THE CELL CYCLE AND CELL
DIVISION (MITOSIS)
Hillis Textbook
Chapter 7
The lifespan of an organism is linked to cell
reproduction — usually called cell division.
Organisms have two basic strategies for reproducing
themselves:
• Asexual reproduction
• Sexual reproduction
 Cell division is also important in growth and repair of
tissues.
 Ex: root growth, bone growth, regeneration of body parts or
repair of injury.

In asexual reproduction the offspring are clones—
genetically identical to the parent.

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Any genetic variations are due to mutations.
A unicellular prokaryote may reproduce itself by binary
fission.

Single-cell eukaryotes can reproduce by mitosis.

Other eukaryotes are also
able to reproduce through
asexual or sexual means.
DNA in eukaryotic cells is organized into chromosomes.

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A chromosome consists of a
single molecule of DNA and
proteins.
Somatic cells—body cells not
specialized for reproduction
Each somatic cell contains two
sets of chromosomes (homologs)
that occur in homologous
pairs.
Four events must occur for cell division:
1. Reproductive signal—to initiate cell division
2. Replication of DNA
3. Segregation—distribution of the DNA into the two
new cells
4. Cytokinesis—division of the cytoplasm and
separation of the two new cells
In prokaryotes, cell
division results in
reproduction of the
entire organism.
The cell:
•
Grows in size
•
Replicates its DNA
•
Separates the DNA and
cytoplasm into two cells
through binary fission

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•
•
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Most prokaryotes have
one chromosome, a
single molecule of
DNA—usually
circular.
Two important regions
in reproduction:
ori - where replication
starts
ter - where replication
ends
As replication
proceeds, the ori
complexes move to
opposite ends of the
cell.
Eukaryotic cells divide by mitosis followed by
cytokinesis.
The cell cycle—the period between cell divisions that
consists of a long interphase, mitosis (M phase) and
cytokinesis.
 During interphase, the cell
nucleus is visible and cell
functions, including replication,
occur


Interphase begins after cytokinesis
and ends when mitosis starts.
The process in plant cells (which
have cell walls) is different than
in animal cells (which do not have
cell walls).
Interphase has three
subphases: G1, S, and
G2.

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G1 (Gap 1)—variable, a
cell may spend a long
time in this phase
carrying out its
functions
S phase (Synthesis)—
DNA is replicated
G2 (Gap 2)—the cell
prepares for mitosis,
synthesizes
microtubules for
segregating
chromosomes
Three structures appear in
prophase:
•
The condensed
chromosomes
•
Centrosome
•
Spindle forms

The karyotype of an
organism reflects the
number and sizes of its
condensed chromosomes


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During prometaphase,
the nuclear envelope
breaks down.
Chromosomes consisting
of two chromatids attach
to the kinetochore
mictotubules.

During metaphase,
chromosomes line up at
the midline of the cell.

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During anaphase, the
separation of sister
chromatids
After separation, they
move to opposite ends
of the spindle and are
referred to as
daughter
chromosomes.
Microtubules also
shorten, drawing
chromosomes toward
poles.

Telophase occurs
after chromosomes
have separated:
•
Spindle breaks down
•
Chromosomes uncoil
•
Nuclear envelope and
nucleoli appear
•
Two daughter nuclei
are formed with
identical genetic
information
Cytokinesis:

•
Division of the cytoplasm differs in plant and animals
In animal cells, plasma membrane pinches between the
nuclei because of a contractile ring of microfilaments of
actin and myosin.
Plant cells:

Vesicles from the Golgi apparatus appear along the
plane of cell division
•
These fuse to form a new plasma membrane.
•
Contents of vesicles form the cell plate—the beginning of
the new cell wall.
After cytokinesis:

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Each daughter cell contains all of the components of a
complete cell.
Chromosomes are precisely distributed.
The orientation of cell division is important to
development, but organelles are not always evenly
distributed.
Cell Cycle Regulation:


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Progression is tightly
regulated—the G1-S
transition is called R,
the restriction point.
Passing this point
usually means the cell
will proceed with the
cell cycle and divide.
Specific signals trigger
the transition from one
phase to another.

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Transitions also depend on activation of cyclindependent kinases (Cdk’s).
A protein kinase is an enzyme that catalyzes
phosphorylation from ATP to a protein.
Phosphorylation changes the shape and function of a
protein by changing its charges.
Cdk is activated by binding to cyclin (by allosteric
regulation); this alters its shape and exposes its active
site.
The G1-S cyclin-Cdk complex acts as a protein kinase
and triggers transition from G1 to S.
Other cyclin-Cdk’s act at different stages of the cell
cycle, called cell cycle checkpoints.