General Biology/Cells/How Cells Divide

How cells divide Prokaryote cell division Binary fission Doubling of cell contents, including DNA Fission to divide contents Segregation of replicated genomes by growth of membrane between attachment points Partitioning of cytoplasmic components Escherichia coli Capable of cell division every 20 minutes under optimal conditions (DNA in continuous state of replication) Model organism of bacterial cell division Bacterial DNA replication Replication follows rules of base pairing, with each polynucleotide chain serving as template for synthesis of its complement. Genetic evidence showed that the bacterial chromosome is circular long before there was corroborating physical evidence. Eukaryotic chromosomes Discovered by Walther Fleming in 1882 in dividing cells of salamander larvae, following improvements in microscopes and staining technology He called division mitosis (mitos = “thread”) Chromosome number is constant in a species Ranges from 2 to >500 (46 in human somatic cells) Homologous pairs, one contributed by each parent Change in number is cause and consequence of speciation Chromosome constancy and their precise division in mitosis and meiosis led biologists to postulate that they were carriers of hereditary information Chromosome number 1N = number of chromosomes in gamete 1N = haploid chromosome number 2N = number of chromosomes in somatic cells (cells that are not egg or sperm) 2N = diploid Deviations from N or 2N are usually lethal in animals Chromosome numbers Eukaryotic chromosomes Consist of chromatin DNA and associated proteins, mainly histones Nucleosomal organization Euchromatin: unwound chromatin, in basic nucleosomal configuration; genes available for expression Heterochromatin: highly condensed except during replication Karyotype: array of chromosomes an individual possesses Clinical importance (Down syndrome; cancer) Evolutionary importance (speciation) Chromosome organization Human karyotype stained by chromosome painting Chromosomes Homologous pairs Inherited one from each parent Identical in length and position of centromere Contain identical or similar genes Homologous pair = homologs Morphology After replication, consist of two sister chromatids attached to a centromere Human chromosomes Diploid number = 2n = 46 = 23 pairs of homologs Haploid number = 23 (gametes) Each replicated chromosome contains 2 sister chromatids = 92 chromatids Cell cycle Growth and division cycle of cells Precisely controlled by biochemical and gene activity, except in cancer Phases G1: primary growth phase S: DNA replication; chromosome replication G2: second growth phase; preparation for mitosis M: mitosis; nuclear division C: cytoplasmic division Mitotic cell cycle Cells exiting the cell cycle are said to be in G0 Cell cycle time varies with stages of life cycle and development, with G1 the most variable DNA replication occurs during S phase of the cell cycle following G1. - at this point the chromosomes are composed of two sister chromatids connected by a common centromere. Replicated human chromosomes Mitosis Nuclear division equational division of replicated chromosomes chromatids move to opposite poles Continuous process prophase metaphase anaphase telophase Driven by motors and microtubules No change in chromosome number N → N by mitosis 2N → 2N by mitosis May be accompanied by cytokinesis Kinetochore Microtubules attach to kinetochores. Metaphase Momentary alignment of chromosomes in center of cell Anaphase Plant mitosis Similar to animal mitosis New cell wall formed between cells from membrane partition Cell cycle control Cell cycle events are regulated by protein complexes and checkpoints Discovered by microinjection of proteins in to eggs, by mutational analysis and by techniques of molecular biology Molecular control of cell cycle: Cdk and cyclin Cyclin dependent protein kinase (Cdk) Phosphorylate serine/threonine of target regulatory proteins Function only when bound to cyclin Cyclin: short-lived proteins that bind to cdks Controlling the cell cycle External signals initiate cell division in multicellular organisms Growth factors: extracellular regulatory signals Usually soluble; bind to cell surface receptor Sometimes membrane bound, requiring cell-cell contact with receptor E.g., upon wound, platelets release PDGF which stimulates fibroblasts to enter cell cycle (exit G0), to heal wound Cancer Unregulated cell proliferation Cancer cells have numerous abnormalities >46 chromosomes Mutations in proto-oncogenes Encode proteins stimulating the cell cycle May be regulated by phosphorylation Often over expressed in cancer cells Mutations in tumor-suppressor genes Encode proteins inhibiting the cell cycle Often bind to products of proto-oncogenes May be regulated by phosphorylation Mutations and cancer References This text is based on notes very generously donated by Paul Doerder, Ph.D., of the Cleveland State University.