cell growth and division. cell division is needed to… grow – most organisms grow by producing...
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Cell Growth and Division
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Cell division is needed to…
• Grow – most organisms grow by producing more cells
• Repair wounds and damaged cells
• Develop and change
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Why do cells divide?(aka, why are cells small?)
• Two main reasons why cells can’t grow indefinitely:– A larger cell places more demands on its DNA (more
things to control and not enough DNA)– Surface-to-volume ratio decreases as size increases
• Thus, before the cell becomes too large, it divides
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When do cells divide (or stop)?• When cells crowd each other, they stop dividing• When internal factors signal the cell to start or stop
dividing – e.g., p53 gene makes sure the cell doesn’t divide until
chromosomes have doubled; cyclin enzymes start/stop the cell cycle
• When external chemical or physical signals (growth regulators) stimulate or inhibit growth and division
(Cancer is an example of uncontrolled cell growth….we will discuss cancer later in the unit)
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Cells divide at different rates
• The rate of cell division varies with the need for the type of cell…
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What is the cell cycle?• The series of phases that cells go through as they
grow and divide.• A continuous process, but we divide it into 3 stages:
1. Interphase2. Mitosis3. Cytokinesis
http://images1.clinicaltools.com/images/gene/celldivision/cellcycle.jpg
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Stage 1: Interphase• Cell life between divisions• The longest stage!• Nucleus is still present• Phases:
– Gap 0 (G0) – cells like nerves leave the cell cycle
– Gap 1 (G1) – growth– Synthesis (S) – duplicates
DNA (replication)– Gap 2 (G2 ) – doubles
organelles, prepares to divide
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Stage 2: Mitosis• Division of the cell nucleus and DNA
• Phases:1. Prophase2. Metaphase3. Anaphase4. Telophase
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Stage 3: Cytokinesis
• Division and splitting of the cytoplasm and cells
Mitosis + cytokinesis result in two identical daughter cells!
YouTube:Mitosis
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(Review…) Interphase• Nucleus is still intact and the nucleolus is visible.• DNA is in chromatin form• Includes G1, S, and G2
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Mitosis step 1: Prophase• Centrioles separate and produce spindle fibers • Chromatin condenses to form chromosomes• Nuclear envelope and nucleolus break down
prophase → pro (#1!)
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Chromatin = DNA unwound (looks like spaghetti)
Chromosome = DNA condensed and organized; looks like this: 2 chromosomes
Sister chromatids = a duplicated chromosome
Centromere = the place where chromotids are connected
Some useful DNA vocabulary!
Chromatin? Chromosomes?
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To recap…During prophase, chromatin condense into chromosomes… which have duplicated into sister chromatids, attached to each other at their centromeres.
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Mitosis step 1: Prophase• Centrioles separate and produce spindle fibers • Chromatin condenses to form chromosomes• Nuclear envelope and nucleolus break down
prophase → pro (#1!)
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Mitosis step 2: Metaphase• Chromosomes line up at the middle of the cell• A spindle fiber attaches to each sister chromatid at
the centromere
metaphase → middle
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Mitosis step 3: Anaphase• Spindle fibers contract and pull apart the
chromosomes to opposite ends of the cell
anaphase → apart
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Mitosis step 4: Telophase• Chromosomes return to chromatin form• Nuclear membranes and nucleolus reform• Spindle fibers disappear• Nuclear division is complete
telophase → telephone
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After mitosis…Stage 3: Cytokinesis• Pinching of the cytoplasm, resulting in two
identical daughter cells.
YouTube:Mitosis Video
Mitosis Review
The cell cycle:
IPMATCI
Passed
My
Algebra
Test with a
“C”
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Mitosis in plants• Plants don’t have centrioles• Cell plate forms during telophase• During cytokinesis, cell plate separates the daughter
cells and becomes the new cell wall
Blood lily mitosis
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Mitosis in real cells…PMAT
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Where doesn’t mitosis happen?• Nerve and blood cells are different• Sex cells (gametes) undergo a different
division process