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Cell reproduction notes
 

Cell reproduction notes

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    Cell reproduction notes Cell reproduction notes Presentation Transcript

    • Cell Reproduction Chapter 8
    • 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
    • Chromosome Structure• Chromosomes are rod shaped structures made of DNA and proteins found in the nucleus of cells
    • 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
    • Chromosome Structure• Between cell divisions, DNA is not so tightly coiled• This form is called chromatin
    • Chromosome Numbers• Each species has a characteristic number of chromosomes in each cell
    • 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
    • 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
    • Chromosome Numbers• Karyotype – picture of the chromosomes in a normal dividing cell found in a human
    • Diploid and Haploid Cells• Diploid – cells that have • Haploid – cells that only 2 sets of chromosomes have 1 set of (46 total in humans) chromosomes (23 total in humans) • Only sperm and egg cells are haploid
    • Section 2CELL DIVISION
    • Cell Division in Prokaryotes• Binary fission is the division of a prokaryotic cell into two offspring cells
    • 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)
    • 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
    • Interphase1. G1 phase – offspring cells grow to mature size2. S phase – the cell’s DNA is copied3. 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)
    • Stages of Mitosis
    • Prophase • DNA condenses into chromosomes • Nuclear membrane breaks down • Centrosomes form and microtubules grow from them (called the mitotic spindle)
    • Metaphase • Chromosomes line up in the middle of the cell • Spindle fibers attach to the centromere of each chromosome
    • Anaphase • Chromatids are pulled apart and move to opposite ends of the cell
    • Telophase • Chromosomes reach opposite ends of the cell • Spindle fibers disassemble • Chromosomes unravel • Nuclear membranes reform
    • Cytokinesis• Animal cells – • Plant cells – cell plate cytoplasm divides in forms between new two cells
    • 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
    • Control of Cell Division1. Cell growth (G1) checkpoint – controls whether the cell will divide2. DNA synthesis (G2) checkpoint – make sure DNA was copied properly3. Mitosis checkpoint – signals tell the cell to exit mitosis
    • 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
    • Section 3MEIOSIS
    • Meiosis• A process of nuclear division that reduces the number of chromosomes in new cells to half the number in the original cell
    • 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
    • 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
    • Meiosis I
    • Meiosis II
    • 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
    • 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
    • Prophase I
    • Metaphase I• Tetrads line up along the middle of the cell• Spindle fibers attach to the centromere of each homologous chromosome
    • 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
    • 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)
    • Prophase II• Spindle fibers form and begin to move the chromosomes toward the midline of the dividing cell
    • Metaphase II• Chromosomes move to the midline of the dividing cell
    • Anaphase II• Chromatids separate and move toward opposite ends of the cell
    • 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
    • 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)
    • Development of GametesSPERMATOGENESIS• 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
    • Development of GametesOOGENESIS• 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
    • 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