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Why Cells divide?
• In unicellular organisms, division of one cell
reproduces the entire organism
• Multicellular organisms depend on
cell division for:
– Development from a fertilized cell
– Growth
– Repair of injured tissues.
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DNA is the genetic material of the cell
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Most of the DNA of the Eukaryotic Cell is
Located in the Nucleus
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Cellular Organization of the Genetic Material
• A cell’s endowment of DNA (its genetic
information) is called its genome
• DNA molecules in a cell are packaged into
chromosomes.
• The number of chromosomes in a
eukaryotic cell depends on the species. For
example, a human body cell has 46
chromosomes. A dog’s body cell has 78
chromosomes.
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What is a Chromosome?
• One long DNA molecule and a number
of protein molecules attached to it form
one chromosome.
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Chromatin Fibers&
Chromosomes
• In nondividing cells, chromosomes
exist as a diffuse mass of long and thin
fibers called CHROMATIN.
• As a cell prepares to divide, its
chromatin coils up forming compact
and distinct (short and thick)
chromosomes that are visible under
the light microscope.
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Concept 12.1: Cell division results in genetically
identical daughter cells
• Cells duplicate their DNA
before they divide
• So one DNA molecule in the
mother cell doubles ensuring
that each daughter cell
receives an exact copy of the
genetic material, DNA, and
consequently the exact # of
chromosomes as in the mother
cell.
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In preparation for cell division:
•DNA is replicated and
•the two new DNA molecules and associated
proteins stay attached while condensing to form
two sister chromatids.
•The two sister chromatids form a duplicated
chromosome.
•The narrow “waist” of the duplicated
chromosome, where the two sister chromatids are
most closely attached, is called the
CENTROMERE
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DNA Replication
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The Cell Cycle
• The cell cycle is an ordered sequence
of events in the life of a eukaryotic cell
from its origin in the division of a parent
cell until its own division into two
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Phases of the Cell Cycle:
– The Interphase (about 90% of the cell cycle) has three
subphases:
• G1 phase (“first gap”) centers on the growth of the
cell.
• S phase (“synthesis”) duplicates the chromosomes.
• G2 phase (“second gap”) completes the preparation
for cell division before the mitotic phase starts.
– The Mitotic (M) phase has two subphases:
• Mitosis (the nuclear division)
• Cytokinesis (the cytoplasmic division)
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Phases of the Cell Cycle
Interphase
S
(DNA Replication)
G1
(Cell Growth)
M: Mitosis
(Nuclear Division)
C: Cytokinesis
(Cytoplasmic Division)
C
M
Mitotic Phase
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G2
(Cell Preparation
for Mitosis)
Concept 12.2: The mitotic phase alternates with interphase in the cell cycle
INTERPHASE
G1
S
(DNA synthesis)
G2
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The
Cell
Cycle
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Interphase
Two pairs of centrioles
Chromatin
Nucleolus
- Growth of the
baby cell.
- Synthesis of
cellular
components
needed for cell
division,
Nuclear
envelope
Plasma
membrane
Nucleus with
chromatin
including
replication of
DNA and the
centrosome
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15
Phases of the Mitotic Phase of the Cell Cycle
• 1- Mitosis is conventionally divided into the following
phases:
– Prophase
• Early prophase
• Late prophase (Prometaphase)
– Metaphase
– Anaphase
– Telophase
• 2- Cytokinesis is well underway by late telophase
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Chromosome
(two sister chromatids
joined at centromere)
Prophase
Sister
chromatids
Centromere
- Chromosomes appear
due to coiling of
chromatin.
G2 OF INTERPHASE
- Nucleolus breaks
down.
Developing
spindle
Nucleus with
dispersed chromosomes
- Spindle fibers begin
to form from centrioles.
- Centrioles move
toward opposing cell
poles.
- Nuclear envelope
breaks down at the
end of this stage.
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PROPHASE
PROMETAPHASE
17
G2 of interphase → Prophase → Prometaphase
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Cell Division: The Cell Cycle
• Metaphase
Equatorial plate
Spindle fibers
– Chromosomes aligned
on equatorial plate of
cell
– Spindle fibers extending
from centriole attach at
centromere of
chromosome
Chromosomes aligned
on equatorial plate
Spindle fibers
– Total array termed
mitotic spindle
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19
Anaphase
• Centromeres that held
chromatid pairs together
separate
• Spindle fibers move sister
chromatids apart toward poles
Sister chromatids
being pulled apart
Sister chromatids being pulled apart
• The microtubules shorten by
depolymerizing at their
kinetochore ends
• Each chromatid is now a
chromosome of one DNA
molecule
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Spindle fibers
(d) Anaphase
20
Telophase
– Arrival of new
chromosomes at each pole
Re-forming
nuclear
envelope
Cleavage furrow
of cytokinesis
Nucleolus
– Chromosomes begin to
uncoil and return to
chromatin (long and thin)
Cytokinesis occurring
– A nucleolus reforms within
each nucleus.
– Spindle fibers break up and
disappear.
– New nuclear envelope forms
around each set of
chromosomes.
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Cleavage furrow
(e) Telophase
21
Metaphase
→
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Anaphase
→Telophase &
Cytokinesis
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Cytokinesis (Cytoplasmic Division)
• In animal cells, cytokinesis occurs by a
forming a cleavage furrow
• In plant cells, cytokinesis occurs by a
forming a cell plate.
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Cytokinesis in Animal Cells
100 µm
Cleavage furrow
Contractile ring of
microfilaments
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Daughter cells
Cytokinesis
in Plant Cells
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Vesicles
forming
cell plate
Wall of
parent cell
Cell plate
1 µm
New cell wall
Daughter cells
Cell plate formation in a plant cell (TEM)
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Mitosis
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Binary Fission
• Prokaryotes (bacteria and archaea) reproduce by a type
of cell division called binary fission
• In binary fission:
– the chromosome replicates
– and the two daughter chromosomes actively move
apart
– The plasma membrane grows inward, and
– new cell wall is deposited resulting in two daughter
cells
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Binary Fission
Cell wall
Origin of
replication
E. coli cell
Chromosome
replication begins.
Soon thereafter,
one copy of the origin
moves rapidly toward
the other end of the cell.
Replication continues.
One copy of the origin
is now at each end of
the cell.
Replication finishes.
The plasma membrane
grows inward, and
new cell wall is
deposited.
Two daughter
cells result.
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Plasma
membrane
Bacterial
chromosome
Two copies
of origin
Origin
Origin