Meiosis is an important aspect of sexual reproduction

Gametes & Fertilization Gametes – specialized cells necessary for reproduction Males – sperm, Females – egg The zygote goes on to develop into an embryo, and on into a complete individual. When fertilization takes place: (Sperm, 23 chromosomes) + (Egg, 23 chromosomes) = (Zygote, 46 chromosomes) Normal body cells have a diploid (2n) chromosome number, meaning chromosomes occur in pairs. In humans, the male and female each contribute half (haploid (n)), 23 chromosomes.

Sexual Reproduction 2 parents; offspring not genetically identical The process of meiosis shuffles genetic information (DNA) and results in variation in genes. Therefore producing a variation in offspring

Meiosis Note: occurs in reproductive organs and only produce gametes with half the chromosome compared to body cells 46 chromosomes, 23 pairs 23 chromosomes Interphase Prophase 1 Metaphase 1 Anaphase 1 Telophase 1 & Cytokinesis Prophase 2 Metaphase 2 Anaphase 2 Telophase 2 & Cytokinesis Meiosis 1 Meiosis 2

Centrioles Spindle fibers Prophase I Corresponding homologous chromosomes from each parent pair up to form homologous pairs When sister chromatids exchange DNA its called crossing over. Crossing over creates a number of genetic variations in gametes Homologous Pairs (Humans have 23 pairs making 46 total chromosomes)

Importance of crossing over Siblings look alike but not exactly the same…. Crossing over increases genetic diversity

Metaphase I The centrioles send out spindle fibers to line up homologous pairs in the middle of cell along the metaphase plate Centrioles Spindle fibers Homologous Pairs

Anaphase I The centrioles use the spindle fibers to separate the homologous pairs Each homologous chromosome is pulled to the opposite pole of the cell Centrioles Spindle fibers Homologous Chromosomes

Independent Assortment A shuffling of the 23 homologous pairs, in which there are two possibilities for how chromosomes will sort themselves into daughter cells. “independent assortment” leads to variations in species as there are more than 8 million combinations possible for the 23 pairs in any egg or sperm cell. resulting in 70 trillion different zygotes possibilities

Telophase I & Cytokinesis Telophase I – the cell creates a temporary nucleus around the two homologous chromosome sets Cytokinesis – the cell divides into two cells Centrioles Homologous Chromosomes Nuclear Membrane

Meiosis I - Homologous Pair Separate Prophase I - Homologous chromosomes pair up, non-sister chromatids exchange genetic material. (crossing over) Metaphase I - Homologous chromosomes pair up at the equator. (one chromosome from each parent) Anaphase I - Homologous chromosomes separate and are pulled to opposite poles by the spindle fibres. Telophase I - One chromosome from each homologous pair is at each pole of the cell. Interkinesis - results in 2 daughter cells with ½ # of chromosomes. This stage between cell divisions the cell will grow and make proteins as in interphase of mitosis. Unlike interphase in mitosis, there is no replication of DNA during this stage.

Prophase II Each of the Meiosis II stages are running in 2 cells at the same time. Similar to Prophase of Mitosis Centrioles attach spindle fibers to the chromosomes Centrioles Spindle fibers Chromosomes

Prophase II Each of the Meiosis II stages are running in 2 cells at the same time. Similar to Prophase of Mitosis Centrioles attach spindle fibers to the chromosomes Centrioles Spindle fibers Chromosomes

Metaphase II - The X-shaped chromosomes form a single line across the middle of the cell

Telophase II & Cytokinesis Telophase II – the cells creates a permanent nucleus around the two haploid chromosome sets Cytokinesis – the cells divides into four haploid daughter cells Chromatids Nuclear Membrane

Meiosis II - Separates sister chromatids Meiosis II is like mitosis because in both processes the chromatids of each chromosome are pulled to opposite poles. Each daughter cell inherits one chromatid from each chromosome resulting in four haploid cells each with half the N of chromosomes. Prophase II - There is one chromosome of the homologous pair in each cell. Metaphase II - The X-shaped chromosomes form a single line across the middle of the cell. Anaphase II - Sister chromatids move to opposite poles of the cell. Once they separate, each sister chromatid is considered to be a chromosome. (nondisjunction could occur here) Telophase II - Spindle fibres begin to disappear, and a nuclear membrane forms around each set of chromosomes. Cytokinesis - In cytokinesis, the two daughter cells are separated.

Meiosis 23 chromosome 46 chromosomes 23 chromosomes

Male and Female Gametes In males, all 4 cells resulting from meiosis develop into sperm In females, 1 cell gets most of the cytoplasm and becomes the egg while other 3 die, due to unequal cytoplasm and organelles

Mitosis vs. Meiosis

Differences in Mitosis and Meiosis Review of Mitosis vs. Meiosis Comparison animation Differences in Mitosis and Meiosis Mitosis # daughter chromosomes = # parent chromosomes Two identical daughter cells Occurs in all cells except reproductive cells Produces diploid cells Diploid cells = 2n Somatic Body Cells (skin) Meiosis # daughter chromosomes = half the # parent chromosomes Four non-identical daughter cells Occurs only in reproductive cells Produces haploid cells Haploid cells = n Gamete Sex Cells (sperm or egg)