3.a.2 – in eukaryotes, heritable information is passed to the next generation via processes that...
TRANSCRIPT
Chapter 12 The Cell Cycle
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).
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
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
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
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
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
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
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
Interphase - parts
In order: G1 - first gap S - synthesis G2 - second gap
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.
Mitotic Phase - parts
1. Mitosis - division of replicated chromosomes
2. Cytokinesis - division of the cell’s cytoplasm
Mitosis - Purpose
To divide the 2 copies of the DNA equally
To separate the sister chromatids into separate cells
Mitosis Steps
1. Prophase2. Prometaphase3. Metaphase4. Anaphase5. Telophase
Mitosis Video #
1Mitosis Video #
2
Prophase
Nucleoli disappearChromatin condenses into
chromosomes Centrioles separate to
opposite ends of the cellMitotic spindle begins to form
Prometaphase
Prometaphase
Nuclear envelope dissolvesSpindle fibers join with the kinetochore of the centromeres
Metaphase
Centrioles now at opposite ends of the cell
Chromosomes line up on the metaphase plate
Spindle apparatus fully developed
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
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
Telophase
Telophase
Chromosomes uncoil back to chromatin
Nuclear envelope reformsNucleoli reappearSpindle fibers disappearCytokinesis usually starts
Cytokinesis
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
Animal Cell - Mitosis
Plant Cell - Mitosis
Evolution of Mitosis
Evolution, cont.
Hypothesis: Mitosis has origins in prokaryotic cells How do we know this?▪ Proteins involved are identical▪ Ex: kinetochores, protein kinase
checkpoints, etc.
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
Checkpoints
A critical control point in the cell cycle
Several are knownCells must receive a “go-ahead”
signal before proceeding to the next phase
G1 Checkpoint
Also called the “restriction point” in mammalian cells
Places cells in a non-dividing phase called the Go phase
GO
Go Phase
Non-dividing stateMost cells are in this stateSome cells can be reactivated
back into M phase from the Go
phase
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
GO
Protein Kinase
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
GO
Protein KinaseMPF
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
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
Growth Factors
External signals that affect mitosis
Examples: PDGF Density-dependent inhibition Anchorage dependence
PDGF
Platelet-Derived Growth Factor
Stimulates cell division to heal injuries
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
Anchorage Dependence
Inhibition of cell division unless the cell is attached to a substratum
Prevents cells from dividing and floating off in the body
Cancer Cells
Do not stop dividing. The control mechanisms for
cell division have failed
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
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.