chapter 9. why do cells divide? dna overload: the genetic material is only capable of...
TRANSCRIPT
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CHAPTER 9
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Why do cells divide?
DNA Overload: the genetic material is only capable of “managing” a limited amount of cellular activity
Materials Exchange: As a cell get bigger, the volume increases faster than the surface area, making it difficult to get material in and wastes out at a pace fast enough to allow the cell to survive.
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Surface Area to Volume Ratio
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How does cell division help cells?
• Keeps surface area to volume ratio high.
• Before cell becomes too large it splits, maintaining nuclear control and efficient transport of materials
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Chromosomes
What are they?• Segments of
condensed DNA.
• DNA is coiled around proteins for organization.
• Only visible during division.
Structure:
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The Cell Cycle – Events a cell goes through between one
division and the next
– During this time a cell:1. Grows & Develops2. Prepares for Division3. Divides to form two daughter cells
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Phases of the Cell Cycle (visual)
G1
S
G2
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Phases of the Cell Cycle (words)
Stage (in order)
Description
G1 Initial growthIncrease in size & make new organelles etc.
S DNA replication
G2 Prepare for division
M Nuclear (mitosis) & Cell division (cytokinesis)
*G0 Resting stage following G1“Waits” here until it is ready to divide againSome cells (nerve cells) cannot come out of G0
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Interphase
• Non-dividing stage• Cell engaged in
metabolic activity & preparing for mitosis
• DNA is not condensed (chromatin)
• Nucleolus may be visible
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Prophasea. Nuclear membrane
breaks down
a. Chromosomes condense and become visible
a. Centrioles move to opposite ends and begin to form spindle
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Metaphase
a. Chromosomes line up in the middle
b. Microtubules connect the centromere of each chromosome to the poles of the spindle
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Anaphasea. Chromatids split at
centromere (each is now an individual chromosome)
a. The new, single-stranded, chromosomes begin to move to opposite ends
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Telophase
a. Chromosomes reach opposite ends and begin to de-coil.
a. 2 new nuclear membranes begin to form.
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Cytokinesisa. Division of the cell
b. Often occurs during or directly after telophase
c. Mitosis can occur independently of cytokinesis, this results in a multinucleated cell, such as muscle cells.
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Cytokinesis: Plants v. Animals
1. Plants: Cell plate forms and gives rise to a cell wall. No centrioles involved.
2. Animals: membrane pinches in and cell splits.
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Controls on Cell Division Some cells stop dividing when mature (stay in G0)
ex) many neurons (nerve) & muscle cells
Others divide for growth & repair
Normally, cells will stop dividing when they come in contact with other cells
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Cell Cycle Regulators:
• Proteins called cyclins start/stop the cell cycle.
• The cycle is regulated by various factors:
1. Intracellular factors: - All chromosomes are
replicated- All spindles are attached
2. Extra-cellular factors: - Growth factors- Cell to cell contact
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Uncontrolled Cell Growth: CANCER
• Tumor Suppressor Genes: (p53) - genes that normally halt the cell
cycle - stuck OFF (cells keep dividing)
• Oncogenes: - genes that tell cells to divide - “stuck ON” (cells keep dividing)
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Stem Cells & Development• Cells take on specialized roles through the
process of differentiation
• Differentiated cells develop from undifferentiated cells called stem cells
• Fully undifferentiated stem cells are totitpotent– (ex. Fetilized egg through first few divisions)
• Mostly undifferentiated are pluripotent – (ex. Early embryonic cells)
• Partially differentiated are multipotent – (ex. Adult marrow cells)
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• When/Why do organisms rely on mitosis/cytokinesis?
• How do the daughter cells formed as a result of mitosis/cytokinesis compare to the original parent cell?
• Can you think of a time when new cells need to be formed that do not compare to the parent cell in this way?
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Gamete ProductionGamete Production• Sexual reproduction
begins with the fusion of egg and sperm to produce a zygote.
• Each organism must inherit a single copy of each gene from each parent.
• Therefore, when an organism produces its gametes (egg or sperm) their gene pairs must be separated.
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Chromosome NumberChromosome Number
1. The fruit fly (drosophila) has 88 chromosomes
2.2. 4 4 come from the mommom and 44 from the daddad
2. These two sets of chromosomes are called homologous chromosomes.
2. A cell with both sets is called diploid (2n).
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5. Gametes (egg/sperm) only have one set of chromosomes and are referred to as haploid (1N)
6. The process of forming haploid (N) cells, {gametesgametes}, from diploid (2N) cells {body/somatic cellsbody/somatic cells} is called meiosismeiosis.
5. Meiosis involves two distinct stages meiosis Imeiosis I and meiosis IImeiosis II
Chromosome Number Chromosome Number (con’t)(con’t)
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Crossing OverCrossing Over• Genetic material is
exchanged between homologous chromosomes during Prophase 1
• This is referred to as genetic recombination.
• During “synapsis” the crossed over homologous chromosomes are referred to as a tetrad.
What is the benefit of crossing over? HINT: Think from an evolutionary perspective.
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Meiosis I v. Meiosis II
• During Meiosis I, homologus pairs are separated.
• During Meiosis II, sister chromatids of double-stranded chromosomes are separated to form single-stranded chromosomes.
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Gamete FormationGamete Formation
MalesMales“spermatogenesis”
• All 4 haploid cells produced become sperm
FemalesFemales“oogenesis”
• Only one of the cells becomes an egg
• Egg is larger, needs the cytoplasm
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Male v. Female Gamete Male v. Female Gamete FormationFormation
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Mitosis v. MeiosisMitosis v. Meiosis• MitosisMitosis:
– 2 genetically identical diploid cells– produces somatic (regular body) cells– Method of asexual reproduction
• MeiosisMeiosis:– 4 genetically different haploid cells– Reduction division– Produces gametes (reproductive cells)
for sexual reproduction