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Chapter 12 The Cell Cycle “Every cell from a cell” --- the continuity of life is based one the reproduction of cells, or cell division. Cell cycle: --- the life of a cell from the time it is first formed from a dividing parent cell until its own division into two cells. Cell division functions in reproduction, growth, and renewal. 200 µm 20 µm Cell division: --- involves the distribution of identical genetic materialDNA-to two daughter cells. Genome 基因體 --- genetic information, a cell’s endowment of DNA. --- human cell ~ 2 m of DNA Chromosomes 染色體 --- every eukaryotic species has a characteristic number of chromosomes in each cell nucleus. * Somatic cells have two sets of chromosomes * Gametes have one set of chromosomes Chromatin (染色質) Eukaryotic chromosomes are made of chromatin, a complex of DNA and associated protein molecules. Gene (基因) : the units that specify and organism’s inherited traits. chromatin ~ long, thin fiber duplication chromatin condense chromosome Chromosome duplication Chromatin DNA-protein complex Sister chromatids : containing identical copies of the chromosome’s DNA. 0.5 µm A eukaryotic cell has multiple chromosomes, one of which is represented here. Before duplication, each chromosome has a single DNA molecule. Once duplicated, a chromosome consists of two sister chromatids connected at the centromere. Each chromatid contains a copy of the DNA molecule. Mechanical processes separate the sister chromatids into two chromosomes and distribute them to two daughter cells. Chromosome duplication (including DNA synthesis) Centromere Separation of sister chromatids Centromeres Sister chromatids Sister chromatids * Mitosis (有絲分裂) --- the division of the nucleus. * Cytokinesis (細胞質分裂) Two set of chromosome --- the division of the cytoplasm. * Meiosis (減數分裂) --- yields daughter cells that have half chromosomes. one set of chromosome Mitosis is just one part of the cell cycle. Mitotic (M) phase : includes both mitosis and cytokinesis (the shortest part of the cell cycle) * A typical human cell might undergo one division in 24 hours. * G1 is the most variable in length in different 90% 5~6 hr types of cells. 10~12 hr 4~6 hr less than 1 hr The mitotic division of an animal cell Interphase: Prophase prometaphase G2 metaphase telophase & anaphase cytokinesis Longest stage shortest stage The mitotic spindle distributes chromosomes to daughter cells. (紡錘絲) ~ begins to form in the cytoplasm during prophase. ~ * made of microtubules and associated proteins. * elongated by incorporating more subunits of the protein tubulin. * starts in the centrosome. (microtubule-organizing center; MTOC) * centrioles are not essential for cell division. * Spindle includes the centrosomes, the spindle microtubules, and the asters. * Kinetochore: ~ a structure of proteins associated with specific sections of chromosomal DNA at the centromere. * Kinetochore microtubules * Nonkinetochore microtubules (“polar” microtubules) Aster Sister chromatids Centrosome Metaphase Plate Kinetochores Overlapping nonkinetochore microtubules Kinetochores microtubules Microtubules * Microtubules of asters 0.5 µm Metaphase --- the spindle complete contact with the plasma membrane (in metaphase) Centrosome 1 µm Chromosomes Metaphase Anaphase Anaphase commences when proteins holding together the sister chromatids of each chromosome are inactivated How do the kinetochore microtubules function in the poleward movement of chromosomes ? Kinetochores are equipped with motor proteins that “walk” a chromosome along the attached microtubules toward the nearest pole. The microtubules shorten by depolymerizing at their kinetochore ends. What’s the function of the nonkinetochore microtubules ? --- for elongating the whole cell during anaphase. * Nonkinetochore microtubules interdigitate across the metaphase plate Anaphase: 1. Nonkinetochore microtubules orininating from opposite spindle poles move past each other toward their poles. 2. The nonkinetochore microtubules lengthen by the addition of tubulin subunits to their ends. Cytokinesis divides the cytoplasm * Cleavage furrow Actin + Myosin Cleavage furrow Contractile ring of microfilaments * No cleavage furrow 100 µm Vesicles forming cell plate Wall of patent cell 1 µm Cell plate New cell wall Daughter cells Daughter cells (a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (SEM) • Mitosis in a plant cell Chromatine Nucleus Nucleolus condensing 1 Prophase. The chromatin is condensing. The nucleolus is beginning to disappear. Although not yet visible in the micrograph, the mitotic spindle is staring to from. Figure 12.10 Chromosome 2 Prometaphase. 