Chromosomal Basis of Inheritance and Cytogenetics

• Mitosis


  Cell replication, generates somatic cells, results in growth, replacement of worn-out cells, repair of damage
• Meiosis


  Gamete production, daughter cells with half the number of chromosomes as the parent (2n → n), necessary for sexual reproduction
• Homologous chromosomes


  Chromosomes with the same order of genes along their DNA, one pair from the father and one from the mother
• Meiosis


  1. Chromosome number reduced to half

  2. Production of male and female gametes

  3. Two nuclear divisions: Meiosis I (reduction division) and Meiosis II (equation division)

  4. Produces four cells
• Meiosis I


  1. Prophase I: Chromosomes condense, spindle and spindle fibers form, synapsis and crossing-over occur

  2. Metaphase I: Chromosomes align on equatorial plane, random orientation of homologues

  3. Anaphase I: Homologous chromosomes separate, independent assortment

  4. Telophase I: Spindle fibers disassemble, nuclear membrane reforms
• Before meiosis I, the cell is in interphase of the cell cycle that includes DNA replication
• Before DNA replication, chromosomes = one chromatid. After DNA replication: chromosomes = two chromatids
• During the time between telophase I and the beginning of meiosis II, no DNA replication occurs
• Meiosis II


  1. Prophase II: Chromosomes condense, spindle and spindle fibers form

  2. Metaphase II: Chromosomes align at the equator, spindle fibers bind to kinetochores of each sister chromatid

  3. Anaphase II: Spindle fibers contract, centromeres split, sister chromatids pulled to opposite poles

  4. Telophase II: Spindle fibers disassemble, nuclear membrane re-forms
• Cytokinesis, the division of the cytoplasm and its contents, may or may not occur after telophase I
• Cytokinesis occurs after telophase II, maturation of the four daughter cells to gametes (sperm or egg)
• In humans, three of the four cells resulting from meiosis in a female disintegrate, the one that survives is randomly chosen
• Chromosomes


  Long strands of DNA, packaged together with proteins and other molecules
• Chromosome


  • Most chromosomes are about 40% DNA and 60% protein
• Nucleosome


  Complex of DNA and histone proteins
• Solenoids


  Higher order of coils of nucleosomes
• Chromatin loop


  Solenoids organized into chromatin loop
• Condensin


  Aids radial looping of solenoids around a scaffold protein
• Centromere


  A "constriction" found in a chromosome, essential for segregation, region where spindle fibers attach and move the chromosome during mitosis or meiosis
• Kinetochores


  Disc-shaped proteins in the centromere where the spindle fibers attach during cell division
• Cohesin


  Protein complex that hold the two identical DNA molecules
• Chromatid


  Half of a replicated chromosome, two sister chromatids in replicated chromosome
• Karyotype


  Particular array of chromosomes an individual possesses
• Chromosomes in human karyotype

  • 23 pairs
  • Autosomal chromosomes: 22 pairs
  • Sex chromosomes: 1 pair (XX - female, XY - male)
• Somatic cells


  Non-reproductive cells, form the body of the organism, contain two copies of all chromosomes (2n)
• Germ cells


  Sex cells for sexual reproduction, contain one copy of all chromosomes (n)
• Zygote


  Cell formed by the fertilization of an egg and a sperm, contains two copies of all chromosomes (2n)
• Cell division


  1. Binary fission

  2. Mitosis

  3. Meiosis
• Mitosis


  Cell replication, generates somatic cells, results in growth, replacement of worn-out cells, repair of damage, daughter cells with same number of chromosomes as the parent (2n → 2n)
• Meiosis


  Gamete production, generates sex cells, necessary for sexual reproduction, daughter cells with half the number of chromosomes as the parent (2n → n)
• Eukaryotic cell cycle


  1. G1 phase

  2. S phase

  3. G2 phase

  4. Mitosis phase

  5. Cytokinesis
• G1 phase


  Primary growth phase of cell, preparation for DNA synthesis
• G0 phase


  Resting state, cell cycle arrest
• S phase


  Stage of DNA replication
• G2 phase


  Second growth phase, preparation for the separation of the newly synthesized genome
• Mitosis phase


  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase
• Prophase


  Chromosomes condense, cytoskeleton disassembles, spindle and spindle fibers form, nuclear membrane disintegrates, nucleolus disappears
• Spindle


  Structure made of microtubules that spans the cell from both sides
• Spindle fibers


  Individual strands of the spindle consisting of microtubules
• Asters


  Microtubules that extend outward from centrioles, hypothesized to be aiding the centriole to brace to the plasma membrane by making it stiffer