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Cell Reproduction 8.1 – Chromosomes 8.2 – Cell Division 8.3 - Meiosis
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What factors limit cell size? Diffusion DNA Content Surface area-to-volume ratio
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DIFFUSION Certain materials needed for the survival of the cell can only enter through diffusion. Ex. waste, carbon dioxide, oxygen Diffusion is only efficient over a short distance. A 20 cm cell would have to wait several months to receive the above listed molecules
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DNA CONTENT Most cells have only 1 nucleus The DNA in the nucleus makes RNA and releases it to the cytoplasm where it directs the production of enzymes Lack of enzymes = Lack of cellular metabolism Some large cells have compensated by having multiple nuclei.
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Surface area-to-volume ratio If you double the size of a cell… -It has 8x as much volume -Gives rise to 8x as much waste and need for nutrients -It becomes impossible for diffusion to keep up with the cellular demands
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Chromosomes Rod-shaped structures made of DNA and protein Coiled chromatin =chromosomes Histones = proteins that help maintain the shape of the chromosome
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Types of Chromosomes Sex Chromosomes X or Y chromosomes Determine sex of individual Autosomes Non-sex chromosomes Determine all other characteristics or traits Ex. Hair color, eye color, hemophilia
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Chromosome Number Diploid Having 2 sets of chromosomes (2n) Creates genetic diversity Hapliod Contain one set of chromosomes (n) Sex cells
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Two major stages of the cell cycle Interphase – the growth period of time where cell activities are carried out -most of the cells life Mitosis – process of nuclear division, followed by the division of the cytoplasm
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INTERPHASE What is going on during this time? G 1 Phase Metabolism – making ATP Cell growth, maintenance, repair S Phase Chromosomes are replicated (ready for mitosis) Make new proteins Synthesizing new organelles Conducting photosynthesis (autotrophs) Storing excess glucose (starch, glycogen) G 2 Phase Final preparation before cell division (mitosis)
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Stages of Mitosis Prophase Metaphase Anaphase Telophase
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PROPHASE First stage of mitosis Longest phase of mitosis Chromatin the nucleus becomes visible chromosomes Nucleus disappears The chromatin, which was copied during interphase forms 2 complete sets of chromosomes called sister chromatids Spindle fibers form between pairs of centrioles
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METAPHASE Second stage of mitosis Chromosomes move to the equator of the cell Spindle fibers attach to each chromatid by its centromere
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ANAPHASE Third stage of mitosis Chromosomes separate at the centromere Each sister chromatid begins to move to opposite ends of the cell
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TELOPHASE Fourth stage of mitosis Two daughter cells are formed Each new cell has a complete set of chromosomes The cytoplasm then divides (cytokinesis) Nucleus reappears Chromosomes uncoil to form chromatin Interphase begins
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Control of the Cell Cycle Cellular enzymes and genes play a key role in checking and balancing cell division Tells cells when to divide/stop Proto-oncogene – regulates cell growth, division and the ability for cells to adhere to one another Mutation of the proto-oncogene causes an oncogene ONCOGENE = UNCONTROLLED CELL DIVISION
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Control of the Cell Cycle (2) Contact inhibition – cell to cell communication between neighboring cells…stops the cellular division Eliminates an over-production of certain types of cells within the body.
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CANCER – a loss of control Cancer – an uncontrolled dividing of cells Caused by a change in one or more of the genes synthesizing enzymes to control the cell cycle (genetic factor) The cancer genes are often expressed when environmental conditions change (environmental factor)
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How can cells regain control Some genes act as brakes to suppress cancer expression Tumor-suppressor genes – code for proteins that prevent cell division from occurring too often To get cancer, these tumor suppressor genes (all 3 of them) must be damaged
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Carcinogen Any substance that can induce or promote cancer Most carcinogens are mutagens Cause mutation within the cell Known carcinogens include: Chemicals in tobacco smoke Radiation UV light Certain viruses
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MEIOSIS
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INTERPHASE
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INTERPHASE MAIN EVENTS Chromatin replicates Just like in mitosis Forms 2 identical chromatids Centriole pairs replicate (animal cells only)
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PROPHASE I
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PROPHASE I MAIN EVENTS Chromatin condenses into chromosomes Tetrad forms by synapsis Crossing over occurs between homologous chromosomes Centriole pairs move apart Nuclear envelope and nucleoli disappear
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METAPHASE I
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METAPHASE I MAIN EVENTS Tetrads align along the metaphase plate Centromeres of homologous chromosomes point toward opposite poles Attach to spindle fibers
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ANAPHASE I
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ANAPHASE I MAIN EVENTS Homologous chromosomes separate Sister chromatids move as a unit remain attached at the centromere
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TELOPHASE I AND CYTOKINESIS
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TELOPHASE I MAIN EVENTS Chromosomes reach poles Still as sister chromatids Cytokinesis occurs simultaneously Interkinesis No DNA replication before meiosis II
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MEIOSIS II MAIN EVENTS Very similar to mitosis PII – spindles form MII – chromosomes align AII – centromeres split TII – nuclei reform Cytokinesis – 4 haploid cells
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PROPHASE II
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METAPHASE II
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ANAPHASE II
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TELOPHASE II AND CYTOKINESIS
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KEY DIFFERENCES BETWEEN MEIOSIS AND MITOSIS… Meiosis is reduction division 2n to n Meiosis creates genetic variation Meiosis is 2 successive nuclear divisions Meiosis I separates pairs of chromosomes; centromeres do not divide
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GAMETE PRODUCTION THROUGH MIEOSIS Spermatogenesis Creates 4 sperm cells Oogenesis Creates 1 ootid (egg cell) 3 polar bodies (nonfunctional)
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ASEXUAL REPRODUCTION Production of offspring from one parent without the union of gametes Occurs only by mitosis
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SEXUAL REPRODUCTION Production of offspring through meiosis and the union of gametes Offspring genetically different due to genetic recombination Parts of chromatids can be exchanged (crossing over) Homologous pairs separate
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