Cell Division
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Cell division is the basis for all forms of organismal
reproduction.
Single-celled organisms divide to reproduce. Cell division
in multicellular organisms produces specialized
reproductive cells, such as egg and sperm.
In order for a cell to divide, the genome must also divide,
so, in all types of cell division in all organisms, DNA
replication precedes cell division.
Cell division can be grouped into asexual and sexual cell
division.
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Types of cell division
In prokaryotes there is only one simple
type of cell division, which produces
two identical daughter cells from one
progenitor cell (asexual cell division).
Eukaryotes also show asexual cell
division; this converts a single fertilized
egg cell, a zygote, into a multicellular
organism, or a single unicellular
organism into a population or a colony.
The asexual cell division in eukaryotes
is called mitosis (M). Both haploid (n)
and diploid (2n) cells can divide
asexually.
The sexual cell division in eukaryotes is
called meiosis (Mei) and occur in
specialized cells, the meiocytes, which
divides twice, resulting in four haploid
cells called a tetrad.
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Life cycles of humans, plants, and fungi
In humans and many
plants, three cells of
the meiotic tetrad
abort. The
abbreviation n
indicates a haploid
cell, 2n a diploid
cell; gp stands for
gametophyte, the
small structure of
haploid cells that
will produce
gametes.
In flowering plants the gametophyte stage is radically reduced, but in others (such as
mosses) the gametophyte is the main vegetative stage
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G1, S, G2, M
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Cells spend most of their life in interphase, the period between nuclear divisions,
and comparatively little time in mitosis. Interphase is divided into three stages, G1,
S, and G2.
In G1cells are growing and synthesizing the materials necessary for their proper
functioning. The cells are "doing their thing", so if they are nose cells they are
producing mucus, if they are muscle cells they are contracting and relaxing, etc.
Some cells, such as our nerve cells (neurons) and red blood cells, never leave this
stage and it is then called G0. A cell which is in the G0 stage will not divide. It will
not grow, either, but will continue to function until it dies.
Cells which will divide pass through a specific phase in
G1 which acts as a gateway into the S stage. Once cells
pass this "point of no return" they will proceed through
S, G2 and mitosis. S is the stage when DNA synthesis
(chromosome replication) occurs. The chromosomes
consist of two identical strands once replication is
completed. Each of these strands is called a chromatid.
During mitosis the chromatids will separate and each
chromatid will become a separate chromosome.
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Mitosis
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Mitosis (M) is usually the shortest segment of the cell cycle, lasting for
approximately 5 to 10 percent of the cycle. DNA synthesis takes place during the S
period. G1 and G2 are gaps between S and M. Together, G1, S, and G2 constitute
interphase, the time between mitoses. (Interphase used to be called "resting period";
however, cells are active in many ways during interphase, not the least of which, of
course, is DNA replication.) The chromosomes cannot be seen during interphase,
mainly because they are in an extended state and are intertwined with one another
(chromatine).
For the sake of study, biologists divide mitosis into four stages called prophase,
metaphase, anaphase, and telophase. It must be stressed, however, that any nuclear
division is a dynamic process on which we impose such arbitrary stages only for our
own convenience.
In each of the resultant daughter cells, the chromo-some complement is identical
with that of the original cell. Of course, what were referred to as chromatids now
take on the role of full-fledged chromosomes in their own right.
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The four stages of mitosis
Prophase
 Metaphase
 Anaphase
 Telophase
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Prophase
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The onset of mitosis is characterized by the chromosomes becoming distinct. They
get progressively shorter through a process of contraction, or condensation, into a
series of spirals or coils; the coiling produces structures that are more easily moved
around.
As the chromosomes become visible, they appear double-stranded, each
chromosome being composed of two longitudinal halves, the sister chromatids. The
two chromatids formed by one chromosome each contain one of the replicated DNA
molecules.
Because of semiconservative replication these replicate DNA molecules are each
"half old and half new"; that is, in each double helix one of the nucleotide strands is
newly polymerized. These sister chromatids are joined at a region called the
centromere. At this stage the centromere has already divided into a pair of sister
centromeres.
The nuclear membrane begins to break down, and the nucleoplasm and cytoplasm
become one.