3 Metaphase. The 4 We now see discrete spindle is complete, chromosomes; each and the chromosomes, consists of two attached to microtubules identical sister at their kinetochores, chromatids. Later are all at the metaphase in prometaphase, the plate. nuclear envelop will fragment. Anaphase. The 5 chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of cell as their kinetochore microtubles shorten. Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divided the cytoplasm in two, is growing toward the perimeter of the parent cell. Prokaryotes (bacteria) reproduce by a type of cell division called binary fission, meaning literally “division in half”. Cell wall Origin of replication E. Coli cell 1 Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell. 2 Replication continues. One copy of the origin is now at each end of the cell. 3 Replication finishes. The plasma membrane grows inward, and new cell wall is deposited. 4 Two daughter cells result. Two copies of origin Origin Plasma Membrane Bacterial Chromosome Origin Mitosis in eukaryotes may have evolved from binary fission in bacteria. * A hypothetical sequence for the evolution of mitosis Prokaryotes. Bacterial chromosome Chromosomes Microtubules ~ Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis carried out by most eukaryotic cells Dinoflagellates. (腰鞭毛蟲) Intact nuclear envelope Kinetochore microtubules Diatoms. (矽藻) Intact nuclear envelope Kinetochore microtubules Most eukaryotes. Centrosome Fragments of nuclear envelope Concept 12.3: The cell cycle is regulated by a molecular control system The frequency of cell division varies with the type of cell Ex. Human skin cells: divide frequently throughout life. liver cells: keep in reserve (to repair a wound) mature nerve cells and muscle cells: do not divide at all. These cell cycle differences ~ Result from regulation at the molecular level What drives the cell cycle ? The cell cycle is driven by specific chemical signals present in the cytoplasm. Experiment 1 Experiment 2 S S G1 S the G1 cell immediately entered the S phase— DNA was synthesized. M M G1 M the G1 cell immediately began mitosis— a spindle formed and chromatin condensed, even though the chromosome had not been duplicated. The sequential events of the cell cycle are directed by a distinct cell cycle control system, a cyclically operating set of molecules in the cell that both triggers and coordinates key events in the cell cycle. ~ similar to a clock G1 checkpoint Control system G1 M G2 M checkpoint G2 checkpoint S The cell cycle is regulated at certain checkpoints by both internal and external controls. Cell cycle “checkpoints” * A checkpoint is a critical control point where stop and go-adhead signals can regulate the cycle. (~ signal transduction pathways) Animal cells generally have built-in stop signals that halt the cell cycle at checkpoints until overridden by go-ahead signals. * G1, G2, and M phase checkpoint * G1 checkpoint: (restriction point) ~ the most important checkpoint. * G1 checkpoint: (restriction point) x Go-ahead signal Complete the cell cycle and divide. Exit the cycle, switching into a nondividing state (“G0 phase”) G0 G1 checkpoint G1 G1 (a) If a cell receives a go-ahead signal (b) If a cell does not receive a go-ahead signal at the G1checkpoint, the cell exits the cell cycle at the G1 checkpoint, the cell and goes into G0, a nondividing state. continues on in the cell cycle. What kinds of molecules make up the cell cycle control system (the molecular basis for the cell cycle clock)? Two types of regulatory proteins are involved in cell cycle control: Cyclins and cyclin-dependent kinases (Cdks) constant concentration inactive cdk active cdk cyclin cyclically fluctuating concentration The activity of cyclins and Cdks ~ Fluctuates during the cell cycle ~ Ex. MPF “ maturation-promoting factor” “M-phase-promoting factor” ~ first identified cyclin-cdk complex ~ triggers the cell’s passage past the G2 checkpoint into M phase. Cdk Degraded Cyclin Cyclin is degraded G2 checkpoint MPF Cdk Cyclin Stop and Go Signs: Internal and External Signals at the Checkpoints • Both internal and external signals – Control the cell cycle checkpoints Internal signals: originating inside the cell ~ ex. Messages from the kinetochores Anaphase onset: When the kinetochores of all the (M