Transcript RACC BIO cell cycle/mitosis

• The Cell Cycle
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Pre-Unit Questions
• What types of cells undergo mitosis?
• What is a haploid cell?
– Give an example of this type of cell
• What is a diploid cell?
– Give an example of this type of cell
• True or False – the cell cycle consists of prophase,
metaphase, anaphase, and telophase only. (if false,
correct the statement)
• What are the parts of a duplicated chromosome
• Can you identify the following stages of mitosis
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The Cell Cycle
• Overview: The Key Roles of Cell Division
• The continuity of life
– Is based upon the reproduction of cells, or cell
division
100 µm
• Unicellular organisms
– Reproduce by cell division
(a) Reproduction. An amoeba,
a single-celled eukaryote, is
dividing into two cells. Each
new cell will be an individual
Figure 12.2 A
organism (LM).
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Cell Division
• Multicellular organisms depend on cell division
for
– Development from a fertilized cell
– Growth
– Repair
200 µm
20 µm
(b) Growth and development.
(c) Tissue renewal. These dividing
This micrograph shows a
bone marrow cells (arrow) will
sand dollar embryo shortly
give rise to new blood cells (LM).
after the fertilized egg divided,
Figure 12.2 B, C forming two cells (LM).
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Cell Division
• Cell division results in genetically identical
daughter cells
• Cells duplicate their genetic material
– Before they divide, ensuring that each
daughter cell receives an exact copy of the
genetic material, DNA
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Cellular Organization of the Genetic Material
• A cell’s endowment of DNA, its genetic
information
– Is called its genome
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Genetic Material
• The DNA molecules in a cell
– Are packaged into chromosomes
Figure 12.3
50 µm
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Genetic Material
• Eukaryotic chromosomes
– Consist of chromatin, a complex of DNA and
protein that condenses during cell division
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Distribution of Chromosomes During Cell Division
•
In animals
–
Somatic cells have two sets of chromosomes
• diploid
–
Sex cells (gametes) have one set of chromosomes
• haploid
•
In preparation for cell division
–
DNA is replicated and the chromosomes condense
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Phases of the Cell Cycle
• The cell cycle consists of
– The mitotic phase
– Interphase – the cell grows, duplicates proteins,
organelles and chromosomes
INTERPHASE
The mitotic phase
alternates with
interphase
G1
S
(DNA synthesis)
G2
Figure 12.5
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Interphase – and its subphases (preparation for cell division)
G1 phase – cell grows, produces proteins and organelles
requires structural proteins and enzymes
S phase – DNA synthesis (form sister chromatids)
(see next slide)
G2 phase – 2nd growth phase. During G2, organelles replicate,
chromosomes get ready to condense, and microtubules start to form at
centrioles.
INTERPHASE
M phase –cell division and cytokinesis
G1
Mitosis – division of the nucleus
S
(DNA synthesis)
G2
Cytokinesis – division of the
cytoplasm
Figure 12.5
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Duplicated Chromosomes
• Each duplicated chromosome
– Has two sister chromatids, which separate
during cell division
0.5 µm
A eukaryotic cell has multiple
chromosomes, one of which is
represented here. Before
duplication, each chromosome
has a single DNA molecule.
Once duplicated, a chromosome
consists of two sister chromatids
connected at the centromere. Each
chromatid contains a copy of the
DNA molecule.
Mechanical processes separate
the sister chromatids into two
chromosomes and distribute
them to two daughter cells.
