium, but grow multiple layers and fill the
dish. They do not exhibit contact inhibition: they don't stop growing when
touching on all sides. Also cancer cells appear to have no limit to the
number of generations they can produce. Back in the mid-1950s, a biopsy of
cervical cancer was removed from a woman named Henrietta Lacks and grown in
tissue culture.
While Ms. Lacks died long ago, HeLa cells are a widely-
cultured research "organism" available through a number of biological
supply companies. Within the past few years, an interesting issue has
arisen regarding these cells: are they still "human"? While HeLa cells
currently being grown in tissue culture are descendents of the original
human cancer cells, by now they have mutated so much that it's questionable
whether they can still be considered "human" tissue, especially since they
were abnormal, cancer cells to begin with.Tissue culture is now a widely-used means of more effectively and quickly
finding the right drugs to treat cancer. Typically, in the past, people
with cancer were subjected to one toxic drug after another in hopes of
finding one that would be effective against that particular cancer.Unfortunately, by the time the right drug was found, it frequently was too
late to do any good.
Now, when a person is diagnosed with cancer, a biopsy
can be taken and a number of cultures of cells can be grown. Each of these
cultures can be subjected to a different drug, thus enabling doctors to
find the right drug sooner, while it may still be of help, and without
needlessly subjecting the person to many kinds of toxic chemicals.Within our bodies, different cells do mitosis at different rates. Skin
cells continuously do mitosis and divide, thus our skin is constantly
renewed and repaired. In sharp contrast, most nerve cells stop doing
mitosis soon after birth (Caution: overconsumption of alcohol can kill
nerve/brain cells, and they can never be replaced, they will never "grow
back.").
Liver cells are somewhere in between. In a healthy adult, liver
cells normally do not divide, but can divide to repair minor damage. Major
liver damage or a disease like cirrhosis is too much damage to be repaired
through mitosis. In contrast, it is possible to use one adult liver to do
liver transplants for four babies, and if all goes well, these pieces can
eventually regenerate whole livers.|mitosis|
| (m?t?s?s, m?-) , process of nuclear division in a living cell by |
|which the carriers of hereditary information, or the chromosomes, |
|are exactly replicated and the two copies distributed to identical |
|daughter nuclei.
Mitosis is almost always accompanied by cell|
|division (cytokinesis), and the latter is sometimes considered a|
|part of the mitotic process. The pattern of mitosis is |
|fundamentally the same in all cells. However, while animal cells|
|apparently divide by pinching into two separate cells, plant cells |
|develop a cell plate, which becomes a cellulose cell wall between |
|the two daughter cells. The importance of mitosis is the|
|maintenance of the chromosomal set; each cell formed receives|
|chromosomes that are alike in composition and equal in number to|
|the chromosomes of the parent cell.
|
|The Stages of Mitosis|
|Mitosis is simply described as having four stages-prophase, |
|metaphase, anaphase, and telophase; the steps follow one another|
|without interruption. The entire four-stage division process |
|averages about one hour in duration, and the period between cell|
|divisions, called interphase or interkinesis, varies greatly but is|
|considerably longer.|
|During interphase the chromosomes are dispersed in the nucleus and |
|appear as a network of long, thin threads or filaments, called the |
|chromatin. At some point before prophase begins, the chromosomes|
|replicate themselves to form pairs of identical sister chromosomes,|
|or chromatids; the deoxyribose nucleic acid (DNA) of the|
|chromosomes is synthesized only during interphase, not while |
|mitosis is in process.|
|During prophase the two chromatids remain attached to one another |
|at a region called the centromere, but each contracts into a |
|compact tightly coiled body; the nucleolus and, in most cases, the |
|nuclear envelope break down and disappear.
Also during prophase the|
|spindle begins to form. In animal cells the centrioles separate and|
|move apart, and radiating bundles of fibers, called asters, appear |
|around them. Some sets of fiber run from one centriole to the|
|other; these are the spindle fibers. In plant cells the spindle|
|forms without centrioles.
|
|During metaphase the chromosomes congregate at a plane midway|
|between the two ends to which the spindle tapers. This is called|
|the equatorial plane and marks the point where the whole cell will |
|divide when nuclear division is completed; the ends of the spindle |
|are the poles to which the chromatids will migrate. The chromatids |
|are attached to the spindle fibers at the centromeres. |
|During anaphase the two chromatids of each chromosome separate and |
|move to opposite poles, as if pulled along the spindle fibers by|
|the centromeres. During telophase new nuclear envelopes form around|
|the two groups of daughter chromosomes (as they are now called),|
|the new nucleoli begin to appear, and eventually, as the formation |
|of the two daughter nuclei is completed, the spindle fibers |
|disappear.
The chromosomes uncoil to assume their dispersed |
|distribution within the interphased nucleus. Cytokinesis, which may|
|begin before or after mitosis is completed, finally separates the |
|daughter nuclei into two new individual daughter cells.|
|A considerable variance in the degree and timing of these stages|
|exists across species, and cells can be classified by their mitotic|
|characteristics. Despite the relative ease of observation of the|
|physical stages of mitosis under the microscope (primarily because |
|the chromosomes stain readily when in their coiled state), the|
|exact chemical and kinetic nature of mitosis is not yet fully|
|understood. For instance, the spindle has been determined to |
|consist largely of thin, elongate tubules called microtubules, but |
|their functions have yet to be understood. |
|Meiosis and Amitosis|
|Mitotic division is the method of nuclear division of the somatic |
|(body) cells, as distinguished from the gametes, or sex cells (eggs|
|and sperm).
In sexual reproduction, i.e., by the union of two|
|gametes, the complex process of meiosis takes place, which produces|
|cells that each contain only half the normal number of chromosomes.|
|Direct cell division, in which the nucleus simply cleaves in two|
|(sometimes but not always followed by division of the cytoplasm), |
|is called amitosis and is very rare.
|
