3.a.2 – in eukaryotes, heritable information is passed to the next generation via processes that...
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Chapter 12 The Cell Cycle
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Essential Knowledge
3.a.2 – In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis, or meiosis plus fertilization (12.1-12.3).
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Roles of Cell Division
1. Reproduction (uni vs. multi)2. Growth3. Repair/replace old or damaged cells4. To distribute identical genetic material
to two new daughter cells
Goal: To split the sister chromatids and give one to each new cell – to make two new cells with the correct amount of genetic info
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Two cell types
1. Somatic Non-sex cells (all body cells
EXCEPT sex) Ex: red blood, skin, muscle
2. Sex Otherwise known as gametes Ex: sperm, egg
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Genome
The cell's hereditary endowment of DNA
Usually packaged into chromosomes (easier to manage during the cell cycle)
Prokaryotes: Single stranded, in cytoplasm, circular in shape
Eukaryotes: Double stranded, in nucleus, helix/spiral in shape
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Chromosomes
Made of a DNA and protein complex called chromatin
During cell division, the chromatin becomes highly condensed into the chromosomes
# of chromosomes: Sex cell (haploid cell)▪ 23 (no pairs)—only contains one set of
chromosomes Somatic cell (diploid cell)▪ 46 (in 23 pairs)—contain two sets of chromosomes
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Chromosomes vs. Chromatin
Chromatin Long, thin fiber Uncondensed Before/after cell division Genetic material usually in this form
Chromosome Characteristic “X” shape Condensed into this shape Only during cell division
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Chromosomes - Structure
At cell division, each chromosome has been duplicated
The duplicated chromosome consists of two sister chromatids
Centromere – the point where two sister chromatids are connected
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Cell Cycle – 2 parts
1. Interphase - (90% of cycle) - when the cell grows and duplicates the chromosomes
2. Mitotic Phase (M) - when the chromosomes are split into separate cells
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Interphase - parts
In order: G1 - first gap S - synthesis G2 - second gap
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G1, S, G2
G1: Cell grows and carries out regular
biochemical functionsS:
DNA is replicated or synthesized - chromosomes are replicated
G2: Cell completes preparations
Comment: A cell can complete S, but fail to enter G2.
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Mitotic Phase - parts
1. Mitosis - division of replicated chromosomes
2. Cytokinesis - division of the cell’s cytoplasm
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Mitosis - Purpose
To divide the 2 copies of the DNA equally
To separate the sister chromatids into separate cells
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Mitosis Steps
1. Prophase2. Prometaphase3. Metaphase4. Anaphase5. Telophase
Mitosis Video #
1Mitosis Video #
2
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Prophase
Nucleoli disappearChromatin condenses into
chromosomes Centrioles separate to
opposite ends of the cellMitotic spindle begins to form
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Prometaphase
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Prometaphase
Nuclear envelope dissolvesSpindle fibers join with the kinetochore of the centromeres
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Metaphase
Centrioles now at opposite ends of the cell
Chromosomes line up on the metaphase plate
Spindle apparatus fully developed
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Anaphase
Centromeres break and the duplicate chromosomes are pulled away from each other toward opposite ends of the cell
Cell elongates; poles move slightly further apart
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Kinetochores
Specialized regions of the centromeres where spindle microtubules attach
Appear to “ratchet” the chromosome down the spindle fiber microtubule using a motor protein
Microtubules dissolve behind the kinetochore
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Telophase
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Telophase
Chromosomes uncoil back to chromatin
Nuclear envelope reformsNucleoli reappearSpindle fibers disappearCytokinesis usually starts
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Cytokinesis
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Cytokinesis – Animal vs. Plant
Animal Cleavage furrow forms Microfilaments contracts and divides the
cytoplasm into two partsPlant
Cell plate develops in between the two new daughter cells (from Golgi vesicles)
New cell wall developed around the cell plate
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Animal Cell - Mitosis
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Plant Cell - Mitosis
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Evolution of Mitosis
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Evolution, cont.
Hypothesis: Mitosis has origins in prokaryotic cells How do we know this?▪ Proteins involved are identical▪ Ex: kinetochores, protein kinase
checkpoints, etc.
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Regulation of Cell Division
Must be controlledRate of cell division depends on
the cell type Ex - skin: frequently liver - as needed brain - rarely or never
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Checkpoints
A critical control point in the cell cycle
Several are knownCells must receive a “go-ahead”
signal before proceeding to the next phase
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G1 Checkpoint
Also called the “restriction point” in mammalian cells
Places cells in a non-dividing phase called the Go phase
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GO
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Go Phase
Non-dividing stateMost cells are in this stateSome cells can be reactivated
back into M phase from the Go
phase
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Protein Kinase Checkpoint - G2
Uses protein kinases to signal “go-ahead” for the G2 phase
Activated by a protein complex whose concentration changes over the cell cycle
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GO
Protein Kinase
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MPF
M-phase Promoting FactorProtein complex required for a
cell to progress from G2 to Mitosis
Role of MPF - to trigger a chain of protein kinase activations
Active MPF has: cdk and cyclin
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GO
Protein KinaseMPF
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CDK and Cyclin
CDK: Protein Kinase Amount remains constant during cycle Inactive unless bound with cyclin
Cyclin: Protein whose concentration builds up
over G1, S and G2 When enough cyclin is present, active
MPF is formed
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Active MPF
Triggers MitosisActivates a cyclin-degrading
enzyme, which lowers the amount of cyclin in the cell
Result - no active MPF to trigger another mitosis until the cycle is repeated
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Growth Factors
External signals that affect mitosis
Examples: PDGF Density-dependent inhibition Anchorage dependence
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PDGF
Platelet-Derived Growth Factor
Stimulates cell division to heal injuries
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Density-Dependent Inhibition
The number of cells in an area force competition for nutrients, space, and growth factors
When density is high - no cell division
When density is low - cells divide
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Anchorage Dependence
Inhibition of cell division unless the cell is attached to a substratum
Prevents cells from dividing and floating off in the body
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Cancer Cells
Do not stop dividing. The control mechanisms for
cell division have failed
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Cell division balance
Regulation of cell division is a balance between:
Mitosis - making new cells Apoptosis - cell suicideCancer can result if either
process doesn’t work
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Summary
Identify the roles of cell division Identify the composition of a chromosome. Recognize the phases of the cell cycle. Identify the stages and characteristics of
Interphase. Identify the stages and characteristics of
Mitosis. Recognize the mechanisms of Cytokinesis. Recognize factors that control cell division. Recognize the results when the regulation of
cell division goes wrong.