Chapter Review 10 1 Bacterial Cell Division Binary fission is a simple form of cell division Prokaryotic cell division is clonal resulting in two identical cells Bacterial DNA replication and partitioning of the chromosome are concerted processes Proteins control chromosome separation and septum formation DNA replication begins at a specific point the origin and proceeds bidirectionally to a specific termination site Newly replicated chromosomes are segregated to opposite poles at the same time as they are replicated New cells are separated by septation which involves insertion of new cell membrane and other cellular materials at the midpoint of the cell A ring of FtsZ and proteins embedded in the cell membrane expands radially inward pinching the cell into two new cells 10 2 Eukaryotic Chromosomes Chromosome number varies among species The gain or loss of chromosomes is usually lethal Eukaryotic chromosomes exhibit complex structure Chromosomes are composed of chromatin a complex of DNA and protein Heterochromatin is not expressed and euchromatin is expressed The DNA of a single chromosome is a very long double stranded fiber The DNA is wrapped around a core of eight histones to form a nucleosome which can be further coiled into a 30 nm fiber in interphase cells During mitosis chromosomes are further condensed by arranging coiled 30 nm fibers radially around a protein scaffold Newly replicated chromosomes remain attached at a constricted area called a centromere consisting of repeated DNA sequences After replication a chromosome consists of two sister chromatids held together at the centromere by a complex of proteins called cohesins figure 10 7 10 3 Overview of the Eukaryotic Cell Cycle figure 10 8 The cell cycle is divided into five phases The phases of the cell cycle are gap 1 G1 synthesis S gap 2 G2 mitosis and cytokinesis C G1 S and G2 are collectively called interphase and mitosis and cytokinesis together are called M phase The duration of the cell cycle varies depending on cell type The length of a cell cycle varies with age cell type and species Cells can exit G1 and enter a nondividing phase called G0 the G0 phase can be temporary or permanent 10 4 Interphase Preparation for Mitosis G1 S and G2 are the three subphases of interphase G1 is the primary growth phase during S phase DNA synthesis occurs G2 phase occurs after S phase and before mitosis The centromere binds proteins assembled into a disklike structure called a kinetochore where microtubules attach during mitosis The centromeric DNA is replicated but the two DNA strands are held together by cohesin proteins 10 5 M Phase Chromosome Segregation and the Division of Cytoplasmic Contents figure 10 11 During prophase the mitotic apparatus forms In prophase chromosomes condense the spindle is formed and the nuclear envelope disintegrates In animals cells centriole pairs separate and migrate to opposite ends of the cell establishing the axis of nuclear division During prometaphase chromosomes attach to the spindle In metaphase chromosomes align at the equator Chromatids of each chromosome are connected to opposite poles by kinetochore microtubules They are held at the equator of the cell by the tension of being pulled toward opposite poles At anaphase the chromatids separate At this point cohesin proteins holding sister chromatids together at the centromeres are destroyed and the chromatids are pulled to opposite poles This movement is called anaphase A and the movement of poles farther apart is called anaphase B During telophase the nucleus re forms Telophase reverses the events of prophase and prepares the cell for cytokinesis In animal cells a belt of actin pinches off the daughter cells A contractile ring of actin under the membrane contracts during cytokinesis In plant cells a cell plate divides the daughter cells Fusion of vesicles produces a new membrane in the middle of the cell to produce the cell plate In fungi and some protists daughter nuclei are separated during cytokinesis 10 6 Control of the Cell Cycle figure 10 18 Research uncovered cell cycle control factors Experiments showed that there are positive regulators of mitosis and that there are proteins produced in synchrony with the cell cycle cyclins The positive regulators are cyclin dependent kinases Cdks Cdks are complexes of a kinase and a regulatory molecule called cyclin They phosphorylate proteins to drive the cell cycle The cell cycle can be halted at three checkpoints Checkpoints are points at which the cell can assess the accuracy of the process and stop if needed The G1 S checkpoint is a commitment to divide the G2 M checkpoint ensures DNA integrity and the spindle checkpoint ensures that all chromosomes are attached to spindle fibers with bipolar orientation Cyclin dependent kinases drive the cell cycle The cycle progresses by the action of Cdks Yeast have only one CDK enzyme vertebrates have more than four enzymes During the G1 phase G1cyclin combines with Cdc2 kinase to form the Cdk that triggers entry into S phase The anaphase promoting complex cyclosome APC C activates a protease that removes cohesins holding the centromeres of sister chromatids together the result is to trigger anaphase separating the chromatids and drawing them to opposite poles The APC C also triggers destruction of mitotic cyclins to exit mitosis In multicellular eukaryotes many Cdks and external signals act on the cell cycle Growth factors like platelet derived growth factor PDGF stimulate cell division This acts through a MAP kinase cascade that results in the production of cyclins and activation of Cdks to stimulate cell division in fibroblasts after tissue injury Cancer is a failure of cell cycle control Mutations in proto oncogenes have dominant gain of function effects leading to cancer Mutations in tumorsuppressor genes are recessive loss of function of both copies leads to cancer Vocabulary anaphase In mitosis and meiosis II the stage initiated by the separation of sister chromatids during which the daughter chromosomes move to opposite poles of the cell in meiosis I marked by separation of replicated homologous chromosomes anaphase promoting complex APC A protein complex that triggers anaphase it initiates a series of reactions that ultimately degrades cohesin the protein complex that holds the sister chromatids together The sister chromatids are then released and move toward opposite poles in the cell aster In animal cell mitosis a radial array of