5.1 - Meiosis

Meiosis ensures the formation of haploid gamete cells in sexually reproducing diploid organisms

• Diploid: a cell with two full sets, or pairs, of chromosomes

  • chromosomes differ in size, shape, genetic information, and centromere location

  • cell contains one set from each parent

  • represented by 2n

  • body cells are diploid (e.g., skin cells, leaf cells, hypha cell)

• Haploid: a cell with only one set of chromosomes

  • represented by n

  • gametes (sex cells) are haploid (e.g., egg, pollen)

Meiosis results in daughter cells with half the number of chromosomes as the parent cell

• The diploid parent cell produces four haploid daughter cells (sex cells)

Meiosis involves two rounds of a sequential series of steps (meiosis I and meiosis II)

(Meiosis I)

• Prophase I

  • nuclear envelope begins to disappear

  • fibers begin to form

  • DNA coils into visible duplicated (or double) chromosomes made up of sister chromatids

  • double chromosomes pair up based on size, shape, centromere location, and genetic information

  • while paired, chromatids exchange genetic information with chromatids from the other chromosome (non-sister chromatids exchange genetic information)

• Metaphase I

  • double chromosomes remain in pairs

  • fibers align pairs across the center of the cell

• Anaphase I

  • fibers separate chromosome pairs

  • each double chromosome, from the pair, migrates to opposite sides of the cell

• Telophase I

  • nuclear envelope reappears and establishes two separate nuclei

  • each nucleus contains only one double chromosome from each pair

    • nucleus only contains half of the total information the parent nucleus contained

  • chromosomes will begin to uncoil

• Cytokinesis will separate the cell into two daughter cells

• Daughter cells are haploid and genetically different from each other and the parent cell

(Meiosis II)

• Prophase II

  • nuclear envelope begins to disappear

  • fibers begin to form

• Metaphase II

  • fibers align double chromosomes across the center of the cell

• Anaphase II

  • fibers separate sister chromatids

  • chromatids (single chromosomes) migrate to opposite sides of the cell

• Telophase II

  • nuclear envelope reappears and establishes separate nuclei

  • each nucleus contains single chromosomes

  • chromosomes will begin to uncoil

• Cytokinesis will separate the two cells into four daughter cells

• Daughter cells are haploid and genetically different from each other and the parent cell

Mitosis and meiosis are similar in the way genetic information is passed to daughter cells

• Both processes involve:

  • nuclear envelope disappearing

  • DNA coiling into chromosomes

  • aligning chromosomes in the center of the cell

  • using fibers to separate chromosomes

  • nuclear envelope reappearing

  • chromosomes uncoiling

  • followed by cytokinesis and production of daughter cells

Mitosis and meiosis differ in the number of resulting cells and the genetic content of the cells

• Mitosis produces two daughter cells that are genetically identical to the parent

• Meiosis produces four haploid cells that are genetically varied from each other and the parent cell

Key Takeaways:

1. Diploid cells have pairs of chromosomes (a whole set), one from each parent, and are represented by 2n. Haploid cells have a single set of chromosomes, represented by n

2. The purpose of meiosis is to produce haploid gametes

3. Meiosis involves two rounds of cell division. In meiosis I, pairs of chromosomes separate, resulting in two haploid cells containing only one of the double chromosomes from each pair. In meiosis II, double chromosomes separate, resulting in four haploid cells, each with single chromosomes

4. Mitosis and meiosis are similar in the overall process (PMAT) of how genetic information is passed on to daughter cells. However, mitosis produces two genetically identical cells and meiosis produces four haploid genetically varied cells

5.2 - Meiosis and Genetic Diversity

Meiosis generates genetic diversity

• Meiosis results in four haploid gametes (sex cells) that are genetically different

• Certain processes that generate genetic diversity take place during and after meiosis

Crossing over increases genetic diversity among gametes

• Crossing over occurs in prophase I of meiosis I

• Non-sister chromatids of double homologous chromosomes exchange segments

• Results in recombinant chromatids

• Formation of recombinant chromatids increases genetic diversity

Random assortment of chromosomes serves to increase variation

• The order of the homologous pairs during metaphase I affects which chromosomes end up in each gamete

• Different combinations of chromosomes in each gamete increase genetic variation

Fertilization of gametes serves to increase variation

• When fertilization occurs, information from each parent is contributed to the fertilized egg

• Typically one gamete from each parent fuse together to form a diploid offspring

• Fertilization is random in that any gamete can contribute to the diploid nature of genomes in offspring; this increases the potential for genetic diversity

Key Takeaways:

1. Crossing over in prophase I occurs when non-sister chromatids exchange segments. This results in recombinant chromosomes

2. Random assortment of chromosomes in metaphase I can result in different combinations of chromosomes in gametes

3. During sexual reproduction, any gamete from one parent can combine with any gamete from another parent, resulting in genetically different offspring. This increases the genetic diversity within a population of organisms