Gametes sex cells Gametes fuse fertilization zygote Gametes are formed by meiosis Somatic cells all other cells but sexual cells Every species has a specific number of chromosomes Humans have 46 chromosomes or 23 pairs (one from each parent) Homologous chromosomes the pairs of chromosomes, similar in size, shape and genetic material. Diploid cell that has both pairs of homologous chromosomes (2N) Haploid one set of chromosomes (N) Cell division that results in gametes with the number of chromosomes as the parent cell Process of reduction because the homologous chromosomes are separated Have meiosis I and meiosis II End result produces 4 haploid cells Have Interphase first! Same as mitosis DNA replication occurs Prophase 1 Chromosomes pairs with its corresponding homologous chromosome to form a tetrad They are fastened together at their centromeres There are 4 chromatids in a tetrad Crossing over will occur with the homologous chromosomes The chromosomes will exchange portions of their chromatids Nuclear membrane disappears and spindle fibers form Section 11-4 Crossing-Over They do the genetic TANGO! Metaphase 1 Centromeres of the tetrads line up on the equator in pairs Tetrads are hooked to the spindle microtubules at the centromeres Pairs are connected to opposite poles Anaphase 1 Spindles help move to poles The homologous chromosomes of each tetrad separate from each other and move to opposite ends of the cells Sister chromatids attached at centromeres move together Anaphase 1 Disjunction process of separation There are as many chromosomes as in the original cell but the chromosome is double stranded. Telophase I Movement of homologous chromosomes continues until there is a haploid set at each pole Each chromosome = linked sister chromatids Nuclear membrane reforms Cytokinesis Divides into haploid daughter cells Section 11-4 Figure Meiosis Meiosis I There is no interphase Each chromosome has 2 chromatids Each daughter cell now has the haploid number (just 2 chromosomes each) Same as Mitosis; except having non- identical chromatids. Meiosis II Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. Prophase II Metaphase IIAnaphase IITelophase II The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells. Section 11-4 Figure Meiosis II Prophase II- Each daughter cell forms a spindle and the double stranded chromosome moves toward the center Metaphase II Chromosomes line up on the metaphase plate Chromosome is still represented as chromatids Chromatids are fastened to the spindle microtubules Anaphase II The 2 chromatids are separated and move to opposite poles. Telophase II Both daughter cells divide so the product is 4 haploid cells Nuclear membrane reforms Cytokinesis Cytoplasm divides The Overview of Meiosis Mitosis produces 2 identical diploid cells. Meiosis produces 4 genetically different haploid cells. -Is the production of gametes -at the end of Meiosis the haploid cells are incapable of fertilization until they go through more changes. Spermatogenesis: -In males; production of 4 sperm cells -Is much smaller than the female egg. -main contribution is DNA. -needs to be able to move, swim to an egg to fertilize it. Oogenesis -In females; production of 1 egg cell and 3 polar bodies. -large than the sperm. -not only contributes DNA, but contains all the other components needed for the embryo to grow. Egg Sperm