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Review Chromosome Patterns At the end of mitosis’ telophase and cytokinesis each new cell has an unreplicated copy of each chromosome. The chromosomes look like sticks. During interphase each chromosome is replicated. The replicated chromosomes look like X’s and have 2 sister chromatids joined at a centromere. This diagram reviews the shapes of the chromosomes during mitosis. Meiosis – V2 Meiosis Meiosis is the process by which sex cells or gametes are made. This process is also called gametogenesis. During meiosis the number of chromosomes per cell is reduced by half. Meiosis has two distinct stages Meiosis I Meiosis II The starting cells in the ovary or testes are diploid and the chromosomes have replicated. Overview Note that the big difference from mitosis is during the first meiosis I metaphase, when the replicated chromosomes line up on the equator with their homologous chromosome – IN PAIRS! Prophase I Nuclear membrane dissolves, and the centrioles move to opposite poles. Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. There are 4 sister chromatids in a tetrad. The pairing of homologous chromosomes is the key to understanding meiosis. Crossing-over probably occurs here. Crossing-over is when homologous chromosomes overlap and exchange portions of their chromatids. Metaphase I Homologous pairs of chromosomes line up at the equatorial plate. Spindle fibers from the centrioles attach to the centromeres of the chromosomes. Anaphase I The spindle fibers pull the homologous chromosomes toward opposite ends of the cell. Telophase I Nuclear membranes might reform. The cell separates into 2 cells. Cytoplasm divides after telophase, resulting in 2 haploid daughter cells. Prophase II The centrioles are replicated. Centrioles in the 2 new cells move to opposite poles. New spindle fibres form and attach to centromeres. Metaphase II The chromosomes are in a single line on the equatorial plate. This is similar to metaphase in mitosis. Anaphase II Sister chromatids separate and move to opposite ends of the cell. Telophase II Cytoplasm separates, resulting in 4 haploid cells. Gamete Formation In males, meiosis or spermatogenesis results in 4 sperm cells In females, meiosis or oogenesis results in 1 egg cell for reproduction and three polar bodies, which are not used in reproduction. The one egg cell has all the cytoplasm, while the polar bodies only have a haploid copy of the chromosomes. Summary Comparing Mitosis and Meiosis Mitosis Starting # cells? Where does this occur? Are ending cells genetically identical? Ending # of cells? Ending N? Ending cells haploid or diploid? Meiosis Comparing Mitosis and Meiosis Starting # cells? Mitosis 1 diploid Meiosis 1 diploid Starting N? 4N 4N Where does this occur? Body or somatic cells Are ending cells Genetically genetically identical? identical Ending # of cells? 2 Ending N? 2N Ending cells haploid Diploid or diploid? Ovary or testes Genetically different 4 N Haploid