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Cell Reproduction • • • • • • Understand why cells reproduce Describe the structure of the chromosome Describe the cell cycle Describe each phase of mitosis Distinguish between mitosis and meiosis Describe the process and phases of meiosis Terms • • • • • • • • • Anaphase Asexual reproduction Autosomes Cell cycle Cell plate Centromere Centrioles Centrosome Cleavage furrow • • • • • • • • • • • Chromosome Crossing over Cytokinesis Diploid Gamete G1 G2 Haploid Histone Interphase Meiosis • • • • • • • • • • • • • Metaphase Mitosis Microtubules Prophase Restriction point S phase Sexual reproduction Sex chromosome Spindle Somatic cell Synapsis Telophase Tetrad Why Cells Must Reproduce • Cells are limited in size due to: – Surface area to volume ratio: S.A./volume • Volume increases 8x faster than surface area • The area in the ‘middle’ of the cell cannot get nutrients in or wastes out because there is not enough surface area for diffusion – Amount of DNA – limited amount of instructions, cell cannot keep up with demand Why Cells Must Reproduce • To grow an organism must produce more cells • Each new cell must have an accurate copy of the DNA (genome) • Mitosis – Growth – Replace cells that wear out – Replace damaged cells – Asexual reproduction Purposes of Mitosis • Asexual reproduction: – Unicellular organisms • Amoeba, paramecium – Multicellular organisms • Corals, sponges, plants Genome • Genome – cell’s genetic information – Prokaryote = single, long DNA molecule (?) • E. coli – 1700 genes • Binary fission – bacteria cell division – Eukaryote = lots of DNA • Humans – 3.2 b nucleotides – 25,000 genes (est. HGP ’09) – Coils up to form 46 chromosomes Chromosomes • Genome – genes • Genes - sections of DNA that are the code for making the cell’s proteins • Chromatin – DNA + histones (proteins) ‘at work’ – Making proteins Chromatin Chromosomes • Chromatin coils up to become chromosomes – Chromosomes are ‘gene packages’ Chromosomes • Each chromosome = two sister chromatids (identical DNA) • Centromere = connects the two chromatids Chromosomes • Each species has a characteristic number of chromosomes – Human somatic cells (body cells) have 46 (diploid number) – Dogs = 78, goldfish = 96 – Human gametes (sperm or eggs) have 23 (haploid) Karyotype • Karyotype – arrangement of chromosomes – Picture taken during metaphase – Size – Centromere location – Staining pattern Normal male Cell Cycle • Life history of the cell • Interphase • Mitosis Cell Cycle - Interphase • Cell growth • 90% of cell cycle – Three subphases: • G1 (“first gap”) growth • S (“synthesis”) DNA is copied • G2 (“second gap”) cell completes preparations for division Cell Cycle - Interphase • Some cells may enter G0 – Don’t return to mitosis – Liver, nerve, muscle Cell Cycle – M-phase • Mitosis – division of the nucleus and chromosomes • Cytokinesis – division of the cytoplasm and organelles Cell Cycle – M-phase • Mitosis is a continuum: – For description, mitosis is usually broken into five subphases: • Prophase • Prometaphase • Metaphase • Anaphase • Telophase Cell Cycle - Interphase • Late interphase (G2) • DNA has been copied – ‘S’ phase – Chromatin – not chromosomes yet • Centrosomes have been duplicated – Microtubules (cytoskeleton) – Will help form the spindle Cell Cycle – M-phase • Prophase - the chromosomes are coiled, with sister chromatids • The nucleoli disappear • The mitotic spindle begins to form • Centrosomes move toward opposite poles • Spindle fibers (microtubules) begin to grow between the centrosomes Cell Cycle – M-phase • Late Prophase - nuclear envelope disintegrates so that spindle fibers can attach to the chromosomes • Kinetochores – special regions of the spindle, from each pole attach to the centromere Cell Cycle – M-phase • Metaphase - spindle fibers push the sister chromatids until they are all arranged at the metaphase plate • Shortest phase of cycle Cell Cycle – M-phase • Anaphase - centromeres divide, separating sister chromatids • Each chromatid is pulled toward the pole to which it is attached by spindle fibers Cell Cycle – M-phase • Telophase - cell continues to elongate – Two nuclear envelopes begin to reform – Chromosome uncoils – Nucleolus reforms • Cytokinesis begins Fig. 12.9 Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Cytokinesis Divides Cytoplasm • Animal cells: • Cleavage furrow – microfilaments of actin and myosin form and contract like a drawstring • Contraction of the ring pinches the cell in two Cytokinesis Divides Cytoplasm • Plants have cell walls • Cell plate - vesicles from the Golgi coalesce at the metaphase plate – Plate