Meiosis and Genetic Inheritance

Genetic Disease

  • Genetic diseases are caused by changes to the DNA and can be inherited.
    • Sickle cell mutation can be passed on to offspring.
    • This is unlike non-genetic diseases like the flu or cancer.
  • The mutation is present in reproductive cells (sperm & ova).

Sickle Cell Disease

Chromosome Terminology

  • Humans are diploid (2n), having chromosome pairs.
    • One chromosome in each pair comes from each parent.
    • Homologous pairs contain the same genes (sections of DNA).
    • Information can vary between homologues (e.g., brown/blonde hair).
  • Each chromosome initially exists as a single copy (1 chromatid).
    • It gets copied before cell division through DNA replication.
    • Identical copies of a chromosome are called sister chromatids.
    • Sister chromatids are joined at the centromere.

How Cells Make a Person

  • Humans reproduce sexually through the fusion of sex cells.
  • Gametes are specialized cells used in sexual reproduction.
    • Female gametes = ova (eggs).
    • Male gametes = sperm.
    • They contain half the number of chromosomes found in body cells.
  • Body cells are diploid (2n), with 2 representatives of each chromosome.
  • Gametes are haploid (1n), with 1 representative of each chromosome.
  • During fertilization, gametes fuse & form a new cell called a zygote.

Meiosis Builds Gametes

  • Meiosis is a reduction-division cell cycle.
    • Ensures the same number of chromosomes in each generation.
  • Meiosis is essential for sexual reproduction.
    • 2Mom2+2Dad2=1+1=2Offspring\frac{2Mom}{2} + \frac{2Dad}{2} = 1 + 1 = 2Offspring
  • Without meiosis, chromosomes would double with each generation.
    • Humans: gametes would be diploid (2n).
    • Offspring would be tetraploid (4n).
    • The 3rd generation would be octaploid (8n), the 4th would be 16-ploid, and so on.

Two Cycles in Meiosis

  • Chromosomes are replicated during interphase.
    • Each chromosome consists of 2 sister chromatids.
  • Meiosis I: recombines & separates chromosomes.
  • Meiosis II: separates sister chromatids.
  • Meiosis includes the same 4 stages as mitosis, but occurs twice.
    • Prophase = “first or before”.
    • Metaphase = “middle”.
    • Anaphase = “apart”.
    • Telophase = “end”.
    • The cell divides and the process repeats.

Stages of Meiosis

Prophase I

  • Chromosomes condense, the nucleus breaks down, and the spindle forms.
  • Crossing over occurs between chromosome pairs.
    • Enzymes break chromatids & exchange parts.
    • Each chromatid becomes a unique combination of parental genes.

Metaphase I

  • Spindle fibers push/pull chromosomes.
    • Pairs line up along the cell's middle.
  • Homologous pairs exhibit independent assortment.
    • Orientation of chromosome pairs is random.
    • Either chromosome can align on either side of the middle.
    • Helps create gametes with different combinations of genes.
  • There are 2232^{23} ways that 23 chromosome pairs can align, resulting in 8,388,608 combinations (not including crossing over).

Anaphase I

  • Chromosome pairs separate.
    • Each chromosome is pulled to the opposite side.

Telophase I

  • A nucleus forms around each group of chromosomes.

Cytokinesis

  • Daughter cells separate.
    • Each has half of the total chromosomes.
    • In humans, these cells are haploid (1n).
    • Each contains 1 chromosome from each pair.
    • Each chromosome has 2 sister chromatids, which are no longer genetically identical due to crossing over.

Meiosis II

  • Prophase II - the nucleus breaks down & the spindle forms and binds to chromosomes.
  • Metaphase II - chromosomes align along middle.
  • Anaphase II - sister chromatids separate.
    • Centromeres are split by enzymes.
    • Chromatids pulled to opposite sides.
  • Telophase II - a nucleus forms around chromosomes.
  • Cytokinesis divides cells.
    • Daughter cells develop into gametes (gametogenesis).
    • Each contains 1 set of unique chromosomes.