Cellular Division

• Cell division


  Essential process that produces a continuous supply of replacement cells
• Reasons for cell division

  • Growth and development
  • Reproduction to propagate species and pass on genetic information
• Asexual reproduction

  Most straightforward and ancient form of reproduction, generates genetically identical offspring (clones)
• Sexual reproduction


  Offspring's genetic makeup is derived from two parents, traits are mixed up/recombined so genetically different offspring are produced
• Sexual reproduction process

  1. Fertilization

  2. Zygote formation

  3. Zygote growth and mitotic division into trillions of cells
• Cell division


  Meiosis to create unique gametes, mitosis to regenerate a cell
• Apoptosis


  Programmed cell death, works in concert with cell division to shape new structures during development
• DNA replication


  1. Helicases unwind DNA

  2. DNA polymerase builds new DNA

  3. Ligases seal adjacent DNA molecules

  4. Proofreading and error repair mechanisms ensure accuracy
• Binary fission

  Asexual reproduction process in prokaryotes
• Eukaryotic cell division


  • DNA is divided among many chromosomes
  • Chromosomes are made of chromatin (DNA + histone proteins)
  • 6 ft of DNA is packed into each nucleus via chromatin winding
• Eukaryotic cell division


  1. Chromosomes condense before division

  2. Replicated chromosomes have two sister chromatids and a centromere

  3. Humans are diploid with 23 pairs of chromosomes
• Homologous chromosomes


  Contain the same genes but may have different alleles
• Cell cycle


  1. Interphase (G1, S, G2)

  2. Mitosis

  3. Cytokinesis
• Mitosis

  1. Prophase/Prometaphase

  2. Metaphase

  3. Anaphase

  4. Telophase

  5. Cytokinesis
• Meiosis


  Cell division that yields haploid gametes (sex cells)
• Meiosis increases genetic diversity via crossing over, independent assortment, and random fertilization
• Polyploidy


  Extra chromosomal sets, fatal in humans but important to plant evolution
• Nondisjunction


  Failure of sister chromatids to separate properly during meiosis, results in gametes with extra or missing chromosomes
• Down syndrome is the most common example of an extra autosome resulting from nondisjunction
• DNA replication


  1. Helicases unwind DNA at origins of replication

  2. DNA polymerase builds new DNA (5' to 3') off of each template strand

  3. Ligases seal up adjacent DNA molecules
• DNA replication is incredibly accurate because of proofreading and error repair mechanisms
• Only 1 in a billion nucleotides are incorrect
• Mutations do occur
• Mutations to proofreading or repair enzymes (e.g., BRCA1/2)


  Can be cancerous
• Eukaryotic Cell Division


  • DNA is divided among many chromosomes
  • Chromosomes are made of chromatin (DNA + histone proteins)
  • 6 ft of DNA is packed into each of our nuclei via packing; chromatin is wound up tight into fibers of nucleosomes
• Eukaryotic Cell Division


  1. After replication and only before division do chromosomes condense

  2. Condensed replicated chromosomes have two sister chromatids and a visible centromere
• Humans


  Diploid (2n); we have two copies of each chromosome
• Chromosomes in humans


  • 23 pairs of chromosomes total
  • 22 pairs of autosomes
  • 1 pair of sex chromosomes (designated X and Y chromo's)
• Homologous pair of chromosomes

  Two members of a pair
• Cell cycle


  The sequence of events between cell divisions
• Cell cycle


  1. Interphase

  2. Mitosis

  3. Cytokinesis
• Interphase


  The time between cell divisions; cell growth and DNA replication
• Interphase


  1. G1 (Gap 1)

  2. S (Synthesis)

  3. G2 (Gap 2)
• Not all cells cycle; some arrest in G1 or permanently arrest in G0 and never divide
• Mitosis


  Prepares cells for division by distributing DNA
• Replicated chromosomes (46 x 2) must be given to two daughter cells accurately (46 each) for proper cellular function
• Mitosis


  1. Prophase

  2. Prometaphase

  3. Metaphase

  4. Anaphase

  5. Telophase

  6. Cytokinesis
• Prophase/Prometaphase


  • Chromosomes condense into two longitudinal strands or chromatids; nuclear envelope breaks down; spindle fibers (microtubules) attach
• Because chromosomes were replicated during interphase, similar chromatids are joined at a centromere forming sister chromatids
• Our cells alternate between two states: division and nondivision