Transcript The Mitotic Cell Cycle-2004

The Mitotic Cell Cycle
Functions of Cell Division
• Reproduction—some unicellular organisms
divide to form duplicate offspring
• Growth—multicellular organisms grow and
develop from single cell (fertilize egg)
• Repair—replace cells
that die from normal wear & tear or
accidents
Fig. 12.1
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Cell Division
• Genome—a cell’s total hereditary
endowment of DNA
– Genome is specific to species
• Human DNA extends about 3 meters, so
how is it possible to copy all of it and
ensure cells get even distribution?
-DNA molecules are packaged into
chromosomes which are more
manageable
• Every eukaryotic organism has a
characteristic number of
chromosomes
– Human somatic cells (all body cells
except reproductive cells) contain 46
chromosomes (23 pairs)
– Human reproductive cells, gametes—
sperm and egg cells—have
23 chromosomes
Fig. 12.2
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Chromosome
• Each duplicated
chromosome consists of
two sister chromatids
which contain identical
copies of the
chromosome’s DNA.
• As they condense, the
region where the strands
connect shrinks to a
narrow area, is the
centromere.
Fig. 12.3
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Mitotic Cell Cycle
• In a dividing cell, the mitotic phase (M)
phase alternates with interphase, a growth
period.
–Mitotic phase—
usually the
shortest part of
cell cycle
–Interphase—
accounts for –90%
of the cycle
Interphase Subphases
• G1 phase (first gap)—cell grows by
producing proteins and cytoplasmic
organelles
• S phase (synthesis of DNA)—cell
continues to grow as in G1 phase, while
duplicating chromosomes
• G2 phase (second gap)—grows more as it
completes preparations for cell division
Mitosis
• Prophase
• Metaphase
• Anaphase
• Telophase
G2 of Interphase
• Nucleus well-defined and
bounded by nuclear envelope
• Contains one or more nucleoli.
• 2 centrosomes (with centriole
pairs) visible
• Chromosomes duplicated
– Still seen as chromatin (DNA
+ protein)
– No individual chromosomes
seen
Prophase
• Chromatin fibers become more tightly coiled,
condensing into discrete chromosomes
• Nucleoli disappear
• Chromosomes appear as 2 identical sister
chromatids joined together by centromere
• Mitotic spindle begins to form (made of
microtubules), radiating from centrosomes
• Centrosomes move to opposite poles
Late Prophase/Prometaphase
• Nuclear envelope fragments-disintegrates
• Microtubules of spindle extend from poles and
invade nucleus and interact with chromosomes
• Kinetochore forms on chromatids
• Some spindle fibers connect with
kinetochores; some attach to
opposite pole
Metaphase
• Centrosomes at opposite poles of cell
• Chromosomes convene on the metaphase plate
• Centromeres of all chromosomes are aligned
with one another, and sister chromatids straddle
metaphase plate
• Mitotic spindle completely
formed
Anaphase
• Paired centromeres
of each chromosome
separate
• Each chromatid is
now considered a fullfledged chromosome
and move to opposite
poles as kinetochore
microtubules shorten
Telophase and Cytokinesis
• Nonkinetochore microtubules
elongate the cell
• Daughter nuclei form at two poles of
cell
• Nuclear envelopes arise from
fragments of parent cell’s nuclear
envelope and other portions of
endomembrane system
• Chromatin fibers become less tightly
coiled
• Cytokinesis—division of cytoplasm
– Separate from mitosis
– Formation of cleavage furrow,
which pinches cell in two
Cytokinesis in Plants
• No cleavage furrow
• During Telophase, vesicles derived from
Golgi apparatus move along microtubules to
middle of cell producing cell plate
• Cell plate enlarges until its surrounding
membrane fuses with the plasma membrane
The mitotic spindle distributes
chromosomes to daughter cells:
a closer look
• The mitotic spindle = fibers composed of
microtubules and associated proteins
• As the spindle assembles during prophase,
the elements come from partial disassembly
of the cytoskeleton.
• The spindle fibers elongate by incorporating
more subunits of the protein tubulin.
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• Assembly of the spindle microtubules starts
in the centrosome.
– The centrosome (microtubule-organizing
center) of animals has a pair of centrioles at
the center
Fig. 12.6a
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• As mitosis starts, the two centrosomes are
located near the nucleus.
• As the spindle fibers grow from them, the
centrioles are pushed apart.
• By the end of prometaphase they develop
as the spindle poles at opposite ends of the
cell.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Each sister chromatid has a kinetochore
of proteins and chromosomal DNA at the
centromere.
• The kinetochores of the joined sister
chromatids face in opposite directions.
• During prometaphase,
some spindle
microtubules
attach to the
kinetochores.
Fig. 12.6b
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• When a chromosome’s kinetochore is
“captured” by microtubules, the
chromosome moves toward the pole from
which those microtubules come.
• When microtubules attach to the other pole,
this movement stops and a tug-of-war
ensues.
• Eventually, the chromosome settles
midway between the two poles of the cell,
the metaphase plate.
• Other microtubules from opposite poles
interact as well, elongating the cell.
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• One hypothesis for the movement of
chromosomes in anaphase is that motor
proteins at the kinetochore “walk” the
attached chromosome along the
microtubule toward the opposite pole.
– The excess microtubule sections
depolymerize.
Fig. 12.7a
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• Nonkinetichore microtubules are
responsible for lengthening the cell along
the axis defined by the poles.
– These microtubules interdigitate across the
metaphase plate.
– During anaphase motor proteins push
microtubules from opposite sides away from
each other.
– At the same time, the addition of new tubulin
monomers extends their length.
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Bar = 100 Microns
Onion Root Tip
Rapidly dividing cells give us the opportunity to study the various
stages of cell division.
Notice that these plant cells, unlike animal cells, have cell walls.
30 microns
50 microns
Mitosis in eukaryotes may have
evolved from binary fission in bacteria
• Prokaryotes reproduce by binary fission, not
mitosis.
• Most bacterial genes are located on a single
bacterial chromosome which consists of a circular
DNA molecule and associated proteins.
• While bacteria do not have as many genes or DNA
molecules as long as those in eukaryotes, their
circular chromosome is still highly folded and coiled
in the cell.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• In binary fission, chromosome replication begins
at one point in the circular chromosome, the
origin of replication site.
• These copied regions begin to move to opposite
ends of the cell.
Fig. 12.10
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• Cell division
involves inward
growth of the
plasma
membrane,
dividing the parent
cell into two
daughter cells,
each with a
complete genome.
Fig. 12.10
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings