Transcript Meiosis
I. Sexual Reproduction --two parents combine 1. The sex cell from the male parent is sperm. a. Sperm head is almost all nucleus (genetic info) Gametes: Sex Cells 2. The sex cell from the female parent is the egg a. Eggs are large and contain food material. Diploid / Haploid •Cells that have 2 sets of DNA are diploid. • Diploid is 2n. • All of your body cells are 2n •A sex cell with one set of DNA is haploid. • Haploid is n • sperm & egg (gametes) are haploid Fertilization-- joining of egg and sperm •n + n = 2n or 23 + 23 = 46 chromosomes (23 chromosomes from your dad join with 23 chromosomes from your mom (n=23).) • fertilization forms a zygote - grows to become an embryo by mitosis •The zygote has the diploid (2n = 46) chromosome number for that organism. •The zygote (or 1st body cell) does mitosis to grow and develop. Stages of Meiosis Prophase I 1. double stranded chromosome and spindle fibers appear. 2. The nuclear membrane and nucleolus disappear. 3. Tetrads form (4 chromatids). 4. Crossing over occurs. Metaphase I - The pairs of chromosomes (tetrads) line up in the center of the cell. Anaphase I - double stranded chromosomes (tetrad) separate from its twin. Each one is pulled to opposite ends of the cell. Telophase I - The cytoplasm divides and two cells form. Each chromosome is still double stranded. NOT FOLLOWED BY INTERPHASE! Prophase II - the double stranded chromosomes and spindle fibers reappear in each new cell. Metaphase II - the double stranded chromosomes move to the center of the cell: Anaphase II - The centromere divides and the chromatids separate and move to opposite ends of the cell. Telophase II - the spindle fibers disappear and a nuclear membrane forms around the chromosomes at each end of the cell. - followed by cytokinesis II Meiosis Animation Meiosis (sperm) (egg) III. The Importance of Sex Chromosomes A. Body cells have chromosomes that are found in pairs B. The 46 human chromosomes form 23 pairs of chromosomes C. The pairs form because the chromosomes are alike The 23rd pair determines the sex or gender of the child. XX= female XY= male