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MITOSIS AND CELL DIVISION
Why are cells
small?
If cell is too big?
Asexual
reproduction
 avoid DNA overload
 nucleus can efficiently control all cell processes.
 efficient material exchange with environment
 food, water, oxygen into cell
 wastes and products out of cell
 larger SA:Volume ratio
 maximum amount of cell membrane
 cell divides!
 single-celled organisms
 results in 2 genetically identical cells
 simple, efficient cell division
 multi-celled organisms
 small part of organism breaks off
 grows into genetically identical organism
Sexual Reproduction
Chromosomes
in Prokaryotes
Chromosomes in
Eukaryotes
 most animals and plants
 offspring have DNA from both parents
 provides genetic diversity
 single circular chromosome
 floats in cytoplasm
 multiple chromosomes
 made of chromatin = DNA coiled around
histones
 nucleosomes: coils of chromatin/histones
 coiled again and packed tightly in chromosomes.
 helps DNA separate precisely during cell
division
Cell Cycle
 events in life cycle of cell
 cell grows
 DNA replicates
 cell splits into 2 identical daughter cells.
Purpose
 new cells for organism growth and development.
 repair of damaged tissues.
Prokaryotes
 binary fission
Eukaryotes
 4 stages: Interphase = G1 S
G2
Mitosis = M
 Interphase
 G1 phase: cell growth
 S phase: DNA replicates
 G2 phase: preparation for cell division
 M phase: cell division
 mitosis: division of cell nucleus
 cytokinesis: division of cytoplasm
4 stages of Mitosis
Prophase
1.
2.
3.
4.
DNA coils, chromosomes become visible.
nucleolus disappears, nuclear membrane breaks down
centrioles migrate to opposite ends of cell.
chromosomes attach to spindles at centromeres
 sister chromatids: duplicate
chromosomes
 centromere: attachment point of
chromatids
 centrioles: animal cells only
Metaphase
Anaphase
Telophase
1. chromosomes line up in middle of cell.
2. spindle fibers connect chromosomes to centrioles
1. sister chromatids split, pulled toward centrioles.
1. nuclear envelope forms around each new group of
chromosomes.
2. chromosomes unwind into chromatin
3. spindles disappear and nucleolus reappears.
CYTOKINESIS
Happens during
telophase
Regulation of cell
cycle
 daughter cells split after mitosis
 animal cells: cell membrane pinches inward to
divide cytoplasm (cleavage furrow).
 plant cells: cell wall forms cell plate to divide
cytoplasm
 regulatory proteins called cyclins.
 internal regulators
 respond to events inside cell
 checkpoints during cell cycle (go or stop)
 eg. are chromosomes all replicated?
 eg. are spindle fibers attached?
 external regulators
 respond to events outside cell
 speed up or slow down cell cycle
 eg. growth factors speed up cell division for
wound healing.
Apoptosis
 cell death process
 response to cell injury
 programmed as part of development
How often do cells
divide?
 different types of cells divide at different rates.
 bacteria: 20 mins.
 yeast 2 hours
 skin: 24 hours
 liver: 1 year
 nerve: never, once mature
How long can cells
live?
 telomeres: repeated DNA sequences at both ends of
chromosomes.
 worn away every time cell divides.
 when too short, cell dies.
 telomerase: enzyme that repairs telomeres.
 cancer and stem cells have higher levels.
 some cells can become “immortal”
What is cancer?
 uncontrolled CELL DIVISION.
 group of more than 100 diseases.
 develops over TIME.
 causes: defects in genes that control division.
What is a tumor?
 mass of cancer cells.
 benign: does not spread.
 malignant: spreads to healthy tissues.
How does cancer
start?
 cell does not respond to signals to regulate growth.
 breaks away from normal cell cycle.
 divides uncontrollably.
Genes
 regions on chromosome that
 hold instructions for cell functions.
 have DNA code for proteins that cell makes.
 switch cell functions on and off.
 control cell cycle: G1, S, G2, M, C.
What is a
mutation?
Genes that control
cell division:
Proto once genes
 damage in the DNA causes defect in gene(s)
 caused by -radiation exposure
-tobacco products
-alcohol, drugs
- some viral infections
 Normal situation = gas pedal
 tell cells when to divide.
 regulate cell growth.
 regulates size of tissues and organs
 Mutated = onco genes = gas pedal stuck to floor
 uncontrolled cell growth and division
 cancer.
Tumor suppressor
genes
Backup mechanisms
DNA repair genes
Apoptosis
 Normal situation = brakes
 inhibit cell division to control growth.
 Mutated = no brakes
 uncontrolled cell growth and division.
 cancer.
 code for proteins that detect and repair DNA errors
 active in G2 phase after DNA copied.
 MANY DNA repairs/cell/day
 errors not corrected become permanent.
 cell suicide mechanism
 some tumor cells evade apoptosis and do not
respond well to conventional treatment.