UNIT 3 ORGANIZATION AND DEVELOPMENT
THE CELL CYCLE AND MITOSIS
PHASES OF THE CELL CYCLE
The cell cycle consists of
•  Interphase – normal cell activity
•  Cell Division – mitosis and cytokinesis
INTERPHASE
Growth
G1
(DNA synthesis)
Growth
G2
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DNA
•  Genetic information – makes up an
organism’s genome
•  Packaged into chromosomes
Figure 12.3
50 µm
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MITOSIS AND CELL DIVISION
•  Mitosis makes new cells for repair; to replace
old, damaged, or dead cells.
•  Mitosis makes new cells for growth.
•  Somatic (non sex cells) undergo mitosis.
•  In every mitotic division, 2 cells are made.
•  These cells are genetically identical to the
parent cell.
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CHROMOSOMES
Chromosomes in Humans: 23 pairs, 46 total
•  Autosomes: non-sex chromosomes, chromosomes 1 – 22
•  Sex chromosomes: determine sex, X and Y, chromosome 23
Sets of chromosomes
•  Diploid: two sets of chromosomes, 2n (46 in humans)
•  Haploid: one set of chromosomes, n (23 in humans)
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STRUCTURE OF
CHROMOSOMES
•  DNA is an extremely long double-stranded
molecule.
•  One strand of DNA makes up a chromosome.
•  Before mitosis, the DNA exists in long,
uncondensed strands called - chromatin
•  Each unduplicated chromosome contains one DNA
molecule.
•  Duplicated chromosomes are called sister
chromatids.
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STRUCTURE OF CHROMOSOMES
•  DNA wraps around a group of histone proteins to form a
nucleosome.
•  Higher order coiling and supercoiling also help condense and
package the chromatin inside the nucleus:
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Chromosome Duplication
•  Because of duplication, each condensed chromosome
consists of 2 identical sister chromatids joined by a
centromere.
•  Each duplicated chromosome contains 2 identical
DNA molecules (unless a mutation occurred), one in
each chromatid:
Non-sister
chromatids
Centromere
Duplication
Sister
chromatids
Two unduplicated
chromosomes
Sister
chromatids
Two duplicated chromosomes
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STRUCTURE OF CHROMOSOMES
•  The centromere is where sister chromatid are connected.
•  The kinetochore serve as points of attachment for microtubules
that move the chromosomes during cell division.
Metaphase chromosome
Centromere
region of
chromosome
Kinetochore
Kinetochore
microtubules
Sister Chromatids
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PHASES OF THE CELL
CYCLE - AGAIN
Interphase
•  G1 - primary growth
•  S – DNA synthesis (replication)
•  G2 - secondary growth
M - mitosis
C - cytokinesis
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INTERPHASE
G1 – Growth 1 - Cells undergo majority of growth
S – Synthesis - Each chromosome replicates
(Synthesizes) to produce sister chromatids
•  Attached at centromere
•  Contains attachment site (kinetochore)
G2 – Growth 2 - Chromosomes condense Assemble machinery for division such as
centrioles
G0 – cells exit the cell cycle from G1 (they are not
preparing for division.
** During interphase, the cells are also carrying out
their function(s).
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G2 of Interphase
•  A nuclear envelope bounds
the nucleus.
•  Centrosome duplicated (one
for each end of the cell).
•  Chromatin is not fully are
condensed, but DNA has
been replicated during S
phase.
•  Chromosomes are not yet
visible.
G2 OF INTERPHASE
Centrosomes
(with centriole pairs)
Nucleolus
Chromatin
(duplicated)
Nuclear
Plasma
envelope membrane
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Prophase
•  Chromatin has condensed -
chromosome are visible.
•  Each duplicated chromosome
appears as two identical sister
chromatids joined together at the
centromere.
PROPHASE
Early mitotic
spindle
Aster
Centromere
•  The mitotic spindle begins to form. It
is composed of the centrosomes
and the microtubules that extend
from them.
•  The centrosomes move to opposite
ends of the cell and microtubules
become longer.
