The Cell Cycle
Introduction to Biology
The Key Roles of Cell
Division
• The ability to reproduce is one of the key
features that separates life from non-life.
• All cells have the ability to reproduce, by
making exact copies of themselves.
• In unicellular organisms, division of one cell
reproduces the entire organism
• In multicellular organisms, cell division is needed
for:
o Development of an embryo from a sperm/egg
o Growth
o Repair
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100 µm
Reproduction
200 µm
Growth and development
20 µm
Tissue renewal
Asexual Reproduction
• Asexual reproduction is reproduction that
involves a single parent producing an
offspring.
o The offspring produced are, in most cases,
genetically identical to the single cell that
produced them.
o Asexual reproduction is a simple, efficient, and
effective way for an organism to produce a large
number of offspring.
o Prokaryotic organisms (like bacteria) reproduce
asexually, as do some eukaryotes (like sponges)
Sexual Reproduction
• In sexual reproduction, offspring are
produced by the fusion of two sex cells – one
from each of two parents. These fuse into a
single cell before the offspring can grow.
o The offspring produced inherit some genetic
information from both parents.
o Most animals and plants, and many single-celled
organisms, reproduce sexually.
Contrasting
Reproduction Types
Cell Division
• Cells duplicate their genetic material before
they divide, ensuring that each daughter cell
receives an exact copy of the genetic
material, DNA.
• A dividing cell duplicates its DNA, allocates
the two copies to opposite ends of the cell,
and only then splits into daughter cells.
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.
Chromosomes
• The genetic information that is passed on from
one generation of cells to the next is carried
by chromosomes.
• Every cell must copy its genetic information
before cell division begins.
• Each daughter cell gets its own copy of that
genetic information.
• Cells of every organism have a specific
number of chromosomes.
Prokaryotic
Chromosomes
• Prokaryotic cells lack nuclei. Instead, their
DNA molecules are found in the cytoplasm.
• Most prokaryotes contain a single, circular
DNA molecule, or chromosome, that contains
most of the cell’s genetic information.
Eukaryotic Chromosomes
• In eukaryotic cells, chromosomes are located
in the nucleus, and are made up of
chromatin.
• Chromatin is composed of DNA and histone proteins.
• DNA coils around histone proteins to form
nucleosomes.
• The nucleosomes interact with one another to
form coils and supercoils that make up
chromosomes.
Chromosomes During
Cell Division
• In preparation for cell division, DNA is replicated and
the chromosomes condense
• Each duplicated chromosome has two sister
chromatids, which separate during cell division
• The centromere is the narrow “waist” of the
duplicated chromosome, where the two chromatids
are most closely attached
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0.5 µm
Chromosome
duplication
(including DNA
synthesis)
Centromere
Sister
chromatids
Separation
of sister
chromatids
Centromeres
Sister chromatids
Phases of the Cell Cycle
• The cell cycle consists of
o Mitotic (M) phase (mitosis and cytokinesis)
o Interphase (cell growth and copying of
chromosomes in preparation for cell division)
• Interphase (about 90% of the cell cycle) can
be divided into subphases:
o G1 phase (“first gap”)
o S phase (“synthesis”)
o G2 phase (“second gap”)
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INTERPHASE
G1
S
(DNA synthesis)
G2
G1 Phase: Cell Growth
• In the G1 phase, cells
increase in size and
synthesize new proteins
and organelles.
S Phase: DNA Replication
• In the S (or synthesis)
phase, new DNA is
synthesized when the
chromosomes are
replicated.
G2 Phase: Preparing for
Cell Division
• In the G2 phase, many of
the organelles and
molecules required for
cell division are
produced.
M Phase: Cell Division
• In eukaryotes, cell
division occurs in two
stages: mitosis and
cytokinesis.
o Mitosis is the division of
the cell nucleus.
o Cytokinesis is the
division of the
cytoplasm.
Important Cell Structures
Involved in Mitosis
• Chromatid – each strand of a duplicated
chromosome
• Centromere – the area where each pair of
chromatids is joined
• Centrioles – tiny structures located in the cytoplasm of
animal cells that help organize the spindle
• Spindle – long proteins (part of the cytoskeleton) that
the centrioles produce
o Helps move the chromosomes into place.
Prophase
• During prophase, the
first phase of mitosis,
the duplicated
chromosome
condenses and
becomes visible.
Prophase
• The centrioles move
to opposite sides of
nucleus and help
organize the spindle.
Prophase
• The spindle forms
and DNA strands
attach at a point
called their
centromere.
Prophase
• The nucleolus
disappears and
nuclear envelope
breaks down.
Metaphase
• During metaphase,
the second phase of
mitosis, the
centromeres of the
duplicated
chromosomes line up
across the center of
the cell.
Metaphase
• The spindle fibers
connect the
centromere of each
chromosome to the
two poles of the
spindle.
Anaphase
• During anaphase, the
third phase of mitosis,
the centromeres are
pulled apart and the
chromatids separate
to become individual
chromosomes.
Anaphase
• The chromosomes
separate into two
groups near the poles
of the spindle.
Telophase
• During telophase, the
fourth and final phase
of mitosis, the
chromosomes spread
out into a tangle of
chromatin.
Telophase
• A nuclear envelope reforms around each
cluster of
chromosomes.
Telophase
• The spindle breaks apart,
and a nucleolus becomes
visible in each daughter
nucleus.
Cytokinesis
• Cytokinesis is the division of the cytoplasm.