phase checkpoint) chromosomes are attached to the spindle Breakdown of cyclin and the inactivation of proteins holding the sister chromatids together. Sister chromatids separate External signals: ex. Growth factors ~ Cells fail to divide if an essential nutrient is left out of the culture medium. ~ GFs trigger a signal transduction pathway that allows the cells to pass the G1 checkpoint and divide. PDGF PDGF receptor cell Signal transduction Cell division External signals: physical factor Density-dependent inhibition of cell division ~ Crowded cells stop dividing single layer Cells anchor to dish surface and divide (anchorage dependence). When cells have formed a complete single layer, they stop dividing (density-dependent inhibition). If some cells are scraped away, the remaining cells divide to fill the gap and then stop (densitydependent inhibition). 25 µm • Most animal cells exhibit anchorage dependence – In which they must be attached to a substratum to divide Anchorage dependence * Cancer cells: ~ Exhibit neither density- dependent inhibition nor Normal cell ~ single layer Cancer cells do not exhibit anchorage dependence or density-dependent inhibition. anchorage dependence 25 µm 25 µm Chapter 13 Meiosis and Sexual Life Cycles The transmission of traits from one generation to the next is called inheritance, or heredity. (遺傳) ~ Genetics • Overview: Hereditary Similarity and Variation • Living organisms – Are distinguished by their ability to reproduce their own kind • Heredity – Is the transmission of traits from one generation to the next • Variation – Shows that offspring differ somewhat in appearance from parents and siblings Figure 13.1 Offspring acquire genes from parents by inheriting chromosomes. ~ Parents endow their offspring with coded information in the form of hereditary units called genes. constitute our “genome” Genes are segments of DNA. ~ DNA is a polymer of four different kinds of monomersnucleotides. (核苷酸) Asexual and sexual reproduction (無性) (有性生殖) In asexual reproduction, ~ a single individual is the sole parent and passes copies of all its genes to its offspring. (the genomes of the offspring are virtually exact copies of the parent’s genome) “clone” (a group of genetically identical individuals) In sexual reproduction, ~ offspring vary genetically from their siblings and both parents. Hydra, 水螅 ~ reproduce by budding. ~ usually genetically identical to its parent. Parent Bud 0.5 mm Somatic cell Gametes (體細胞) (sperm or ovum) ~ 46 chromosomes ~ 23 chromosomes With a light microscope, condensed (mitotic) chromosomes can be distinguished from one another by their appearance. Differ from ~ size, the positions of their centromeres ~ staining pattern * Karyotype ~ the images of the chromosomes are arranged in pairs, starting with the longest chromosomes. Karyotypes ~ ordered displays of an individual’s chromosomes. Pair of homologous chromosomes Centromere Sister chromatids 5 µm Karyotyping can be used to screen for abnormal numbers of chromosomes or defective chromosomes associated with certain congenital disorders, such as Down syndrome. Down syndrome: each body cell has a total of 47 chromosomes. (cells are trisomic for chromosome 21; trisomy 21) The chromosomes that make up a pair-that have the same length, centromere position, and staining pattern -are called homologous chromosomes, or homologues. carry genes controlling the same inherited characters. Key A gene’s specific location along the length of a chromosome is called the gene’s locus. Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) Exception of the homologous chromosome: X & Y chromosome: Only small parts of the X and Y are homologous. Female: XX Male: XY sex chromosomes autosomes The occurrence of homologous pairs of chromosomes in our karyotype is a consequence of our sexual origins. 