The Formation of Sex Cells

You simply cannot combine an egg and sperm if they contain the normal number of chromosomes! The chromosome # must be reduced to half! This is Meiosis.

I. Overview of Meiosis A. All organisms inherit one of each gene from each parent, resulting in paired genes. In animals, mom’s set of genes comes from an egg cell; dad’s set of genes comes from a sperm cell In plants, the genes come from pollen and ovule B.Sex cells, or gametes contain half of the normal amount of chromosomes 1.Meiosis is the process of creating haploid (N) gametes from diploid (2N) cells occurs in eukaryotic cells only required by sexual, but not asexual, reproduction

II. Chromosome Number A. Eukaryotic sexual reproduction results in organisms with two sets of chromosomes 1. the two sets are said to be homologous a. chromosomes in one set have a version of themselves in the other set B. A cell with both sets of homologous chromosomes is said to be diploid (2N) 1. Ex. somatic cells (liver cell, heart cell, skin cell etc.) C. A cell with one set of chromosomes is said to be haploid (N) 1. Ex. gametes (egg and sperm or ovule and pollen)

Each body cell of a fruit fly has 8 chromosomes, 4 from the father and 4 from the mother Each body cell of a human has 46 chromosomes, 23 from the father and 23 from the mother

Homologous Chromosome Pairs

III. Phases of Meiosis Overview: Diploid cells destined to become gametes (germ cells) go through two rounds of cell division 1. DNA replication a. occurs before the first round of cell division (recall the S phase of mitosis) 2. Meiosis I a. newly synthesized chromosomes pair up with their homologes forming a tetrad Ex. The chromosome containing the gene for eye color from mom will pair up with the chromosome containing the gene for eye color from dad This is different from mitosis! Tetrad

2. Meiosis I cont. b. the tetrads may exchange portions of their chromatids, in a process called crossing-over genes are exchanged between homologous chromosomes this introduces variation! c.homologous chromosomes separate and the cell divides (recall cytokinesis) d.two new haploid (N) cells are formed, each having a unique set of genes

Meiosis I and Meiosis II vs. Mitosis: Start: PAIRED chromosomes, each with 2 sister chromatids 2 sister chromatids split (like Mitosis) End: UNPAIRED chromosomes, each with 1 sister chromatid (RE-PAIRING occurs at ferilization). End: PAIRED chromosomes, each with 1 sister chromatid Start: UNPAIRED chromosomes, each w/ 2 sister chromatids

Images of Meiosis I note the similarities and differences to mitosis

3. Meiosis II a. Performed by the two new cells produced in Meiosis I each of the new haploid cell’s chromosomes contains 2 unique chromatids b. DNA does not replicate c. Prophase through cytokinesis occurs similar to mitosis d. Four unique haploid (N) cells are formed two from each of the cells produced in meiosis I

Males vs. Females there is a difference! For animals, meiosis in males produces 4 sperm from every germ cell vs. one egg for females 3 polar bodies donate cell materials (except DNA) and disintegrate

Link to Meiosis Animation Compare Mitosis and Meiosis IV. Mitosis vs. Meiosis 1. Mitosis Asexual reproduction Begins with a diploid cell (2N) Results in the formation of 2 diploid cells (2N) that are genetically identical to each other and to the original cell 2. Meiosis Required for sexual reproduction Begins with a diploid cell (2N) Results in the formation of 4 haploid cells (N) that are genetically different from each other and the original cell