Mitosis and Meiosis

• Homologs (= homologous pairs of chromosomes) contain the same set of genes, but can have different alleles for some genes. Non-homologs carry completely unrelated sets of genes.
• Karyotype – micrograph (photo) of stained chromosomes arranged in homologous pairs.
• •Sex chromosomes – unpaired X and Y chromosome.

  •Autosomes – all chromosomes except X and Y.
• Mitosis – process that generates 2 daughter cells containing the same number and type of chromosomes/genes as parent cell. In other words, the 2 daughter cells are identical to each other to the parent cell.
• Meiosis – process that generates gametes (= egg and sperm cell) that have 1/2 the number of chromosomes compared to the parent cell.
• Diploid = double set of chromosomes

  Haploid = 1 set of chromosomes
• Fertilization is the union of haploid gametes to produce diploid zygotes
• Metaphase chromosomes can be classified by centromere position
• • The chromosomes visible only at the metaphase stage of mitosis (more condensed).
  • 22 homologous pairs of autosomes and two sex chromosomes.
  • Each chromosome has a characteristic size and shape in the “normal” cell.
• Children receive an X chromosome from their mother, but either an X or Y chromosome from their father
• Interphase: DNA replication (and transcription)
• • Prophase – chromatin condenses to form chromosomes
  • Prometaphase – spindle forms and sister chromatids attach to microtubules from opposite centrosomes
  • Metaphase – chromosome line up at the metaphase plate
  • Anaphase – sister chromatids separate and move to opposite poles
  • Telophase – chromosomes decondense (form chromatin) and nuclei form again
• Nondisjunction – mistakes in chromosome segregation during Meiosis I or II (usually during Anaphase I, e.g. trisomy 21)
• • May result in gametes or embryos that don’t survive

  • Can also result in abnormal chromosome numbers in surviving individuals (e.g. trisomy 21, Down syndrome; or XXY, Klinefelter syndrome)
• • Many hybrids between species (i.e. donkey x horse à mule) are sterile because chromosomes cannot pair properly
• Meiosis contributes to genetic diversity in two ways

  • Crossing-over between homologs creates diJerent combinations of alleles within each chromosome
  • Independent assortment of non-homologs creates diJerent combinations of allele
• The cell cycle: An alternation between interphase and mitosis

  • G1 and G2 phases: Most of cell growth occurs during these phases
  • Some terminally diJerentiated cells stop dividing and arrest (stop) in G0 stage
  • S Phase: DNA replication occurs. This result in two identical sister chromatids in later stages.
• Two general types of cells in plants and animals

  q Somatic cells make up vast majority of cells in the organism

  • In G0 or are actively going through mitosisq Germ cells are precursors to gametes
  • Set aside from somatic cells during embryogenesis
  • Become incorporated into reproductive organs
  • Only cells that undergo meiosis produce haploid gametes