46 chromosome --- two sets of 23 chromosomes. ~ a maternal set (from mother) and a paternal set (from father) Somatic vs. gametes A diploid cell. (2n=46) a single set of the 22 autosomes + a single sex chromosome A haploid cell. (n=23) Sperm + ovum fertilization Fertilized egg (zygote) • The human life cycle Key The processes of Haploid gametes (n = 23) Haploid (n) Diploid (2n) Ovum (n) fertilization and meiosis are the unique trademarks Sperm Cell (n) of sexual reproduction. FERTILIZATION MEIOSIS ~ alternate in sexual life cycles. Ovary Testis Mitosis and development Figure 13.5 Multicellular diploid adults (2n = 46) Diploid zygote (2n = 46) • In animals – Meiosis occurs during gamete formation – Gametes are the only haploid cells Key Haploid Diploid n n Gametes n MEIOSIS FERTILIZATION Zygote 2n Figure 13.6 A Diploid multicellular organism 2n Mitosis (a) Animals • Plants and some algae – Exhibit an alternation of generations – The life cycle includes both diploid and haploid multicellular stages Alternation of generations Haploid multicellular organism (gametophyte) ~ includes both diploid and haploid multicellular stages. n Mitosis n Mitosis n n n Spores Gametes MEIOSIS Diploid multicellular organism (sporophyte) Figure 13.6 B FERTILIZATION 2n (b) Plants and some algae 2n Mitosis Zygote • In most fungi and some protists – Meiosis produces haploid cells that give rise to a haploid multicellular adult organism – The haploid adult carries out mitosis, producing cells that will become gametes Haploid multicellular organism n Mitosis Mitosis n n n Gametes MEIOSIS FERTILIZATION 2n Figure 13.6 C Zygote (c) Most fungi and some protists n Meiosis reduces chromosome number from diploid to haploid. • Interphase and meiosis I MEIOSIS I: Separates homologous chromosomes INTERPHASE PROPHASE I METAPHASE I ANAPHASE I 2. cross over Sister chromatids Nuclear envelope Chromatin Chromosomes duplicate Figure 13.8 Sister chromatids remain attached Centromere (with kinetochore) Centrosomes (with centriole pairs) Tetrad Chiasmata Spindle Metaphase plate Homologous Microtubule chromosomes attached to separate kinetochore Tertads line up Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example 1. Synapsis (聯會) (synaptonemal complex) Pairs of homologous chromosomes split up • Telophase I, cytokinesis, and meiosis II MEIOSIS II: Separates sister chromatids TELOPHASE I AND CYTOKINESIS PROPHASE II Cleavage furrow Figure 13.8 Two haploid cells form; chromosomes are still double METAPHASE II ANAPHASE II Sister chromatids separate TELOPHASE II AND CYTOKINESIS Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes • A comparison of mitosis and meiosis MITOSIS MEIOSIS Parent cell (before chromosome replication) Chiasma (site of crossing over) MEIOSIS I Prophase I Prophase Chromosome replication Duplicated chromosome (two sister chromatids) Chromosome replication Tetrad formed by synapsis of homologous chromosomes 2n = 6 Metaphase Chromosomes positioned at the metaphase plate Anaphase Telophase Sister chromatids separate during anaphase 2n Tetrads positioned at the metaphase plate Homologues separate during anaphase I; sister chromatids remain together Metaphase I Anaphase I Telophase I Haploid n=3 Daughter cells of meiosis I 2n MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II Sister chromatids separate during anaphase II A Comparison of Mitosis and Meiosis • Meiosis and mitosis can be distinguished from mitosis – By three events in Meiosis l 1. Synapsis (聯會) and crossing over (交換) – Homologous chromosomes physically connect and exchange genetic information 2. Tetrads on the metaphase plate – At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates 3. Separation of homologues – At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell – In anaphase II of meiosis, the sister chromatids separate Chiasmato are the physical manifestations of a genetic arrangement called crossing over. Origins of genetic variation Three mechanisms that contribute to the genetic variation arising from sexual reproduction: 1. Independent assortment of chromosomes 2. Crossing over 3. Random fertilization 1. Independent assortment of chromosomes Key Maternal set of chromosomes Paternal set of chromosomes Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I 2n (n = the haploid number) Metaphase II 223 Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 2. Crossing over Prophase I of meiosis Nonsister chromatids The process called crossing Tetrad over produces recombinant Chiasma, site of crossing over chromosomes, which combine genes inherited from our Metaphase I two parents. Metaphase II Daughter cells Recombinant chromosomes 3. Random fertilization Sperm + ovum fertilization 223 223 Fertilized egg (zygote) 223 X 223 + Cross over Evolutionary adaptation depends on a population’s genetic variation Darwin ~ recognized the importance of genetic variation in the evolutionary mechanism ~ natural selection. Adaption, the accumulation of the genetic variations favored by the environment. Sex and mutations are the two sources of this variation * 英文命題, 中英文回答皆可 ~ * 題型: 是非題 (True & False) 選擇題 (multiple choices) 解釋名詞 (definitions) 共計 110分