Chromosome, consisting
of two sister chromatids
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Metaphase
• The centrosomes are now at
opposite ends of the cell.
• The chromosomes line up on the
metaphase plate, an imaginary
plane that is equidistant between
the spindle’s two poles.
METAPHASE
Metaphase
plate
• The centromeres lie on the
metaphase plate.
•  Kinetochores are attached to
microtubules coming from opposite
poles.
•  The entire apparatus of
microtubules is called the spindle
because of its shape.
Spindle
Centrosome at
one spindle pole
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Anaphase
•  Sister separate. Each chromatid thus
becomes a chromosome.
•  Chromosomes begin moving toward
opposite ends of the cell (their
kinetochore microtubules shorten).
ANAPHASE
•  The chromosomes move centromere
first.
•  The cell elongates as the other
microtubules lengthen.
•  By the end of anaphase, the two ends
of the cell have equivalent—and
complete—collections of chromosomes.
Daughter
chromosomes
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Telophase
•  Nuclei begin to
form in the “new” cells.
•  Nuclear envelopes arise from
the fragments of the parent
cell’s nuclear envelope.
TELOPHASE AND CYTOKINESIS
Cleavage
furrow
Nucleolus
forming
•  The chromosomes become
less condensed – chromatin.
•  Mitosis is completed.
Nuclear
envelope
forming
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CYTOKINESIS
•
Cleavage of cell into two
halves
–  Animal cells
  Constriction of actin
filaments
  Cleavage furrow
–
Plant cells
  Cell plate
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MITOSIS IN AN ANIMAL CELL
G2 OF INTERPHASE
Centrosomes
(with centriole pairs)
Nucleolus
Chromatin
(duplicated)
Nuclear
Plasma
envelope membrane
PROPHASE
Early mitotic
spindle
Aster
Centromere
Chromosome, consisting
of two sister chromatids
PROMETAPHASE
Fragments
of nuclear
envelope
Kinetochore
Nonkinetochore
microtubules
Kinetochore
microtubule
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MITOSIS IN AN ANIMAL CELL
METAPHASE
ANAPHASE
Metaphase
plate
Spindle
Centrosome at Daughter
one spindle pole chromosomes
TELOPHASE AND CYTOKINESIS
Cleavage
furrow
Nucleolus
forming
Nuclear
envelope
forming
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MITOSIS IN A PLANT CELL
Chromatine
Nucleus
Nucleolus condensing
1 Prophase.
The chromatin
is condensing.
The nucleolus is
beginning to
disappear.
Although not
yet visible
in the micrograph,
the mitotic spindle is
staring to from.
Chromosome
Metaphase. The
2 Prometaphase.
3
4
spindle is complete,
We now see discrete
and the chromosomes,
chromosomes; each
attached to microtubules
consists of two
at their kinetochores,
identical sister
are all at the metaphase
chromatids. Later
plate.
in prometaphase, the
nuclear envelop will
fragment.
Anaphase. The
5
chromatids of each
chromosome have
separated, and the
daughter chromosomes
are moving to the ends
of cell as their
kinetochore
microtubles shorten.
Telophase. Daughter
nuclei are forming.
Meanwhile, cytokinesis
has started: The cell
plate, which will
divided the cytoplasm
in two, is growing
toward the perimeter
of the parent cell.
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CYTOKINESIS IN ANIMAL AND PLANT CELLS
Cleavage furrow
Contractile ring of
microfilaments
100 µm
Vesicles
forming
cell plate
Wall of
patent cell
1 µm
Cell plate
New cell wall
Daughter cells
Daughter cells
(a) Cleavage of an animal cell (SEM)
(b) Cell plate formation in a plant cell (SEM)
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CELL DIVISION IN PROKARYOTES
•  Prokaryotes lack nuclei and membrane-bound organelle
•  DNA is a circular chromosome
•  Binary Fission:
•  DNA is copied  two circular chromosomes
•  New membrane begins to develop
•  Cells grows to double its size
•  Pinches off into two identical cells
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