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Chapter Eight: Cell Reproduction 8-1 Chromosomes 8-2 Cell Division 8-3 Meiosis 8-1 Chromosomes I. Chromosomal Structure (chromatin UNCOILED, chromosome compact) • Rod-shaped, made of DNA and proteins, found in NUCLEUS as chromatin (numbers VARY between species). PROKARYOTE CHROMOSOMES vs. EUKARYOTE CHROMOSOME (1) Histone Protein • ROUND protein that DNA wraps tightly around to CONSERVE SPACE in nucleus (i.e., the core in a ball of yarn) (2) Nonhistone protein • Nuclear protein involved in REGULATING the ACTIVITY of specific REGIONS of DNA (i.e., genes). (3) Chromatid • Forms as DNA CONDENSES and COPIES itself BEFORE cell division; (NOTE: Each half of a chromosome = 1 chromatid) (4) Centromere (binding protein) • Holds TWO chromatids together UNTIL they separate during cell division. II. Chromosome Numbers • VARIES between species, although some SPECIES (not necessarily closely related) CAN contain the SAME number of chromosomes. Critical Thinking (1) Do you suppose there is a correlation between the NUMBER of chromosomes and the COMPLEXITY of an organism? Support your answer. (A) Sex Chromosomes and Autosomes • Chromosomes can be linked to SEX development (sex chromosomes XX OR XY) OR can be responsible for NON-sex traits (autosomes) (1) Homologous Chromosomes (1maternal, 1 paternal) • A PAIR of chromosomes that carry GENES for SAME traits (1 chromosome is inherited from EACH parent). (2) Karyotype • Photomicrograph of chromosomes EXTRACTED from a dividing DIPLOID cell (2N); (typically derived from a white blood cell) (B) Diploid and Haploid Cells • Can be IDENTIFIED by the NUMBER of chromosomes it contains; TWO types exist—(N, 2N) (1) Diploid (2N, Human Number is ?) • Cell contains FULL set of chromosomes (including all NON –reproductive cells (body or somatic cells), produced via MITOSIS. (2) Haploid (N, Human Number is ?) • Cell contains HALF set of chromosomes (including all REPRODUCTIVE cells (sperm or egg), produced via MEIOSIS. 8-2 Cell Division I. Cell Division in Prokaryotes (i.e., bacteria) • Simpler AND faster than eukaryotes (evolved cells). (1) Binary Fission (cell division of bacteria, i.e., prokaryotes) • DNA is COPIED and bacterium DIVIDES with EQUAL DNA in each cell. II. Cell Division in Eukaryotes (2 divisions: Nuclear AND Cytoplasmic) • Cells divide either through: (1) Mitosis, or (2) Meiosis (1) Mitosis (M phase NUCLEUS divides) • Results in DIPLOID cells IDENTICAL to original parent cell (i.e., growth, replacement of dead cells, 46 46) (2) Meiosis (a.k.a. reduction division, 46 23) • Results in HAPLOID cells with GENETIC VARIATION from parent cell. (i.e., spermatogenesis and oogenesis) (A) The Cell Cycle (5 PHASES G1, S, G2, M, and Cytokinesis) • LIFESPAN of a cell (Cell division is a small phase of the life cycle) (1) Interphase (80% of cell’s LIFE) • Divided into 3 phases (G1, S, and G2) time BETWEEN cell divisions (2) M phase (20% of cell’s LIFE is in M phase precedes CYTOKINESIS) • NUCLEUS divides into 2 GENETICALLY IDENTICAL nuclei. (3) Cytokinesis (after NUCLEUS has completed its division) • Division of CYTOPLASM of a cell END of CELL DIVISION. Critical Thinking (2) What would you PREDICT may happen in a cell if CYTOKINESIS took place BEFORE mitosis? (B) Interphase • Involves CELL GROWTH, DNA REPLICATION, and PREPARING for CELL DIVISION. (1) G1 Phase (G is for “Growth”) • 1ST phase after division, characterized by VIGOROUS cell growth. (2) S phase (S is for “Synthesis”) • Follows G1 phase, DNA is COPIED inside nucleus of cell (chromosomes are copied UNDUPLICATED TO DUPLICATED) (3) G2 Phase (2nd growth phase) • Follows S phase, SLOWED growth and more PREPARATION for DIVISION (i.e., M phase). (4) G0 Phase • Cells can EXIT the cell cycle (by entering G0), NO longer copying their DNA, NO longer preparing to divide. • NOTE: Fully developed cells, including many NEURONS, stop dividing at MATURITY and will NEVER divide again. Critical Thinking (3) If you consider the mass of DNA in a sperm (a haploid cell) to be 1, what would the RELATIVE VALUE be for the DNA mass of a cell in the G2 phase of the cell cycle? (C) Mitosis (M PHASE CYTOKINESIS) • Division of cell NUCLEUS is divided into 4 STAGES (P, M, A, T) (1) Prophase (1st) • CHROMATIN coils into compacted CHROMOSOMES; nuclear MEMBRANE disappears. (2) Metaphase (2nd) • Kinetochore FIBERS pull chromosomes toward CENTER of cell. (3) Anaphase (3rd) • Chromatids SEPARATE at CENTROMERE and migrate to OPPOSITE ENDS of cell. (4) Telophase (4th) • Chromosomes REACH opposite ends and return to chromatin, FIBERS disassemble; NUCLEUS reforms. (5) Centrosomes (Centrioles, in Animal Cells) • Found at POLES, cast out SPINDLE FIBERS toward chromosomes. (6) Spindle Fibers (Mitotic Spindle—2 TYPES of fibers) •EXTEND to EQUALLY DIVIDE chromatids between TWO offspring cells. (7) Kinetochore Fibers (pull APART chromatids) • Extend from CENTROMERE of each CHROMATID to CENTROSOMES. (8) Polar Fibers (extend from CENTROSOME to CENTROSOME) • Fibers extend ACROSS the DIVIDING CELL (pole TO pole). (D) Cytokinesis (FOLLOWS telophase of M Phase) • CYTOPLASM pinches INWARD to yield TWO separate (daughter) cells. (1) Cleavage Furrow (in Animal Cells) • CELL MEMBRANE pinches in (via microtubules) and separates into 2 cells. (2) Cell Plate ( CYTOKINESIS in Plant Cells) • VESICLES formed by GOLGI fuse at MIDLINE of dividing PLANT CELL (produces a CELL WALL). 8-3 Meiosis (Gametogenesis) I. Stages of Meiosis (Meiosis I AND II, reproductive organs) • Sex cells undergo cell cycle, but DIVIDE 2x, requiring 2 phases. (i.e., to make haploid (N) gametes) (A) Meiosis I (PHASE of tetrads ands synapsis) • 1st set of meiotic phases, (PMAT); begin as DIPLOID cells. (1) Synapsis (occurs during PROPHASE I) • Chromosomes PAIR up with HOMOLOGUES (NOTE: This pairing does NOT take place during MITOSIS). (2) Tetrad (PAIR of homologues, during PROPHASE I) • Line so that GENES on 1st chromosome are adjacent to SAME GENES on 2nd chromosome. NOTE: During synapsis, CHROMATIDS may twist around one another (i.e., crossing-over). (3) Crossing-Over (with TETRADS during SYNAPSIS) • Portions of genes may BREAK OFF 1 CHROMOSOME and ATTACH to PARTS of its HOMOLOGUE. NOTE: Allows for EXCHANGE of genes between MATERNAL and PATERNAL chromosomes, increasing genetic variation of SEX CELLS. (4) Genetic Recombination (gives SEX CELLS variation) • RESULT of crossing-over of, increasing genetic diversity of OFFSPRING. (5) Law of Independent Assortment (during Anaphase I) • EACH homologous chromosome can RANDOMLY move to an OPPOSITE side. • NOTE: Independent Assortment results in a RANDOM COMBINATIONS of the maternal & paternal chromosomes, yielding genetic VARIATIONS. (B) Meiosis II (separates CHROMATIDS into opposite poles) • Cell DOUBLED its DNA BUT was reduced (during Meiosis I) by the end to (2N); • NOTE: Meiosis II takes the 2 DIPLOID (2N) cells and REDUCES them into FOUR HAPLOID (N) cells. Critical Thinking (4) Explain why the CHROMSOMES in the haploid cells that are produced by Meiosis I APPEAR DIFFERENT from those produced by Meiosis II. (C) Formation of Gametes (oogenesis AND spermatogenesis) • Meiosis II results in HAPLOID cells called GAMETES, and occurs in reproductive organs. (1) Spermatogenesis (in testes, yields FOUR viable sperm) • A DIPLOID cell divides to form 4 haploid spermatids, each matures (following puberty) to become an active, swimming HAPLOID sperm. (2) Oogenesis (in ovaries, yields ONE viable EGG) • A DIPLOID cell divides to produce ONE mature egg cell (ovum) and THREE polar bodies (immature ova) that disintegrate. (NOTE: ONE mature ovum HOGS cytoplasm, AND therefore nutrients). (D) Asexual and Sexual Reproduction • Two METHODS of cell reproduction: (1) Asexual Reproduction (bacteria, prokaryotes) • Offspring (a clone—LITTLE TO NO VARIATION from this form of reproduction) produced WITHOUT union of sperm and egg. (2) Sexual Reproduction (eukaryotes) • Offspring a result of UNION between gametes, sperm and egg; MUCH VARIATION from this form of reproduction. Extra Slides AND Answers for Critical Thinking Questions (1) No. By using the data table, students should give examples of simple organisms that have more chromosomes than complex organisms. (2) The nucleus and chromosomes would be in one offspring cell only. The cell with the chromosomes would probably be functional, but the cell lacking chromosomes would not be functional. (3) The chromosomes in the new cells produced by Meiosis I have twice the mass of those in the cells made by Meiosis II. (4) Four. The number of chromosomes would double. Each chromosome would consist of two chromatids; thus, the mass would be four times that found in a sperm.