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Meiosis Meiosis

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Meiosis Meiosis MEIOSIS MEIOSIS KEY CONCEPT Gametes have half the number of chromosomes that body cells have. MEIOSIS : CELL TYPES You have Body cells and Gametes Body cells are also called somatic cells. Germ cells develop into gametes. Germ cells are located in the ovaries and testes. Gametes are sex cells: egg and sperm. Gametes have DNA that can be passed to offspring. MEIOSIS : ALL ABOUT CHROMOSOMES Your cells have autosomes & sex chromosomes • Your body cells have 23 pairs of chromosomes. – Homologous pairs of chromosomes have the same structure and genetic material. – For each homologous pair, one chromosome comes from each parent. • Chromosome pairs 1-22 are autosomes. • Sex chromosomes, X and Y, determine gender in mammals. MEIOSIS : DIPLOID AND HAPLOID Diploid (2n) cells have two copies of every chromosome. Body cells are diploid. Half the chromosomes come from each parent. In humans this means we have 2 sets of 23 chromosomes (total of 46) MEIOSIS : DIPLOID AND HAPLOID Haploid (n) cells have one copy of every chromosome. – Gametes are haploid. – In humans gametes have 22 autosomes and 1 sex chromosome. (23 total) MITOSIS REVIEW Goal of Mitosis is Chromosome number must be maintained. • Mitosis and meiosis are types of nuclear division that make different types of cells with different number of chromosomes. • Mitosis makes more diploid cells. • Number of Chromosomes in parent is Equal to the number of chromosomes in offspring. INTRODUCTION TO MEIOSIS Meiosis makes haploid cells from diploid cells. This happens because cells go through TWO rounds of division during meiosis. Meiosis reduces chromosome number and creates genetic diversity. Meiosis Occurs in Sex cells Meiosis Produces gametes. MEIOSIS DETAIL KEY CONCEPT During meiosis, diploid cells undergo two cell divisions that result in haploid cells. homologous chromosomes Meiosis I and Meiosis II Meiosis I and meiosis II each have four phases, similar to those in mitosis. Biggest difference is the how the DNA organizes for each division. In Meiosis I Pairs of homologous chromosomes form tetrads. sister sister chromatids chromatids MEIOSIS DETAIL KEY CONCEPT During meiosis, diploid cells undergo two cell divisions that result in haploid cells. Meiosis I pairs of homologous chromosomes (tetrads) separate. Homologous chromosomes are similar but not identical genetically In Meiosis II SISTER CHROMATIDS separate. MEIOSIS I DETAILS • Meiosis I occurs after DNA has been replicated. • Meiosis I divides homologous chromosomes in four phases. Prophase I Metaphase I Anaphase I Telophase I PROPHASE I Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. There are 4 chromosomes in a tetrad. The pairing of homologous chromosomes is the key to understanding meiosis. Crossing-over may occur here Crossing-over is when chromosomes overlap and exchange portions of their chromatids. CROSSING OVER !!!!! They do the genetic TANGO! PROPHASE I (PICTURE) Homologous Chromosomes (called Tetrads) are held together by a synapse METAPHASE I Tetrads line up along the equator Spindle fibers attach to the centromeres of each chromosome in the tetrad. ANAPHASE I The cell begins lengthening. The spindle fibers pull the homologous chromosomes apart and toward opposite ends of the cell. Sister chromatids attached at centromeres move together Disjunction Occurs – process of separation There are ½ 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 At each pole, now, there is a complete haploid set of chromosomes (but each chromosome still has two sister chromatids). Nuclear membranes reforms. A cleavage furrow appears. The cell separates into 2 daughter cells. TELOPHASE I IMPORTANT NOTE IN BETWEEN MEIOSIS I and MEIOSIS II there is no INTERPHASE!!!! PROPHASE II • Meiosis I results in two haploid (N) cells. • Each cell has half the number of chromosomes as the original cell. • Spindle Reforms • Sister Chromatids are present METAPHASE II The Sister Chromatids line up .along the equator (middle) of the cell similar to metaphase in mitosis ANAPHASE II Sister chromatids separate and move to opposite ends of the cell. TELOPHASE II • Nuclear membranes form around each set of chromosomes at opposite ends of the cell • Spindle fibers break apart • Cell undergoes cytokinesis • End result is four HAPLOID cells with genetically unique information. COMPARE AND CONTRAST • Meiosis differs from mitosis in significant ways. – Meiosis has two cell divisions while mitosis has one. – In mitosis, homologous chromosomes never pair up. – Meiosis results in haploid cells; mitosis results in diploid cells. GAMETOGENESIS Haploid cells develop into mature gametes. • Gametogenesis is the production of gametes. • Gametogenesis differs between females and males. In Males: SPERMATOGENESIS – Sperm become streamlined and motile. – Sperm primarily contribute half of the DNA to an embryo. GAMETOGENESIS In Females: Oogenesis – Eggs contribute half of DNA, all of the cytoplasm, and all organelles to an embryo. – During meiosis, the egg gets most of the contents; the other cells form polar bodies. – So only one functional gamete per cycle of meiosis .
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