• The process of cytokinesis is different in animal
and plant cells.
Cytokinesis in Animal
Cells
• The cell membrane is drawn in until the
cytoplasm is pinched into two equal parts.
• Each part contains its own nucleus and
organelles.
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100 µm
Cleavage furrow
Contractile ring of
microfilaments
Daughter cells
Cleavage of an animal cell (SEM)
Cytokinesis in Animal
Cells
• In plants, the cell membrane is not flexible
enough to draw inward because of the rigid
cell wall.
• Instead, a cell plate forms between the
divided nuclei that develops into cell
membranes.
• A cell wall then forms in between the two new
membranes.
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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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Nucleus
Nucleolus
Chromatin
condensing
Prophase. The
chromatin is condensing.
The nucleolus is
beginning to disappear.
Although not yet visible
in the micrograph, the
mitotic spindle is starting
to form.
Chromosomes
Prometaphase. We
now see discrete
chromosomes; each
consists of two identical
sister chromatids. Later
in prometaphase, the
nuclear envelope will
fragment.
Cell plate
Metaphase. The spindle is
complete, and the
chromosomes, attached
to microtubules at their
kinetochores, are all at
the metaphase plate.
Anaphase. The
chromatids of each
chromosome have
separated, and the
daughter chromosomes
are moving to the ends of
the cell as their
kinetochore microtubules shorten.
10 µm
Telophase. Daughter
nuclei are forming.
Meanwhile, cytokinesis
has started: The cell
plate, which will divide
the cytoplasm in two, is
growing toward the
perimeter of the parent
cell.
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INTERPHASE
PROPHASE
PROMETAPHASE
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INTERPHASE
PROPHASE
PROMETAPHASE
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INTERPHASE
PROPHASE
PROMETAPHASE
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METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS
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METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS
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METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS
Virtual Onion Root Tip
Mitosis Lab
Click here to start
Binary Fission
• Prokaryotes (bacteria and archaea) reproduce by a
type of cell division called binary fission
• In binary fission, the chromosome replicates
(beginning at the origin of replication), and the two
daughter chromosomes actively move apart
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Cell wall
Origin of
replication
Plasma
membrane
E. coli cell
Chromosome
replication begins.
Soon thereafter,
one copy of the origin
moves rapidly toward
the other end of the cell.
Two copies
of origin
Bacterial
chromosome
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Cell wall
Origin of
replication
Plasma
membrane
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.
Bacterial
chromosome
Two copies
of origin
Origin
Origin
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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.
Plasma
membrane
Bacterial
chromosome
Two copies
of origin
Origin
Origin
The Evolution of Mitosis
• Since prokaryotes evolved before eukaryotes, mitosis
probably evolved from binary fission
• Certain protists exhibit types of cell division that seem
intermediate between binary fission and mitosis
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Bacterial
chromosome
Prokaryotes
Chromosomes
Microtubules
Intact nuclear
envelope
Dinoflagellates (Type of plankton)
Kinetochore
microtubules
Intact nuclear
envelope
Diatoms (Type of Algae)
Kinetochore
microtubules
Centrosome
Fragments of
nuclear envelope
Most eukaryotes
The Cell Cycle Control
System
• The sequential events of the cell cycle are directed
by a distinct cell cycle control system, which is similar
to a clock
• The clock has specific checkpoints where the cell
cycle stops until a go-ahead signal is received
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G1 checkpoint
Control
system
G1
M
M checkpoint
G2 checkpoint
G2
S
• For many cells, the G1 checkpoint seems to
be the most important one
• If a cell receives a go-ahead signal at the G1
checkpoint, it will usually complete the S, G2,
and M phases and divide
• If the cell does not receive the go-ahead
signal, it will exit the cycle, switching into a
nondividing state called the G0 phase
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G0
G1 checkpoint
G1
If a cell receives a go-ahead
signal at the G1 checkpoint,
the cell continues on in the
cell cycle.
G1
If a cell does not receive a
go-ahead signal at the G1
checkpoint, the cell exits the
cell cycle and goes into G0, a
nondividing state.
• An example of external signals is densitydependent inhibition, in which crowded cells
stop dividing
• Most animal cells also exhibit anchorage
dependence, in which they must be
attached to a substratum (connective tissue)
in order to divide
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Cells anchor to dish surface and
divide (anchorage dependence).
When cells have formed a complete
single layer, they stop dividing
(density-dependent inhibition).
If some cells are scraped away, the
remaining cells divide to fill the gap and
then stop (density-dependent inhibition).
Normal mammalian cells
25 µm
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Cancer cells do not exhibit
anchorage dependence
or density-dependent inhibition.
25 µm
Cancer cells
Loss of Cell Cycle
Controls in Cancer Cells
• Cancer cells do not respond normally to the
body’s control mechanisms
• Cancer cells form tumors, masses of abnormal
cells within otherwise normal tissue
• If abnormal cells remain at the original site,
the lump is called a benign tumor
• Malignant tumors invade surrounding tissues
and can metastasize, exporting cancer cells
to other parts of the body, where they may
form secondary tumors
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Lymph
vessel
Tumor
Blood
vessel
Glandular
tissue
Cancer cell
A tumor grows from a
single cancer cell.
Cancer cells invade
neighboring tissue.
Cancer cells spread
through lymph and
blood vessels to
other parts of the
body.
Metastatic
tumor
A small percentage
of cancer cells may
survive and establish
a new tumor in another
part of the body.