Download The Cell Cycle

The Cell Cycle
&
The Process of Cell Division
Purpose of Mitosis
• Growth
• Embryonic development
• Tissue repair
• Asexual reproduction
The Role of the
Chromosome
Chromosomes are
a “scaffolding” that
hold, carry and
protect DNA
Chromosomes
• Exist as Chromatin for most of the cell cycle
•
•
•
– Unwound, string-like (like a stretched out Slinky)
– DNA replication and RNA synthesis occur when
like this
• DNA in each cell is approximately 2 meters
long
Prior to cell division, Chromatin begins to coil and
thicken (super coiling)
Now called Chromosomes (sister chromatids)
– Initially joined all along their length by proteins
called cohesins, later centromere
Human somatic cells have 46 chromosomes
– Gametes have 23
The Cell Cycle
• 2 general periods exist: Interphase and
mitotic (M) phase
• 22-24 hour cycle
– 21 hours of Interphase (Growth)
– 1 hour of Mitosis (Division)
The Cell Cycle
Interphase
• 21 hours long
• Three Phases
– G1, S, & G2
• Cells grow in size
• Carries on metabolism ATP synthesis,
excretion of waste, new organelle
synthesis, new proteins
• Chromosomes are duplicated
G1 (5-6 hrs long)
(a.k.a.-Gap 1)
• Rapid growth and metabolic activity
• Multitudes of proteins being synthesized
• Think “Magic ‘E’ word”
S (10-12 hours long)
(a.k.a.-Synthesis)
• More growth
• DNA synthesis/replication
G2 (4-6 hrs long)
(a.k.a.-Gap 2)
• Growth
• Centrosomes replicate
– In animal cells, each
centrosome has 2 centrioles
• Organelle manufacturing
• Final preparations for cell
division
G0: Molecular Control of Cell
Cycle
• Different cells = different cell cycles
• Cytoplasmic signals seem to be most valid
theory of cell division (think ECM; extra
cellular matrix)
– Cell cycle control system
• Checkpoints dictate “stop and go”
– Can be internal and/or external signals
• G1 checkpoint seems to be most important
– Go = G1, S, G2 and Mitosis
– Stop = G0 (Most cells in the human body)
Cyclins and Cyclin-Dependent
Kinases
• Protein kinases (phosphotransferase)
– Enzymes that activate or inactivate other
proteins
• “Go” signal at G1 and G2 checkpoints
• Cyclins (think major milestones)
– Proteins that attach to kinases
– Concentration fluctuates during cell cycle
• Cyclin-dependent kinases (Cdks)
– Example of this relationship shown in
Metaphase to Anaphase conversion
…continued
• Density-dependent inhibition
•
•
– Release of growth factors or growth inhibiting factors
• Surface proteins seem to be the receptors and
messengers of this phenomena
Anchorage dependence
– Substratum is essential for cell division
Cancer cells do not respond to these mechanisms
– HeLa cells (Henrietta Lacks, 1951)
– 20-50 cell divisions is the norm before autophagy
– Transformation forms cancer cells
• Benign: Does not migrate
• Malignant: affects multiple tissues/organs
– metastasis
Mitosis: Cell Division
• Approximately 1 hour long
• Has 5 phases
– Prophase
– Prometaphase
– Metaphase
– Anaphase
– Telophase
Prophase
• Chromatin thickens into chromosomes
(supercoiling)
– Sister Chromatids visible
• Held together by a centromere
• Mitotic spindle begins to form (other
microtubles dissociate to help form spindle)
Prophase (continued)
• Nucleoli disappear
• Centrioles migrate to opposite ends of the cell
– Spindle begins to form (cage-like structure, “aster”)
Prometaphase
• Nuclear envelope fragements
• Microtubles (spindle makes its way
towards nuclear region)
• Kinetochore present on each sister
chromatid
• Microtubels attach to kinetochore and
begin to loosen sister chromatids
Metaphase
• Longest phase of mitosis (20 minutes)
• Centrosomes at opposite poles
• Spindle arranges the chromosomes at
the equatorial plane of cell
• Chromosomes are fully attached to
micortubles at kinetochore
Anaphase
• Shortest stage
• Sister chromatids separate via cohesin
separation
• Microtubles begin dissociating, moving
chromosomes to poles
• Cell lengthens as structural microtubles
lengthen
Telophase
• Chromatids reach opposite ends of the cell
• Nucleus and nucleolus form (nucleus via ER)
• Chromosomes lengthen into chromatin
•
•
again
Spindle breaks down
Plasma membrane begins to separate
(cytokinesis)
– Animal cells pinch (cleavage furrow)
– Plant cells form new cell wall
Telophase in plant and animal cells
Identical Offspring
• Asexual reproduction results in offspring
“identical” to their parents
• Types of asexual reproduction:
• Binary Fission
• Budding
• Spore Formation
• Regeneration
• Vegetative Reproduction
Binary Fission
• Parent cell increases in size and splits into two
•
•
•
•
equal but smaller parts
No parent cell is left
Occurs in single-celled organisms: amoeba,
protozoa and certain algae
Prokaryotes (bacteria and archea) divide via
binary fission without mitosis
One gene containing chromosome
– Origin of replication
– Cell elongates and separates (tubulin? Actin?)
Bacteria & Protozoa Binary
Fission
Evolutionary Mitosis
• Ancestral mechanisms remain intact
– Dinoflagellates: Replicated chromosomes
attach to nuclear envelope and separate with
nuclear division
– Diatoms and Yeasts: Spindle within nucleus
separates chromosomes