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Metaphase
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At this stage, the nuclear spindle becomes prominent. The
spindle is a birdcage-like structure that forms in the nuclear
area; it consists of a series of parallel proteinaceous fibers
that point to each of two cell poles. These spindle fibers are
polymers of a protein called tubulin. The chromosomes
move to the equatorial plane of the cell, where one sister
centromere becomes attached to a spindle fiber from one
pole; the other sister centromere, to the other pole.
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Metaphase
chromosomes
Some of the orders of
chromatin packing
thought to give rise to the
highly condensed mitotic
chromosome.
The folding of naked
DNA into nucleosomes is
the best understood level
of packing.
The structures
corresponding to the
additional layers of
chromosome packing are
more speculative.
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Anaphase and telophase
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Anaphase begins when the pairs of sister chromatids
separate, one of a pair moving to each pole. The
centromeres, which now clearly appear to have divided,
separate first. As each chromatid moves, its two arms appear
to trail its centromere; a set of V-shaped structures results,
with the points of the V's directed at the poles.
At telophase, a nuclear membrane re-forms around each
daughter nucleus, the chromosomes uncoil, and the nucleoli
reappear, effectively re-forming the interphase nuclei. By
the end of telophase, the spindle has dispersed, and the
cytoplasm has been divided into two by a new cell
membrane.
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Cells in active proliferation
Longitudinal section of
an onion root tip (apical
meristem) x40.
The apical meristem x400. Most of the cells even in this
area of active cell division are at interphase. Those with
visible chromosomes are at some stage of mitosis.
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Meiosis: Prophase 1
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Meiosis consists of two nuclear divisions, distinguished as meiosis I and meiosis
II. The events of meiosis I are quite different from those of meiosis II, and the
events of both differ from those of mitosis. Each meiotic division is formally divided
into prophase, metaphase, anaphase, and telophase.
PROPHASE I. The chromosomes become visible as long, thin single threads
(which are composed of pairs of replicated DNA molecules), and continue
contracting during the entire stage. Homologous chromosomes form pairs (this
does not happen in Mitosis); each chromosome has a pairing partner, and the two
become progressively paired, or synapsed, along their lengths. Thus, the number of
homologous pairs of chromosomes in the nucleus is equal to the haploid number n.
The beadlike chromomeres align precisely in the paired homologs, producing a
distinctive pattern for each pair. Since each member of a homologous pair produces
two sister chromatids, the synapsed structure now consists of a bundle of four
homologous chromatids, the tetrad. At this stage, cross-shaped structures called
chiasmata (singular, chiasma) appear between nonsister chromatids. Each
homologous group of four generally has one or more chiasmata. Chiasmata are the
visible manifestations of events called crossovers. A crossover is a precise breakage,
swapping, and reunion between two nonsister chromatids.
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Meiosis: Metaphase I, Anaphase I, and Telophase I
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METAPHASE I. The nuclear membrane and nucleoli disappear, and each
pair of homologs takes up a position in the equatorial plane. The sister
centromeres do not appear to have divided, so they act as one. This
apparent lack of division represents a major difference from mitosis.
The two nonsister centromeres attach to spindle fibers from opposite poles.
ANAPHASE I. Homologous chromosomes move directionally to opposite
poles. This is the stage at which haploid nuclei are formed.
TELOPHASE I. In many organisms, this stage do not exist, no nuclear
membrane re-forms, and the cells proceed directly to meiosis II. In other
organisms, telophase I and the interkinesis are brief in duration; the
chromosomes elongate and become diffuse, and the nuclear membrane reforms. In any case, there is never DNA synthesis at this time, and the
genetic state of the chromosomes does not change.
Genetica per Scienze Naturali
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Meiosis I Stages of Trillium erectum
Late prophase I
Metaphase I
Anaphase I
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Meiosis 2
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PROPHASE II. The presence of the haploid number of
sister chromatid pairs in the contracted state characterizes
prophase II.
METAPHASE II. The pairs of sister chromatids arrange
themselves on the equatorial plane during metaphase II.
Here the chromatids often partly dissociate from each other
instead of being closely pressed together as they are in
mitosis.
ANAPHASE II. Centromeres split and sister chromatids
are pulled to opposite poles by the spindle fibers.
TELOPHASE II. The nuclei re-form around the
chromosomes at the poles.
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Meiosis II Stages of Trillium erectum
Metaphase II
Anaphase II
The centromers have
separated, half chromosomes
are drawn to the poles
Early telophase II.
The nuclei are haploid, the
chromosomes single-stranded
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Comparing mitosis with meiosis
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