cell reproduction notes
TRANSCRIPT
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Cell Reproduction
Chapter 8
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Cell Reproduction
• As a cell prepares to divide, the DNA inside the nucleus becomes organized into chromosomes
• This is to ensure that both of the new cells get all of the genetic information from the original cell
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Chromosome Structure
• Chromosomes are rod shaped structures made of DNA and proteins found in the nucleus of cells
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Chromosome Structure
• Chromosomes consist of 2 identical halves called chromatids
– When a cell divides, each of the two new cells will receive one chromatid
• Two chromatids are attached at the centromere
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Chromosome Structure
• Between cell divisions, DNA is not so tightly coiled
• This form is called chromatin
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Chromosome Numbers
• Each species has a characteristic number of chromosomes in each cell
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Chromosome Numbers
• Animal chromosomes are categorized as either sex chromosomes or autosomes
– Sex chromosomes determine the sex of an organism
• In humans, sex chromosomes are X or Y (females = XX, males = XY)
– All of the other chromosomes are autosomes
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Chromosome Numbers
• Every cell of an organism produced by sexual reproduction has two copies of each autosome (one from each parent)
• The two copies are called homologous chromosomes
– Same size and shape and carry genes for the same traits
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Chromosome Numbers
• Karyotype – picture of the chromosomes in a normal dividing cell found in a human
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Diploid and Haploid Cells
• Diploid – cells that have 2 sets of chromosomes (46 total in humans)
• Haploid – cells that only have 1 set of chromosomes (23 total in humans)
• Only sperm and egg cells are haploid
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CELL DIVISIONSection 2
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Cell Division in Prokaryotes
• Binary fission is the division of a prokaryotic cell into two offspring cells
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Cell Division in Eukaryotes
• Two types of cell division
– Mitosis results in new cells that are identical to the original cell
– Meiosis occurs during the formation of gametes(haploid reproductive cells)
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The Cell Cycle
• A repeating set of events in the life of a cell
• Division is one phase of the cycle
• Time between cell divisions is called interphase
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Interphase
1. G1 phase – offspring cells grow to mature size
2. S phase – the cell’s DNA is copied
3. G2 phase – cell prepares for cell division
• Cells can exit the cycle and enter into the G0
phase to stop dividing (ex: fully developed brain cells)
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Stages of Mitosis
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Prophase
• DNA condenses into chromosomes
• Nuclear membrane breaks down
• Centrosomes form and microtubules grow from them (called the mitotic spindle)
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Metaphase
• Chromosomes line up in the middle of the cell
• Spindle fibers attach to the centromere of each chromosome
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Anaphase
• Chromatids are pulled apart and move to opposite ends of the cell
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Telophase
• Chromosomes reach opposite ends of the cell
• Spindle fibers disassemble
• Chromosomes unravel
• Nuclear membranes reform
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Cytokinesis
• Animal cells –cytoplasm divides in two
• Plant cells – cell plate forms between new cells
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Control of Cell Division
• A cell spends most of its time in interphase
• What tells the cells to exit interphase and begin dividing?
• There are three main checkpoints that act as “traffic signals” for the cell to divide or not to divide
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Control of Cell Division
1. Cell growth (G1) checkpoint – controls whether the cell will divide
2. DNA synthesis (G2) checkpoint – make sure DNA was copied properly
3. Mitosis checkpoint –signals tell the cell to exit mitosis
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When Control Is Lost: Cancer
• If a mutation occurs in one of the genes that regulates the cell cycle, cell growth and division could be disrupted
• This disruption could lead to cancer – the uncontrolled growth of cells
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MEIOSISSection 3
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Meiosis
• A process of nuclear division that reduces the number of chromosomes in new cells to half the number in the original cell
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Formation of Haploid Cells
• Meiosis produces gametes, which are haploid reproductive cells
• Cells preparing to divide by meiosis undergo the G1, S, and G2 phases of interphase
– Cells begin meiosis with a duplicate set of chromosomes, just like mitosis
• Cells undergoing meiosis divide twice, resulting in 4 haploid (1n) cells
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Two Stages of Meiosis
• First cell division = Meiosis I
– Prophase I, Metaphase I, Anaphase I, Telophase I and Cytokinesis I
• Second cell division = Meiosis II
– Prophase II, Metaphase II, Anaphase II, Telophase II and Cytokinesis II
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Meiosis I
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Meiosis II
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Prophase I
• DNA coils tightly into chromosomes
• Spindle fibers appear
• Nuclear membrane disassembles
• Synapsis occurs - homologous chromosomes line up next to each other
– Each pair is called a tetrad
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Prophase I
• Crossing-over occurs – portions of chromatids may break off and attach to adjacent chromatids
• Genetic recombination results – genetic material between maternal and paternal chromosomes is exchanged
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Prophase I
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Metaphase I
• Tetrads line up along the middle of the cell
• Spindle fibers attach to the centromere of each homologous chromosome
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Anaphase I
• Each homologous chromosome moves to an opposite end of the cell
• Random separation of homologous chromosomes is called independent assortment and results in genetic variation
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Telophase I and Cytokinesis I
• Chromosomes reach opposite ends of the cell and cytokinesis begins
• Two new cells are produced, each containing one chromosome from each homologous pair (haploid – 1n)
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Prophase II
• Spindle fibers form and begin to move the chromosomes toward the midline of the dividing cell
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Metaphase II
• Chromosomes move to the midline of the dividing cell
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Anaphase II
• Chromatids separate and move toward opposite ends of the cell
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Telophase II and Cytokinesis II
• Telophase II – nuclear membrane forms around the chromosomes in each of the four new cells
• Cytokinesis II – 4 new cells are formed, each with half of the original cell’s number of chromosomes
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Development of Gametes
• In animals, the only cells that divide by meiosis are those that produce gametes within the reproductive organs
– In humans – testes (males) and ovaries (females)
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Development of Gametes
SPERMATOGENESIS
• In the testes – male gametes known as sperm cells or spermatozoa are produced
• One diploid cell divides meiotically to form four haploid cells called spermatids – each develops into a mature sperm cells
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Development of Gametes
OOGENESIS
• Production of mature egg cells, or ova
• A diploid reproductive cell divides meiotically to produce one mature egg cell (ovum) and three polar bodies which will degenerate – cytoplasm is not evenly distributed in cytokinesis
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Sexual Reproduction
• Production of offspring through meiosis and the union of a sperm and an egg
• Offspring are genetically different because genes are combined in new ways
• Advantage: enables species to adapt rapidly to new conditions