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Chapter 8: Cellular Reproduction 1. The Cell Cycle 2. Mitosis 3. Meiosis 2 Types of Cell Division 2n 1n Mitosis: • occurs in somatic cells (almost all cells of the body) • generates cells identical to original Meiosis: • occurs in gamete production (sperm & egg) • ½ the normal chromosome content (haploid or “1n”) Chromosome Content • somatic cells are diploid (2n) • 2 of each chromosome • i.e., 1 from each parent • gametes are haploid (1n) • 1 of each chromosome 23 pairs of human chromosomes 1 1. The Cell Cycle The Cell Division Cycle M G1 G2 S • all eukaryotic cells progress through the 4 phases of the cell cycle with each division Stages of the Cell Cycle G1: • preparation for DNA replication • non-dividing cells are arrested at this stage of the cell cycle (referred to as G0) S phase: • replication of genetic material (DNA Synthesis) G2: • preparation for cell division M phase: • cell division (Mitosis or Meiosis) ***G1, S & G2 collectively make up “Interphase”*** 2 2. Mitosis 4 Stages of Mitosis 1) Prophase 2) Metaphase 3) Anaphase 4) Telophase & Cytokinesis Interphase G1, S phase & G2 • all events in preparation for cell division • DNA replication • duplication of organelles • preparation of “mitotic apparatus” 3 Prophase 1) duplicated chromosomes condense, remain attached at the centromere 2) break down of nuclear envelope 3) centrosomes move to opposite poles 4) spindle fibers form Duplicated Chromosomes • in prophase, condensed duplicated chromosomes remain attached at the centromere • each “half” of duplicated chromosome referred to as sister chromatids Still considered as 1 chromosome! Metaphase • chromosomes align along the metaphase plate of the cell • mediated by spindle fibers connecting each centromere to a centrosome at ea “pole” of the cell 4 Anaphase • sister chromatids are pulled to opposite poles of the cell • separate at the centromere, each chromatid still attached to a spindle fiber • spindle fibers shorten, pulling each chromatid to opposite poles Telophase Opposite of prophase: • nuclear envelope reforms around chromosomes at each pole • spindle fibers break down • chromosomes decondense Cytokinesis begins… Cytokinesis Division of the cytoplasm completes the process: • the 2 daughter cells are genetically identical to the original parent cell • each daughter cell is now in G1 (the beginning of interphase) 5 3. Meiosis Meiosis Produces 1n Gametes Meiosis requires 2 rounds of cell division: Meiosis I • differs from mitosis in prophase, metaphase, anaphase Meiosis II • basically the same process as mitosis (chromosome #) 1n 2n 1n (DNA content) 4n 2n Meiosis I Differs from mitosis in several key ways: prophase I metaphase I anaphase I Prophase I: homologous chromosomes pair, exchange genetic material by “crossing over” Metaphase I: homologous chromosomes align as pairs Anaphase I: homologous chromosome pairs separate (sister chromatids are NOT separated) 6 Coat-color genes C “Crossing Over” in Prophase I Eye-color genes E Tetrad (homologous pair of chromosomes in synapsis) e c 1 Breakage of homologous chromatids C E c e 2 • an important source of new genetic combinations (genetic variability) Joining of homologous chromatids C E c e Chiasma 3 Separation of homologous chromosomes at anaphase I C E C e E c e c 4 Separation of chromatids at anaphase II and completion of meiosis C E C e c E c e Parental type of chromosome Recombinant chromosome Recombinant chromosome Parental type of chromosome Gametes of four genetic types Independent Assortment of Chromosomes due to alignment in Metaphase I • an additional source of genetic variability Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I 2n possibilities (n = # of homologous pairs, e.g. 23) Metaphase II Gametes Combination 1 Combination 2 Combination 3 Combination 4 Meiosis II prophase II metaphase II anaphase II telophase II/cytokinesis • essentially the same as mitosis • results in four haploid (1n) gametes • each is genetically unique due to 1) “crossing over” and 2) independent assortment of homologous chromosomes 7 Key Terms for Chapter 8 • cell cycle, mitosis, meiosis • haploid, diploid, somatic cell, gamete • interphase, prophase, metaphase, anaphase telophase, cytokinesis • centromere, centrosome, spindle fibers • sister chromatids vs non-sister chromatids • homologous chromosomes • crossing over, independent assortment Relevant Review Questions: 1-6, 8, 9, 13, 17, 20 8