Figure 12.4
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Chromosome
duplication
(including DNA
synthesis)
Centromere
Separation
of sister
chromatids
Centromeres
Sister
chromatids
Sister chromatids
The Mitotic Phase – Mitosis and Cytokinesis
• Mitosis consists of five distinct phases
– Prophase
– Prometaphase
G2 OF
INTERPHASE
Centrosomes
Chromatin
(with centriole pairs)
(duplicated)
Figure 12.6
Nucleolus
Nuclear
Plasma
envelope membrane
PROPHASE
Early mitotic
spindle
Aster
Centromere
Chromosome, consisting
of two sister chromatids
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PROMETAPHASE
Fragments
Kinetochore
of nuclear
envelope
Nonkinetochore
microtubules
Kinetochore
microtubule
Mitosis
– Metaphase
– Anaphase
– Telophase
METAPHASE
ANAPHASE
Metaphase
plate
Figure 12.6
Spindle
Centrosome at Daughter
one spindle pole chromosomes
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TELOPHASE AND CYTOKINESIS
Cleavage
furrow
Nuclear
envelope
forming
Nucleolus
forming
The Mitotic Spindle: A Closer Look
• The mitotic spindle
– Is an apparatus of microtubules that controls
chromosome movement during mitosis
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The mitotic spindle
• The spindle arises from the centrosomes
–
And includes spindle microtubules and asters
–
Within an animal cell centrosome there is a pair of small organelles, the
centrioles
–
Unlike centrosomes in animal cells, plant cells do not have centrioles.
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Mitotic spindle
• An aster is an array of microtubules that has
formed from the centrosome after it moves to
opposite poles.
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Mitotic spindle
• Some spindle microtubules
–
Attach to the kinetochores of chromosomes and move the
chromosomes to the metaphase plate
Aster
Kinetochore microtubules attach to
the kinetochore and the poles.
These fibers will then attach to the
mitotic spindle fibers which will
move the sister chromatids towards
the opposite poles
Sister
chromatids
Centrosome
Metaphase
Plate
Kinetochores
Overlapping
nonkinetochore
microtubules
Kinetochores
microtubules
Microtubules
0.5 µm
Figure 12.7 Centrosome
1 µm
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Chromosomes
Mitotic spindle
•
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Moving chromosomes
• In anaphase, sister chromatids separate and move along
the kinetochore microtubules toward opposite ends of
the cell
– Nonkinetechore microtubules from opposite poles
overlap and push against each other, elongating the
cell
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Movement of chromosomes
• In telophase
– Genetically identical daughter nuclei form at
opposite ends of the cell
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Mitosis Review
•
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Cytokinesis: A Closer Look
• In animal cells
– Cytokinesis occurs by a process known as
cleavage, forming a cleavage furrow
Cleavage furrow
Contractile ring of
microfilaments
Figure 12.9 A
100 µm
Daughter cells
(a) Cleavage of an animal cell (SEM)
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Cytokinesis
• In plant cells, during cytokinesis
– A cell plate forms
Vesicles
forming
cell plate
1 µm
Wall of
patent cell Cell plate New cell wall
Daughter cells
Figure 12.9 B (b) Cell plate formation in a plant cell (SEM)
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Overview of Mitosis
• Mitosis in a plant cell
Chromatine
Nucleus
Nucleolus condensing
Chromosome
Metaphase. The
2 Prometaphase.
3
1 Prophase.
spindle is complete, 4
The chromatin
We now see discrete
and the chromosomes,
is condensing.
chromosomes; each
attached to microtubules
The nucleolus is
consists of two
at their kinetochores,
beginning to
identical sister
are all at the metaphase
disappear.
chromatids. Later
plate.
Although not
in prometaphase, the
yet visible
nuclear envelop will
in the micrograph,
fragment.
the mitotic spindle is
staring to from.
Figure 12.10
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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.
Binary Fission
• Prokaryotes (bacteria)
– Reproduce by a type of cell division called
binary fission
– Results in two genetically identical cells
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Binary Fission
• In binary fission
– The bacterial chromosome replicates.
• Begins at the origin of replication
– The two daughter chromosomes actively move apart
Origin of
replication
1
Origin of replication opens up like a
bubble and the two strands move in
opposite directions
2 Replication continues until one copy of
the origin is now at each end of
the cell.
43
Each replicated strand attached itself to the
cell membrane. The cell elongates,
separating the replicated strands
4 The cell wall grows across the
middle of the cell
5 Two daughter cells result.
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E. coli cell
Two copies
of origin
Origin
Cell wall
Plasma
Membrane
Bacterial
Chromos
ome
Origin
The Evolution of Mitosis
• Since prokaryotes preceded eukaryotes by
billions of years
– It is likely that mitosis evolved from bacterial
cell division
• mitosis is a universal eukaryotic property
– All eukaryotes (except amoeba) undergo
mitosis.
