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Meiosis What now? • We talked about mitosis which is how 99% of your cells divide • What about your gametes? Do they divide? • Yes, they do! • Meiosis – the process of forming haploid gametes from diploid cells • Haploid – have half of the chromosomes • Diploid – have a full set of chromosomes Meiosis • Meiosis has two phases: Phase 1 and Phase 2 • The stages are the same as mitosis now they are followed by 1 and 2 • Example: Prophase 1, Metaphase 1, Anaphase 1, Telophase 1, Cytokinesis, Prophase 2, Metaphase 2, Anaphase 2, Telophase 2 and Cytokinesis • There is no stop at interphase so the DNA doesn’t replicate again!!! Meiosis 1 • Prophase 1 – the first stage of meiosis 1 • A. The nuclear membrane breaks down • B. The centrioles move from the center of the cell to opposite ends • C. Sister chromatids pair up in the cells • D. Crossing over occurs because the chromosomes are close together Metaphase 1 / Anaphase 1 • Metaphase 1 – the chromosomes line up in the middle of the cell • Anaphase 1 – the chromosomes separate and move to the opposite ends of the cell Telophase 1 • • • • • • Again – the opposite of Prophase 1 A. The spindles break down B. Chromosomes uncoil and form two nuclei C. The cell divides Cytokinesis occurs BUT no interphase occurs It goes straight to Prophase 2 Prophase 2 • Prophase 2 is the same as Prophase 1 • A. Chromosomes condense • B. Spindles form in each cell and attach to the chromosomes • Nuclear Membrane breaks down • Crossing over doesn’t occur this time Metaphase 2 / Anaphase 2 • Metaphase 2 – the chromosomes line up in the center of the cell • Anaphase 2 – the chromosomes split apart and move to opposite poles Telophase 2 • • • • • Telophase 2 – opposite of prophase A. Nuclear membranes reform B. Spindles break down C. Cells divide Cytokinesis occurs and splits into 4 separate cells The Purpose of Meiosis •To create 4 new cells from 1 cell. Each new cell has ½ of the chromosomes as the first cell. Compare the Two… Mitosis • Happens in all cells • Creates 2 cells • Cells are diploid • Humans – have 46 chromosomes per cell Meiosis • Happens in sex cells • Creates 4 cells • Cells are haploid • Humans – have 23 chromosomes per cell