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Cell Growth and Division
Cell division is needed to…
• Grow – most organisms grow by producing more cells
• Repair wounds and damaged cells
• Develop and change
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
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)
Cells divide at different rates
• The rate of cell division varies with the need for the type of cell…
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
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
Stage 2: Mitosis• Division of the cell nucleus and DNA
• Phases:1. Prophase2. Metaphase3. Anaphase4. Telophase
Stage 3: Cytokinesis
• Division and splitting of the cytoplasm and cells
Mitosis + cytokinesis result in two identical daughter cells!
YouTube:Mitosis
(Review…) Interphase• Nucleus is still intact and the nucleolus is visible.• DNA is in chromatin form• Includes G1, S, and G2
Mitosis step 1: Prophase• Centrioles separate and produce spindle fibers • Chromatin condenses to form chromosomes• Nuclear envelope and nucleolus break down
prophase → pro (#1!)
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?
To recap…During prophase, chromatin condense into chromosomes… which have duplicated into sister chromatids, attached to each other at their centromeres.
Mitosis step 1: Prophase• Centrioles separate and produce spindle fibers • Chromatin condenses to form chromosomes• Nuclear envelope and nucleolus break down
prophase → pro (#1!)
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
Mitosis step 3: Anaphase• Spindle fibers contract and pull apart the
chromosomes to opposite ends of the cell
anaphase → apart
Mitosis step 4: Telophase• Chromosomes return to chromatin form• Nuclear membranes and nucleolus reform• Spindle fibers disappear• Nuclear division is complete
telophase → telephone
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”
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
Mitosis in real cells…PMAT
Where doesn’t mitosis happen?• Nerve and blood cells are different• Sex cells (gametes) undergo a different
division process