The Cell Cycle and Mitosis
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A. The cell cycle is the
well ordered sequence of events between the time a cell divides to form
two daughter cells and the time those daughter cells divide.
1. Includes a doubling of
a cell's cytoplasms, precise duplication of DNA < mitosis and Cytokinesis.
2. Each daughter cell contains a
single, intact nucleus and some surrounding cytoplasm.
3. Duration of cell cycle varies
with the type of cell. Some cells divide each hour, others take
more than 24 hours.
4. Some cell types, for example,
nerve and muscle cells, never or rarely divide once they are formed
5. Two phases
a. M phase (mitotic phase) = Mitosis + Cytokinesis (usually).
b. Interphase = Remainder of cell cycle excluding M phase
i. Interphase comprises about 90%
of the cell cycle and includes most of a cells growth and metabolic
activities with a very high degree of biochemical activity.
ii. Many components are made continuously
throughout Interphase although DNA synthesis occurs only during a limited
iii. Interphase includes G 1 phase,
S phase (DNA synthesis), and G2 phase.
iv. In brief, what we see in late
The nucleus is well defined and bounded by the
One of more nucleoli are present.
.Two pairs of centrioles are adjacent to nucleus
(formed earlier by replication of original pairs).
Around each pair of centrioles, microtubules
form in a radial array, called as aster.
Chromosomes are duplicated (occurred in S phase),
but cannot be distinguished individually due to loosely packed chromatin
Phases of Mitosis
1. Mitosis is unique to eukaryotes
and may be an evolutionary adaptation for distributing a
large amount of genetic material.
2. Details may vary, but overall
process is similar in most eukaryotes.
3. It is a reliable process with
experimental evidence showing only one error per 100,000 cell divisions.
4. Mitosis and cytokinesis form a
continuum, but for ease of description, mitosis is usually divided into
four stages: prophase, metaphase, anaphase and telophase.
1. During prophase, changes occur
in both the nucleus and cytoplasm.
3. In the cytoplasm:
2 In the nucleus
a. Nucleoli disappear. :
b. Chromatin fibers become tightly
coiled and folded into discrete, observable
c. Each duplicated chromosome is
composed of two identical sister chromatids joined at the centromere.
a. Mitotic spindle forms. It is composed
of microtubules and associated proteins which are arranged between the
4. Each chromatid new has a specialized
structure called the kinetochore located at the
b. Centriole pairs move apart, apparently
propelled along the surface of the nucleus by
lengthening of the microtubule bundles between
c. Nuclear envelope fragments in
d. Absence of nuclear envelope allows
microtubules to interact with the more highly
e. Polar fibers (bundles of microtubules)
extend from each pole toward the equator of the cell.
5. Bundles of micro tubules (kinetochore
fibers) are attached and interact with the polar fibers of the spindle
to put the chromosomes into agitated motion.
1. During metaphase:
a. Centriole pairs are positioned
at opposite ends (pole) of the cell
b. Chromosomes move to the metaphase
plate, the plane equidistant between the spindle poles.
c. Centromeres of all chromosomes
are aligned on the metaphase plate. '\
d. The long axis of each chromosome
is roughly at a right angle to the spindle axis.
e. Kinetochore fibers of sister chromatids
face opposite poles, so identical chromatids are attached to kinetochore
fibers radiating from opposite ends of the parent cell.
f. Entire structure formed by polar
fibers plus kinetochore fibers is called the spindle.
1. Anaphase begins when paired centromeres
of each chromosome move apart.
a. Sister chromatids separate and
are considered chromosomes.
2. At the end of anaphase, the two poles
have complete and equivalent collections of
b. The spindle apparatus starts moving
the separate chromosomes (once joined as sister chromatids) toward opposite
poles of the cell. Due to the attachment of the kinetochore fibers to the
centromeres, the chromosomes move in a "V" shape.
c. The kinetochore fibers begin to
shorten at the chromosomes near the poles.
d. The poles of the cell start to
move farther apart slightly elongating the cell.
1. The main events during telophase:
a. Polar fibers further elongate
2. The spindle poles move away from each
other as a cell elongates during mitosis, and. different hypotheses have
been formulated to explain this:
b. Daughter nuclei begin to form
at the two poles.
c. Nuclear envelopes are formed around
the chromosomes from fragments of the parent cell's nuclear envelope and
other portions of the endomembrane system.
d. Nucleoli reappear.
e. Chromatin fiber of each chromosome
uncoils and the chromosomes become less distinct.
a. Some of the movement may be due
to addition of subunits to the polar fibers and their resulting elongation.
3. Sister chromatid separation and movement
by kinetochore microtubules during anaphase is not fully understood:
b. The poles may be pushed apart
by interdigitating polar fibers that slide past each other in the equatorial
region of spindle overlap. According to this hypothesis, cell elongation
is powered by ATP hydrolysis catalyzed by an ATPase related to dynein.
a. Dynein and ATP do not appear to
b. Low-energy dissociation of kinetochore
microtubules into their protein subunits may play a role in moving chromosomes.
c. One hypothesis holds that the
polar ends of the microtubules depolymerize. As the tubulin subunits dissociate,
the shortening microtubules with their attached chromosomes are thus drawn
to the poles.
d. A second model is based on evidence
that dissociation of microtubules occurs at their kinetochore ends. This
model suggests that depolymerization of microtubules at this end is an
exerbonic reaction that provides the energy for the kinetochore to move
poleward along the remaining tip of the microtubule ahead of the depolymerization
1. Cytokinesis means movement
of the cytoplasm.
a. Important changes can be observed
in the cytoplasm during late anaphase or early telophase.
2. Cytokinesis occurs as cleavage in
b. These changes result in division
of a cell into two cells.
a. First indication of cleavage is
the formation of a cleavage furrow which forms as a shallow groove in the
cell surface near the old metaphase plate.
3. Cytokinesis in plant cells is very
different due to the presence of cell walls.
b. A contractile ring of micro filaments
composed on the protein actin forms on the cytoplasmic side of the furrow.
c. These micro filaments contract,
reducing the diameter of the contractile ring.
I d. This reduction continues, deepening
the cleavage furrow, until the parent cell is pinched in two.
e. The remains of the mitotic spindle,
which is the last connection between the daughter cells, breaks and the
two cells are completely separate.
a. No cleavage furrow forms
4. By the end of telophase
b. Across the midline of the parent
cell (old metaphase plate), a structure called the cell plate forms
c. This cell plate forms from the
coalescing of vesicles derived from the Golgi apparatus.
a. Mitosis, the equal division of
one nucleus into two genetically identical nuclei, is complete.
b. Cytokinesis has begun and the
appearance of two separate daughter cells occurs shortly after mitosis
The Mitotic Spindle
1. The mitotic spindle is
important to the events occurring in mitosis.
a. It forms in the cytoplasm
during prophase and is composed of fibers formed from microtubules and
b. Microtubules of the cytoskeleton
are partially disassembled during assembly of the mitotic spindle
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