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MEIOSIS!!! (A.K.A formation of gametes) 11.4 (p.275) 1.Genes are located on chromosomes in the cell’s nucleus. Mendel’s principles of genetics require at least two things: 2. *Each organism must inherit a single copy of every gene from both its “parents” * When an organism produces its own gametes, those two sets of genes must be separated from each other, so that each gamete contains just one set of genes. 3. Using the fruit fly, Drosophila as an example, we can see how this works. * The body cells of the fruit fly have 8 chromosomes. = maternal chromosomes = paternal chromosomes *It got 4 from its male (paternal) parent and 4 from its female (maternal) parent. *These two sets of chromosomes are homologous, meaning that each of the 4 chromosomes that came from the male parent has a corresponding chromosome from the female parent. 4. A cell that contains both sets of homologous chromosomes is said to be diploid. The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N. Diploid cells contain two complete sets of chromosomes and two complete sets of genes!!! Cells of an adult organism contain two copies of each gene. 5. The GAMETES, however, contain only a single set of chromosomes and therefore a single set of genes. Such cells are said to be haploid. This is usually written as N or 1N. 6. Haploid gametes are produced by the process of Meiosis!! Meiosis: a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell. 7. By the end of meiosis, the one diploid cell has become 4 haploid cells!! 8. Prior to Meiosis I, each chromosome is replicated. Then the cells begin to divide in a way that looks similar to mitosis. BUT, during prophase of Meiosis I, each chromosome pairs with its corresponding homologous chromosome to form a structure called a tetrad. There are 4 chromatids in a tetrad. An overview of meiosis: The Stages of Meiosis Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Figure 13.7 Haploid cells with unreplicated chromosomes IMPORTANT!!! As homologous chromosomes pair up and form tetrads in Meiosis I, they exchange portions of their chromatids in a process called crossing-over!! Crossing-over results in the exchange of alleles between homologous chromosomes and produces new combinations of alleles. Replication of homologous chromosomes and formation of tetrad www.synapses.co.uk Crossing-Over 9. What happens next? The homologous chromosomes separate and two new cells are formed. Each cell has 4 chromatids, but SOMETHING is different. Because each pair of homologous chromosomes was separated, neither of the daughter cells has the two complete sets of chromosomes it would have in a diploid cell. Those two sets have been shuffled and sorted like a deck of cards! THE TWO CELLS PRODUCED BY MEIOSIS I HAVE SETS OF CHROMOSOMES THAT ARE DIFFERENT FROM EACH OTHER AND FROM THE DIPLOID CELL THAT ENTERED MEISOSIS I!!!!! DRAW, COLOR, AND LABEL MEIOSIS I on page 276…..be sure to show how the chromatids are changed after crossing over. 10.MEIOSIS II: the two cells produced by Meiosis I now enter a second meiotic division! NEITHER cell goes through a round of chromosome replication before entering Meiosis II. Each of the cell’s chromosomes has 2 chromatids. During Metaphase II, chromosomes line up in the center of the cell. In Anaphase II, the paired chromatids separate. The result is 4 daughter cells which contain the haploid (N) number of chromosomes. 10.MEIOSIS II: the two cells produced by Meiosis I now enter a second meiotic division! NEITHER cell goes through a round of chromosome replication before entering Meiosis II. Each of the cell’s chromosomes has 2 chromatids. During Metaphase II, chromosomes line up in the center of the cell. In Anaphase II, the paired chromatids separate. The result is 4 daughter cells which contain the haploid (N) number of chromosomes. www.contexo.info/DNA_Basics/Meiosis.htm DRAW, LABEL, AND COLOR MEIOSIS II on page 277 11. In male animals, the gametes are called sperm. In females, gametes are called an egg. There are 4 haploid sperm produced in meiosis. In females, however, there is usually only 1 egg produced!! This occurs because of the uneven division of the cytoplasm at the end of Meiosis I and II. The other three “things” produced by female meiosis are called polar bodies. (they don’t participate in reproduction!) 12. Mitosis results in the formation of 2 diploid cells. Mitosis produces identical cell!! Mitosis allows an organism’s body to grow and develop. It is used in asexual reproduction Meiosis produces 4 haploid cells!!! The cells are genetically different!!!! It is how sexual reproducing organisms produce gametes!! Mitosis and Meiosis are VERY DIFFERENT! (only their names are similar) www.brhs.ab.ca A comparison of mitosis and meiosis MITOSIS MEIOSIS Chiasma (site of crossing over) Parent cell (before chromosome replication) MEIOSIS I Prophase I Prophase Chromosome replication Duplicated chromosome (two sister chromatids) Chromosome replication Tetrad formed by synapsis of homologous chromosomes 2n = 6 Chromosomes positioned at the metaphase plate Metaphase Sister chromatids separate during anaphase Anaphase Telophase 2n Tetrads positioned at the metaphase plate Homologues separate during anaphase I; sister chromatids remain together Metaphase I Anaphase I Telophase I Haploid n=3 Daughter cells of meiosis I 2n MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II Figure 13.9 Sister chromatids separate during anaphase II