enlarges until fused with the plasma membrane Cell Cycle Control • Checkpoints in the cycle are controls – Chemical signals – Checks to be sure all steps are completed – Cells also receive signals from outside (ex. Growth hormones) • 3 checkpoints; G1, G2, and M phases • G1 - Restriction point Cell Cycle Control • Frequency of cell division varies with cell type – Skin cells divide frequently; exercise – Liver cells do not divide unless damaged – Nerve and muscle cells do not appear to divide after maturity • Much is not yet understood – How and why are cells preprogrammed to become __? – How and why do some cells become cancerous? Cell Cycle Control • G1 checkpoint (restriction point) is most important. – Go signal = completes cell cycle and divides. – No go = cell exits cycle; switches to nondividing state, G0 phase Cell Cycle Control • Density-dependent inhibition – Cultured cells normally divide until they form a single layer – Cells will grow to fill a gap – At high densities, not enough growth factor and nutrient for all cells Cell Cycle Control • Anchorage dependence - cells must be anchored to a substrate (extracellular matrix) • Cancer cells do not respond to density-dependent inhibition or anchorage dependence Cancer Cells Not Under Cell Cycle Controls • Cancer cells do not stop dividing when growth factors are depleted – Manufacture their own growth factors (?) – Abnormality in the signaling pathway – Problem in the cell cycle control system • If cancer cells stop dividing, they do so at random, not the normal checkpoints Cancer • Cancer cells may divide indefinitely if they have a continual supply of nutrients • Normal cells divide 20 to 50 times (in vitro) before they stop, age, and die. – Cancer cells may be “immortal” • Cells (HeLa) from a tumor removed from a woman (Henrietta Lacks) in 1951 are still reproducing in culture. Cancer • A cell in a tissue undergoes a transformation converts it from normal to cancer – Normally, immune system recognizes and destroys transformed cells – Some cells escape destruction and reproduce to form a tumor, a mass of abnormal cells Cancer Tumors • Two types: benign, malignant • Benign – tumor remains at original site – surgery • Malignant – cell leave original site, spread to other organs • Metastasis – tumor cells pop ‘loose’ enter lymph Cancer - Treatment • Surgery • Radiation and chemotherapy • Interrupt cell cycle – Mitotic spindle – Block chemical controls Meiosis Cell Reproduction That Produces Gametes Meiosis • Gametes (eggs or sperm) are produced only in gonads (ovaries or testes) • Cells undergo meiosis in the gonads – Produces four daughter cells; half the chromosomes of each parent (haploid) • Fertilization - gametes fuse together; restores the number to 46. (diploid) Meiosis • Chromosomes are homologous pairs – 23 pairs of homologous chromosomes • 22 pairs of autosomes 1 pair of sex chromosomes – Same genes – Requires two parts for every gene – one homologue from one parent and the other from the other parent Meiosis • During ‘S’ each homologue is copied Meiosis • Meiosis takes place in two steps – Meiosis I – Meiosis II • Phases are the same but happen twice – Prophase – Metaphase – Anaphase Interphase – Telophase Meiosis • Prophase I – Same as prophase of mitosis – except: • Synapsis - homologous chromosomes pair up side-by-side • Crossing over - tips of chromosomes may swap places • Value of crossing over? Prophase I Crossing over Meiosis • Metaphase I – Chromosome pairs line up at the middle Metaphase I Meiosis • Anaphase I – Chromosome pairs separate and migrate to opposite poles – In mitosis, chromatids separate and migrate to poles Anaphase I Meiosis • Telophase I – Chromosome pairs arrive at the pole – Chromosomes uncoil – Nuclear envelope forms – Cytokinesis Telophase I Meiosis • Meiosis II – Prophase II – Metaphase II Meiosis II – Anaphase II – Telophase II – 4 new haploid cells called gametes are produced Meiosis • Produces: – 4 non-identical, haploid gametes – Haploid gametes unite to form a zygote • Zygote undergoes mitosis to become a baby – Random chance determines which chromosome you inherit from each parent – Crossing over increases genetic variation Comparison of Mitosis and Meiosis • Mitosis – 2 identical cells – New cells are diploid – Growth, repair, asexual reproduction – No synapsis or crossing over – One cell division • Meiosis – 4, non-identical cells – New cells are haploid – Sexual reproduction – Synapsis and crossing over – Two cell divisions Mitosis and Meiosis • • • • • • How are mitosis and meiosis alike? DNA is replicated Phases are the same Spindle forms Cytokinesis Cells are reproduced