Meiosis and Sexual Reproduction

Haploid vs. Diploid Cells Vocabulary: Haploid (N) - 1/2 the genetic material; 1 set of chromosomes. Diploid (2N) - Normal amount of genetic material; 2 sets of chromosomes. Example: Human Body cell = 46 (diploid) Human sex cell = 23 (haploid)

Meiosis Forms Haploid Cells Gametes are Haploid-they contain one set of chromosomes. One gamete (n) comes from mom and one gamete (n) comes from dad. This is how we make diploid individuals (2n)

Meiosis: form of cell division that halves the number of chromosomes Meiosis: form of cell division that halves the number of chromosomes. The result is haploid gametes or spores. Meiosis: involves two divisions of the nucleus (Meiosis I and Meiosis II) Each division has: prophase, metaphase, anaphase, telophase stages Before Meiosis starts the DNA is replicated in the S phase of interphase.

Meiosis I

Prophase I 1. The chromosomes condense 2. The nuclear envelope breaks down 3. Crossing over occurs

Metaphase I 1. Homologous chromosomes are lined up on the equator of the cell. 2. Spindle fibers attach to the centromeres.

Anaphase I 1. Homologous chromosomes are pulled apart by the spindle fibers. Each chromosome is still composed of two chromatids!!

Telophase I Individual chromosomes gather at each of the poles The cytoplasm divides (cytokinesis) forming two cells. (Note that the chromosomes do not replicate between Meiosis I and II

Prophase II 1. A new spindle forms around the chromosomes.

Metaphase II The chromosomes line up along the equator Spindle fibers attach to the centromeres.

Anaphase II The centromeres divide Chromatids move to opposite poles of the cell

Telophase II A nuclear envelope forms around each set of chromosomes. Spindle breaks down Cell undergoes cytokinesis Result=four haploid cells

Meiosis Contributes to Genetic Variation Contributions: Independent assortment, crossing over and random fertilization

Chromosome pairs separate Independently In humans each gamete receives one chromosome from each of 23 pairs of homologous chromosomes. The chromosome choice is just a matter of chance. Each of the 23 separate independently. So there are about 8 million different combinations from these 23 chromosomes.

Crossing over During Prophase I, the 4 homologous chromosomes are very close together. They can exchange pieces of chromosomes. It is like changing arms with your neighbor.

Importance of generating genetic variation Meiosis is important to evolution. Example: genetics is being used to produce larger animals for food. After time, the “genes” for “large” will be used up and there will be a limit to size. Example: race horses were bred for speed. Years ago the “speed” of horses leveled out. Why?