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Cell Division & Cell Cycle Control
 Cell Division
Cell types in eukaryotic organisms:
 Somatic cells = body cells
 Humans – 46 chromosomes; diploid; 2N
- Two sets of chromosomes
 Gametes = sex cells
 Humans – 23 chromosomes; haploid; N
- One set of chromosomes
Chromosome Anatomy
Sister chromatid
(each ½); each has identical DNA
Only called this when
Attached by
Centromere
Kinetochore: Area where spindle fibers (microtubules)
will attach and pull the chromatids apart in M & M.
 Cell Division (two types)
- Mitosis = process produces to somatic daughter
cells identical to original cell. (Review on own)
 Asexual reproduction
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 One 2N (diploid) cell produces two identical 2N
cells.
 Purpose = growth, repair, maintenance
- Meiosis = produces gametes with half the genetic
information as the parent cell.
 Associated with sexual reproduction
 2N (diploid) cell produce N (haploid) cell(s).
 Purpose = making cells with ½ the parental
genetic information increases variation in a
population
These cell division processes are important parts to the
Cell Cycle
- Has a series of checkpoints that are highly regulated
by their internal and external chemical environments
and size.
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 Stages of the Cell Cycle:
 INTERPHASE – most of cell’s life is spent here
consist of (G1, S & G2).
•
DNA relaxed (Chromatin)
 G0 = after a split = cell arrest
Some cells permanently stuck here
Ex. Nerve cell of CNS
Some cells will enter cycle via signal…
Ex. skin cell, muscle cell
- G1 (1st gap) = first growth phase; organelles duplicate
 G1 check point – controlled by:
• Cell size
• Growth factors
• Environment
- S (synthesis) = Synthesis phase ~ DNA replication
(more to come…)
- G2 (2nd gap) = second growth phase ~ more growth and
prepare for cell division
 G2 check point – controlled by:
•
DNA replication
•
DNA damage/mutations
•
Cell Size
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 MITOSIS or MEIOSIS = division of chromosomes
(PMAT)
 M check point (meta/ana) - controlled by
Spindle fibers (microtubules) attachment to
chromosomes
• DNA condensed (Chromosome)
 CYTOKINESIS = division of cytoplasm
1. Cleavage = animal cells form cleavage furrow
o Microfilaments form a contractile ring around
center of cell and pinch.
2. Cell plate formation = plant cells
o Golgi body forms vesicles - cell wall material =
move to cell plate area…continues until
o Membrane of plate is full; two plasma
membranes formed from vesicle’s membrane.
o At that point, two daughter cells
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 In Prokaryotes:
 Cell division = Binary Fission
- Genes located on a singular circular chromosome.
- As DNA of chromosome replicates at specific
locations (origin of replication…)
- Origins move to opposite ends of cell, stretching
cell
- Cell grows to twice the size…
- Replication done, plasma membrane grows inward
- Cell divides - 2 identical daughter cells results.
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 Evolutionary relationships:
- The DNA sequences & protein structures that
control Binary fission in prokaryotes are the same in
mitosis for eukaryotic cells.
 Molecules of the Cell Cycle Control System:
1. Protein Kinases – activate/inactivate proteins via
phosphorylation
- Present in constant concentration in an inactive
form
- Activated (controlled) by presence of…
2. Cyclins
- Fluctuating concentrations
So, the kinases that help control the cell cycle are called
Cdks (cyclin-dependent kinases)
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 External control of Cell Cycle:
 Growth factors (GF) = proteins released by certain
cells that stimulate other cells to divide.
• Triggers signal transduction pathway that allows a cell
to pass G1
Ex. ~ injury… stimulates platelets in blood to release
specific GF in area; causing fibroblast cells to divide
healing the wound.
 Environment
Ex. ~ When the associated cyclin accumulates during
G2…
• Cdk = MPF (M-phase promoting factor/maturation
promoting factor) triggers cell to pass G2 checkpoint
into the M phase.
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• MPF involved in a phosphorylation cascade within the
nuclear lamina that leads to the degradation of the
nuclear envelope.
• When in M phase (during anaphase) leads to
destruction of its own cyclin.
• Remains inactive until enough cyclin is built up again
to continue past G2 checkpoint,…again.
 Cell density & size
• Density-dependent inhibition = crowded cells stop
dividing.
• Due to not enough supply of GF and nutrients to
supply numbers of cells for division.
And…
• If a cell gets too big, not enough
nutrients/molecules/ ions can cross plasma
membrane to run cell efficiently (SA/V).
• Recalls cell from G0….
 Internal control of Cell Cycle:
Ex.
• During M phase checkpoint…
• All chromosomes must be attached at kinetochores
before separation of sister chromatids
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Cancer
 Cancer cells don’t respond to cell cycle control
mechanisms. Cells keep dividing and can invade other
tissues.
- Do not respond to depletion of GF (they may make
the required growth factor on their own)
- Do not respond to density dependency
- If they do stop at any point in the cell cycle =
random points vs. normal checkpoints.
Transformation = normal cells turn into cancer cells.
 Results in a tumor
• If a tumor remains in the original tissue from
where it started = benign
• If tumor invades another tissue = malignant
- Can impair function of the said other
tissues and the organ(s) they are part of.
 Metastasis – physical spreading of tumor
- Cancer cells may destroy attachments to neighboring
cells (protein = gap junctions)
- Extracellular matrix between cancer cells can spread
into neighboring tissues.
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- May secrete signal molecules that cause blood
vessels to grow toward tumor.
- Nearby cancer cells may separate from original
tumor and enter blood stream/lymph system and
travel to other locations and proliferate there.
 Treatment:
 Chemotherapy = drug given through IV (blood) that
interferes with the cell cycle.
Ex. stops production of protein microtubules of
spindle complex.
 Radiation = localized radiation waves damage cancer cell
DNA (more than normal cells)
- Cancer cells don’t have the DNA repair
mechanisms that normal cells have.
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