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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 chromotids 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.