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Cell Division: Mitosis
Chapter 8
Prokaryotic Cell Division
• Binary fission: dividing in half
• Steps
– Chromosomes duplicate and move to
ends of cell
– Cell elongates
– Plasma membrane pinches off into 2
‘daughter cells’
Chromosomes
• 1 chromosome is 1 DNA
molecule w/ 100’s to 1000’s of
genes
• Composed of chromatin
– DNA and protein
• Number varies within species
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46 in humans
78 in dogs
38 in cats
254 in shrimp
1200 Adders-tongue (fern)
Eukaryotic Cell Division
• Chromosomes replicated to form
2 sister chromatids
– Joined by centromere
• Division splits chromosome apart
– Each becomes a chromosome
– Count centromeres to determine
number
The Eukaryotic Cell Cycle (Overview)
• Interphase
– About 90% of time
– Chromosomes not visible yet
– 3 Subphases
• G1 phase = cell growth and maturation
• S phase = synthesis, chromosomes
replicated
• G2 phase = growth and prep for division
• G0 phase = not dividing or quienescent
• Mitotic (M Phase)
– Actual cell division
– Mitosis – nuclear division
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Prophase
Metaphase
Anaphase
Telophase
– Cytokinesis – cytoplasmic division
• Repeat
Mitotic Phase
Prophase Events
• Sister chromatids condense
• Nuclear envelope begins to disappear
• Centrosomes (centrioles in animal cells)
move to opposite ends of cell
• Mitotic spindles form
Metaphase Events
• Centrosomes at opposite ends of cells
• Sister chromatids line up with
centromere on metaphase plate
• Microtubules attached to each
chromatid at the centromere
Anaphase Events
• Sister chromatids separate
• Single chromosomes move toward opposite ends of
the cell
– Microtubule ‘tug of war’
• Cell elongates
Telophase Events
• Daughter nuclei form
• Nuclear envelope reforms
• Chromosomes begin to uncoil
• Mitosis is complete
Cytokinesis
• Division of cytoplasm
– Begins at the end of telophase (late
anaphase too)
• Animal cells
– Cleavage furrow forms
• Microfilaments interact to pinch plasma
membrane in 2
• Plant cells
– Cell plate forms
• Vesicles collect in the middle until fuse with
plasma membrane
Apoptosis
• Programmed cell death
• Necessary for normal development
– Maintains balance of cell growth
– Organism shape/form development
– Sunburn peeling
• Tightly regulated to maintain balance
Cell Division Control
• Cyclical control system regulates with ‘checkpoints’
– 3 major ones: G1, G2, and M
– Ensures previous processes complete before allowing continuation of
division
• Possible number of divisions varies
– Ends of chromosomes dictate
– Skin cells and those of GI tract divide regularly
– Liver cells rarely divide
• Need growth factors, proteins secreted by the body to stimulate
growth
– Cells respond to specific ones
• Density-dependent inhibition
– Cells stop dividing if become crowded
– Remove cells and dividing restarts
• Anchorage dependence
– Must have contact w/ a solid surface
Cancer
• Cancer is a disease of the cell cycle
– Only 1 cell needs to be transformed into a cancer cell
– Immune system normally detects then destroys
– Undetected can proliferate = tumor
• Benign stay in original location
– Can be removed by surgery so don’t disrupt organs
• Malignant can spread into other areas
– Metastasis: cancer cells spread via the circulatory system from original site
• Don’t follow normal signaling
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No density-dependent inhibition (overlap)
No check points
Make own growth factors to increase proliferation
Unlimited number of divisions
• Often named for site of origin
– 4 categories
Cancerous Cell Growth
Categories of Cancer
• Carcinomas
– External or internal coverings of the body
– E.g. skin or intestinal linings
• Leukemias and lymphomas
– Blood forming tissues
– E.g. bone marrow or lymph bodies
• Sarcomas
– Tissues that support the body
– Eg. Bones or muscle
Cancer Treatments
• Radiation for localized cells
– Damages DNA of cancer cells more than normal
ones
– Lost ability to repair
• Chemotherapy for metastatic tumors
– Drugs interfere with cell division (normal and
abnormal) and cause side effects
• Nausea from intestinal cells
• Hair loss from hair follicle cells
• Infections from immune system cells