Meiosis

• meiosis is used to create genetically different gametes in sexual reproduction.
• meiosis results in four daughter cells with half the number of chromosomes of the parent cell.
• in meiosis, the cell starts as a diploid and produces haploids.
• Meiosis has two divisions, meiosis I and meiosis II. Mitosis only has one division.
• How does meiosis acheive genetic variation?

  Crossing over and independent assortment of chromosomes.
• crossing over in meiosis refers to when homologous pairs of chromosomes line up and exchange genetic information.
• Independent assortment of chromosomes in meiosis is when the chromosomes have various combinations in their arrangement.
• Prophase I

  -DNA condenses

  -paired homologous chromosomes (bivalent)

  -Non-sister chromatids cross over (chiasma)

  -spindles form

  -nuclear envelope breaks down
• metaphase I

  -bivalents line up across the cell equator

  -spindle fibers attach to the centromeres

  -maternal and paternal pairs line up independently of each other (independent assortment)
• anaphase I

  -The homologous pairs are separated

  -They are pulled to opposite poles of the cell by the spindles
• telophase I

  -chromosomes are at the opposite poles

  -spindles break down

  -nuclear envelopes reform around the chromosomes
• Cytokinesis (meiosis I)

  -cytoplasm divides

  -two haploid cells are produced
• DNA is not replicated between meiosis I and meiosis II, as there is no interphase.
• Prophase II

  -nuclear envelope breaks down

  -spindles form at opposite poles
• Metaphase II

  -Chromosomes line up at the cell equator

  -spindle fibres attach to centromeres
• Anaphase II

  -centromeres divide

  -chromatids are pulled to opposite poles by spindles

  -four groups of chromosomes with half the number of the parent cell
• telophase II

  -nuclear envelopes form around the chromosomes
• Cytokinesis (meiosis II)

  -cytoplasm divides

  -four daughter haploid cells are created