The Chromosomal Basis

The unit of heredity is the "allele."

  • Behavior of alleles was predicted by the Mendelian Hypothesis (Theory)
  • Chromosomes are physical structures that behave during gamete formation
    . . - as predicted by Mendelian Theory
  • Eucaryotic chromosome
    . . - DNA double helix
    . . - Chromatin, a protein
    . . - centromere with two "arms" called chromatids
    . . - procaryotic chromosome is "naked" DNA double helix,
    . . . - without chromatin
    . . . - often as a single circular chromosome

    Using the laboratory mouse as an example
    . . - with 40 chromosomes (monoploid level, n = 20)
    . . - to follow the behavior of chrosomes relative to inheritance...

  • Normal Cell Life Cycle, Mitosis
    . . - G-1 stage, single chromatids-
    . . . - 40 chromosomes,
    . . . - 40 chromatids
    . . - S stage, chromatids replicate-
    . . . - 40 chromosomes,
    . . . - 80 chromatids
    . . - M stage metaphase, chromosomes align on equator
    . . - M stage anaphase, chromatids separate, migrate to poles-
    . . . - 2 sets: 40 + 40 chromosomes,
    . . . - 2 sets: 40 + 40 chromatids
    . . - Cytokinesis, cell division to 2 cells-
    . . . - 40 chromosomes,
    . . . - 40 chromatids.

  • Gametogenesis Cell Life Cycle, Meiosis
    . . - G-1 stage, single chromatids-
    . . . - 40 chromosomes,
    . . . - 40 chromatids
    . . - S stage, chromatids replicate-
    . . . - 40 chromosomes,
    . . . - 80 chromatids
    . . - M stage metaphase I, chromosomes align on equator as pairs
    . . - M stage anaphase I, chromosomes separate, migrate to poles-
    . . . - 2 sets: 20 + 20 chromosomes,
    . . . - 2 sets: 40 + 40 chromatids
    . . - M stage metaphase II, chromosomes align on equator
    . . - M stage anaphase, chromatids separate, migrate to poles-
    . . . - 4 sets: 20 + 20 + 20 + 20 chromosomes,
    . . . - 4 sets: 20 + 20 + 20 + 20 chromatids
    . . - Cytokinesis, cell division to 4 cells (gametes)-
    . . . - 20 chromosomes,
    . . . - 20 chromatids.

  • Fertilization
    . . - fusion of two gametes-
    . . . - 20 + 20 chromosomes = 40 chromosomes,
    . . . - 20 + 20 chromatids = 40 chromatids.
    . . - equals normal mouse cell (zygote)
    . . . - entering G-1 stage with 40 chromosomes (monoploid level, n = 20)


    Hypothesis: alleles are on chromosomes
    Physical location (locus) on chromosome matches locus on its pair

  • paired loci are referred to as "genes"
  • allele is coded into DNA sequence


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    © 2004 Prof. LaFrance, Ancilla College