General Biology/Cells/How Cells Divide

From Wikibooks, open books for an open world
< General Biology‎ | Cells
Jump to: navigation, search

General Biology | Getting Started | Cells | Genetics | Classification | Evolution | Tissues & Systems | Additional Material


How cells divide

Prokaryote cell division[edit]

  • 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[edit]

  • 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[edit]

  • 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[edit]

  • 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[edit]

Human karyotype stained by chromosome painting[edit]

Chromosomes[edit]

  • 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[edit]

  • 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[edit]

  • 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 chromotids connected by a common centromere.

Replicated human chromosomes[edit]

Mitosis[edit]

  • 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[edit]

  • 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[edit]

  • 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[edit]

  • 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[edit]

This text is based on notes very generously donated by Paul Doerder, Ph.D., of the Cleveland State University.