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Eukaryotic cell cycle:
cell growth, mitosis, and interphase
G1: Cell prepares for chromosome
replication.
S: DNA replicates and new
chromosomes (sister chromatids)
are formed.
G2: Cell prepares for mitosis and cell
division.
M: Mitosis
http://www.cellsalive.com/
Mitosis:
1.
2.
Occurs in haploid (1N) and diploid (2N) somatic cells.
Continuous process with 4 cytologically distinct stages.
Prophase
Chromosomes shorten, thicken, and become visible by
light microscopy.
Centrioles move apart and mitotic spindle begins to form.
Centrioles migrate to opposite sides of nucleus and
nuclear envelope begins to disappear.
Metaphase Nuclear envelope disappears completely. Replicated
chromosomes held together at the centromere are
aligned on equator of the spindle (metaphase plate).
Anaphase
Centromeres split and daughter chromosomes migrate to
opposite poles. Cell division (cytokinesis) begins.
Telophase
Nuclear envelopes reform, chromosomes become
extended and less visible, and cell division continues.
INTERPHASE
PROPHASE
Centrosomes
(with centriole pairs)
Early mitotic
spindle
Centrosome
Chromatin
Nucleolus Nuclear
envelope
Plasma
membrane
Chromosome,
consisting of two
sister chromatids
Fragments
of nuclear
envelope
Centrosome
Kinetochore
Spindle
microtubules
METAPHASE
ANAPHASE
Cleavage
furrow
Metaphase
plate
Spindle
Figure 8.6 (continued)
TELOPHASE AND CYTOKINESIS
Daughter
chromosomes
Nuclear
envelope
forming
Nucleolus
forming
Fig. 1.15 Interphase and mitosis in an animal cell
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Nuclear Envelope Breakdown
(NEBD)
• Nuclear envelope = a pair of membrane bilayers, which are
joined at the pore complexes, an additional layer called
nuclear lamina (a mesh of filament proteins) underlie the inner
membrane.
• Lamina may organize chromatin into functional domains,
provide structure to nucleus;
• breaks down in mitosis so that mitotic spindle microtubules
can associate with kinetochores.
• NEBD is initiated with one to three holes which expand over
the entire envelope;
• holes occur opposite of invaginations on the envelope.
Centrosome
• Centrosome contains a centriole pair linked by a matrix;
• nine proximal triplets are found at the center of centrosome
• Centrioles are also needed to make cilia and flagella.
centriole
cilia
Centrioles
• they may recruit microtubule nucleating material
(pericentriolar material)
• yeast don’t have centrioles but have spindle pole bodies;
acentriolar mechanisms for spindle formations using
motor proteins
• centrioles duplicate from a template. Each centriole
makes a copy; when cells enter S phase, centriole
duplication takes place.
Centrioles
• Sperm cells contain a pair of centrioles;
eggs have none. The sperm's centrioles are
absolutely essential for forming a
centrosome which will form a spindle
enabling the first division of the zygote to
take place.
Chromosome alignment
• microtubule and chromosome interactions occur at
kinetochores- specialized disk shaped structures
located on each side of the centromere, kinetochore
captures multiple microtubules to build a kinetochore
fiber.
• If a chromosome binds to one kinetochore initially, it will
be attracted to one pole.
• Two opposing forces align the chromosomes along the
metaphase plane.
Microtubules attached to opposite
sides of the dyad shrink or grow
until they are of equal length.
• Microtubules motors
attached to the kinetochores
move them toward the minus
end of shrinking
microtubules (a dynein);
• toward the plus end of
lengthening microtubules (a
kinesin).
• The chromosome arms use a
different kinesin to move to
the metaphase plate.
Taxol
• Chromosome movement in mitosis also involves
polymerization and depolymerization of the
microtubules. Taxol, a drug found in the bark of
the Pacific yew, prevents depolymerization of the
microtubules of the spindle fiber. This, in turn,
stops chromosome movement, and thus prevents
the completion of mitosis. Taxol is being used
with some success as an anticancer drug.
Centromeres
• Centromeres lack common signatures
among chromosomes.
• Centromeric repeat lengths approximate
nucleosome lenghts.
• Centromeres have their own histone which
replace H3.
• Centromeric histone maintains centromeric
structure.