Cell Cycle Notes PDF

CH 12 NOTES, part 1: Chromosomes, the Cell Cycle, and Cell Division (12.1-12.2) The ability of organisms to reproduce best distinguishes living things from nonliving matter The continuity of life is based upon the reproduction of cells, or cell division In unicellular organisms, division of one cell...

CH 12 NOTES, part 1: Chromosomes, the Cell Cycle, and Cell Division (12.1-12.2) The ability of organisms to reproduce best distinguishes living things from nonliving matter The continuity of life is based upon the reproduction of cells, or cell division In unicellular organisms, division of one cell reproduces the entire organism Multicellular organisms depend on cell division for:  Development from a fertilized cell  Growth  Repair 100 µm Reproduction 200 µm Growth and development 20 µm Tissue renewal 12.1 – Most cell division results in genetically identical daughter cells Cells duplicate their genetic material (DNA) before they divide, ensuring that each daughter cell receives an exact copy of the genetic material A dividing cell duplicates its DNA, allocates the two copies to opposite ends of the cell, and only then splits into DAUGHTER CELLS Cellular Organization of the Genetic Material: A cell’s endowment of DNA (its genetic information) is called its GENOME DNA molecules in a cell are packaged into CHROMOSOMES Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus Somatic (nonreproductive) cells have two sets of chromosomes (DIPLOID) Gametes (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells (HAPLOID) Eukaryotic chromosomes consist of CHROMATIN, a complex of DNA and protein (i.e. histone proteins) that condenses during cell division 25 µm Chromosomes = after the DNA replicates in the S phase of interphase, a chromosome consists of tightly coiled chromatin (DNA); a chromosome consists of 2 identical chromatids (sister chromatids) which are connected in the center by a CENTROMERE **a human cell entering mitosis contains 46 chromosomes chromosome centromere chromatin DNA 0.5 µm Chromosome duplication (including DNA synthesis) Centromere Sister chromatids Separation of sister chromatids Centromeres Sister chromatids 12.2 – The mitotic phase alternates with interphase in the cell cycle Eukaryotic cell division consists of: – Mitosis: the division of the nucleus – Cytokinesis: the division of the cytoplasm Gametes are produced by a variation of cell division called meiosis (CH 13) **Meiosis yields nonidentical daughter cells that have only one set of chromosomes, half as many as the parent cell 3 main stages of the cell cycle 1) Interphase: longest stage (90%); includes preparation for cell division 2) Mitosis (10%): nucleus divides into 2 nuclei, each with the same # and kind of chromosomes (DNA) as the parent cell 3) Cytokinesis: cytoplasm divides forming 2 distinct cells INTERPHASE G1 S (DNA synthesis) G2 Cell Cycle G1 = “first gap”; cell growth (producing proteins & organelles) S = DNA “Synthesis” (cell copies its DNA) & more growth G2 = “second gap”; more growth & completes preparation for division Mitosis is one, continuous event, but it can be described as happening in 5 phases: 1) Prophase 2) Prometaphase 3) Metaphase 4) Anaphase 5) Telophase (followed by CYTOKINESIS!) **Cytokinesis is well underway by late telophase G2 OF INTERPHASE PROPHASE PROMETAPHASE METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS **Remember, the cell is coming out of Interphase… PROPHASE chromatin condenses & chromosomes become visible; centrosomes / centrioles separate and move to the opposite sides of the nucleus PROMETAPHASE nuclear envelope breaks down and the nucleolus disappears; spindle fibers (from centrioles of centrosomes) connect to chromosomes at their centromeres (KINETOCHORE) METAPHASE chromosomes line up in the center of the cell (metaphase plate); spindle fibers connect from the poles (end) of the spindle to the centromere / kinetochore of each chromosome ANAPHASE centromeres split, causing the sister chromatids to separate, becoming individual chromosomes chromosomes are pulled apart to opposite ends of the cell as the spindle fibers shorten and “reel them in” to the poles TELOPHASE chromosomes uncoil into chromatin; new nuclear envelope forms around the chromatin; spindle breaks apart; nucleolus reappears in each new nucleus Finally… CYTOKINESIS in animal cells: cell membrane pinches in & divides (cleavage furrow) in plant cells: a cell plate (new cell wall) forms 100 µm Cleavage furrow Contractile ring of microfilaments Daughter cells Cleavage of an animal cell (SEM) Vesicles forming cell plate Wall of parent cell Cell plate 1 µm New cell wall Daughter cells Cell plate formation in a plant cell (TEM) Then the cell returns to Interphase… and the process continues The Mitotic Spindle: A Closer Look The mitotic spindle is an apparatus of microtubules that controls chromosome movement during mitosis Assembly of spindle microtubules begins in the CENTROSOME, the microtubule organizing center The centrosome replicates, forming two centrosomes that migrate to opposite ends of the cell, as spindle microtubules grow out from them An aster (a radial array of short microtubules) extends from each centrosome The spindle includes: the centrosomes, the spindle microtubules, and the asters Some spindle microtubules attach to the kinetochores of chromosomes and move the chromosomes to the metaphase plate Aster Microtubules Sister chromatids Chromosomes Centrosome Metaphase plate Kinetochores Overlapping nonkinetochore microtubules Centrosome 1 µm Kinetochore microtubules 0.5 µm In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell The microtubules shorten by depolymerizing at their kinetochore ends Chromosome movement Microtubule Motor protein Chromosome Kinetochore Tubulin subunits Nonkinetochore microtubules from opposite poles overlap and push against each other, elongating the cell In telophase, genetically identical daughter nuclei form at opposite ends of the cell Nucleus Nucleolus Chromatin condensing Prophase. The chromatin is condensing. The nucleolus is beginning to disappear. Although not yet visible in the micrograph, the mitotic spindle is starting to form. Chromosomes Prometaphase. We now see discrete chromosomes; each consists of two identical sister chromatids. Later in prometaphase, the nuclear envelope will fragment. Cell plate Metaphase. The spindle is complete, and the chromosomes, attached to microtubules at their kinetochores, are all at the metaphase plate. Anaphase. The chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of the cell as their kinetochore microtubules shorten. 10 µm Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divide the cytoplasm in two, is growing toward the perimeter of the parent cell. BINARY FISSION Prokaryotes (bacteria and archaea) reproduce by a type of cell division called BINARY FISSION In binary fission, the chromosome replicates (beginning at the origin of replication), and the two daughter chromosomes actively move apart Cell wall Origin of replication Plasma membrane E. coli cell Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell. Two copies of origin Bacterial chromosome Cell wall Origin of replication Plasma membrane E. coli cell Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell. Replication continues. One copy of the origin is now at each end of the cell. Bacterial chromosome Two copies of origin Origin Origin Cell wall Origin of replication E. coli cell Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell. Replication continues. One copy of the origin is now at each end of the cell. Replication finishes. The plasma membrane grows inward, and new cell wall is deposited. Two daughter cells result. Plasma membrane Bacterial chromosome Two copies of origin Origin Origin The Evolution of Mitosis Since prokaryotes evolved before eukaryotes, mitosis probably evolved from binary fission Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis Bacterial chromosome Prokaryotes Chromosomes Microtubules Intact nuclear envelope Dinoflagellates Kinetochore microtubules Intact nuclear envelope Diatoms Kinetochore microtubules Centrosome Most eukaryotes Fragments of nuclear envelope