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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
CANCER
What is cancer?
 group of more than 100 diseases.
 develops over TIME.
 causes: genetic and/or environmental factors
 cancer is uncontrolled CELL DIVISION.
How does cancer
start?
 cell breaks away from normal cell division and follows
its own agenda.
Normal situation
 Chromosomes are made up of GENES.
 Genes:
 made of DNA.
 instructions for cell and organism functions.
 code for proteins that cell makes.
 switch cell functions on and off.
 control cell cycle: G1, S, G2, M, C.
Mutation
2 types of genes
that control cell
cycle
 damage in the DNA
 caused by:
-sunlight
-tobacco
-alcohol, drugs
 Proto once genes
 Tumor suppressor genes
Proto onco genes
 Normal job: gas pedal.
 Tells cells to divide.
 regulate cell growth.
 allows tissues and organs to stay certain size
and have a certain structure to meet
organism’s needs.
 Mutated: becomes an onco genes.
 uncontrolled cell growth and division.
 cancer.
Tumor suppressor
genes
 Normal job: brakes.
 inhibit cell division to control growth.
 Mutated: no brakes.
 uncontrolled cell growth and division.
 cancer.
Backup mechanisms
 DNA repair system:
 detects and repairs DNA errors.
 if errors not corrected they become
permanent.
 Apoptosis:
 cell suicide mechanism.
 if essential cell component is damaged, cell
self-destructs.
 some tumor cells evade apoptosis and do not
respond well to conventional treatment.