• the key molecules involved in mitosis, have
homologous moleclues in prokaryotes
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Cell Cycle Controls
To grow and develop normally a plant or animal
cell must be able to control the timing of cell
division.
• The frequency of cell division
– Varies with the type of cell
The events are directed by a cell cycle control
system
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G1 phase checkpoint
• The cell has a checkpoint at this phase. The
cellular environment is checked and cell size.
Only when conditions are right will the cell
proceed into the S phase
– If signal is not received it goes to a nondividing stage called G0 phase.
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G0 Phase
• This is mitotic dormancy or differentiation
• Cells are not considered to be in the cycle
of division but become differentiated or
specialized in their function
• Cells can “mature” at this time to play a role
based on the specific genes they express.
• Some cells remain here permanently. Some
may stay here for hours, days, months.
– E.g. nerve cells
• Some can revert back to G1, S and G2
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Other Checkpoints
• Check points will also occur after the S phase
and G2 phase.
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Cell Regulators
• Internal Regulators
– Regulate the timing inside the cell cycle, when
the cell can proceed from one phase to the
next.
• Ex. G1 to S phase
• External Regulators
– Regulate timing external to the cell, like when it
is necessary for cells to divide in general
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Cell Regulation comes from protein signals
Cyclins are a class of proteins that regulate the cell cycle
• Two main types
• Cyclin Dependent Protein Kinases (Cdks)
• Cyclins
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Cyclins
• undergo a constant cycle of synthesis and
degradation during cell division
• cyclins act as an activating protein and bind to
Cdks forming a cyclin-Cdk complex
• This complex then acts as a signal to the cell to
pass to the next cell cycle phase. Eventually,
the cyclin degrades, deactivating the Cdk, thus
signaling exit from a particular phase.
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Cell Regulation-Cyclin dependent kinases
• Cdks
– These are simply found within the cell, not
synthesized
– Only become active when bound with cyclins
– A Cdks is an enzyme that adds negatively
charged phosphate groups to other molecules
in a process called phosphorylation. Through
phosphorylation, Cdks signal the cell that it is
ready to pass into the next stage of the cell
cycle
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In General
• The combinations of cyclins and cyclindependent kinases (CDKs) work together to
phosphorylate (add phosphate) and activate (or
inactivate) target proteins in the steps of the
cell cycle. Different combinations determine
which target protein will be affected
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What are some factors that keep normal cells in
“check”
• Growth Factors
– A protein that stimulates other cells to divide
Density – dependent inhibition
- causes crowded cells to stop dividing
(run out of nutritens)
Anchorage dependence – in order to divide, cells must
be attached to a substrate
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Loss of Cell Cycle Controls
• Cells that do not respond normally to these
controls.
– Cancer cells
– Exhibit neither density-dependent inhibition nor
anchorage dependence
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Loss of Cell Cycle Controls in Cancer Cells
• Cancer cells
– Do not respond normally to the body’s control
mechanisms
– Form tumors
– Benign tumors remain at the original site and
are usually harmless.
• An abnormal mass of essentially normal
cells.
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Tumors
• Malignant tumors invade surrounding tissues
and can affect one or more organs.
• The cells have undergone a transformation (no
longer “normal”)
Figure 12.19
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Cancer
• Cells from a malignant tumor may split off, and
travel to new locations, where they can form
new tumors.
• The spread of cancer cells beyond their original
site is called metastasis
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Cancer Treatments
• Treatments for cancer include:
– Radiation
• Causes Apoptosis – “programmed cell
death”
– Many cells will kill themselves for the
betterment of the organisms
• Like cells damaged by virus
• Cells in the lining of the uterus
– Chemotherapy
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What prevents normal cells from becoming tumor cells
• Genetic damage or errors are often repaired
• The tumor suppressor gene p53 gene
– This gene stops a damaged cell just before the
S phase so that it can be repaired.
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