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MEIOSIS Chapter 10.2 Meiosis This kind of cell division produces gametes containing half the number of chromosomes as a parent’s body cell Meiosis consists of 2 separate divisions known as Meiosis I and Meiosis II Diploid cells Diploid means 2 sets Cell that contains both sets of homologous chromosomes (one set from each parent) 2n Haploid cells Gametes or reproductive cells Haploid means one set Contain only a single set of chromosomes n Homologous chromosomes Paired chromosomes 2 chromosomes of each pair in a diploid cell Each of a pair of homologous chromosomes has genes for the same traits Meiosis I Begins with one diploid (2n) cell During meiosis, a spindle forms and the cytoplasm divides in the same ways they do during mitosis. However, what happens to the chromosomes in meiosis is very different Phases: Prophase I, Metaphase I, Anaphase I, & Telophase I Prophase I Chromosomes coil up and spindle forms Each pair of homologous chromosomes comes together, matched gene by gene to form a 4 part structure called a tetrad. Tetrad consists of 2 homologous chromosomes, each made up of 2 sister chromatids Crossing over Tetrads are paired so tightly that non-sister chromatids from homologous chromosomes sometimes actually exchange genetic material in a process known as crossing over. Crossing over results in new combinations of genes on a chromosome. Metaphase I Centromere of each chromosome becomes attached to a spindle fiber The spindle fibers pull the tetrads into the middle of the spindle Homologous chromosomes are lined up side by side as tetrads Anaphase I Homologous chromosomes, each with its 2 chromatids, separate and move to opposite ends of the cell. Centromeres holding sister chromatids together do not split Telophase I Spindle is broken down, the chromosomes uncoil and cytoplasm divides to yield 2 new cells. Each cell has only half the genetic information of the original cell Another cell division is needed because each chromosome is still doubled, consisting of 2 sister chromatids http://highered.mcgrawhill.com/sites/9834092339/student_view0/chapter11/meiosis_i.html Meiosis II Ends with 4 haploid (n) cells These haploid cells are called sex cells or gametes Male = sperm Female = egg When a sperm fertilizes an egg, a zygote results with a diploid number of chromosomes. Zygote can then develop by mitosis into a multicellular organism. Phases: Prophase II, Metaphase II, Anaphase II, Telophase II Newly formed cells in some organisms undergo a short interphase in which the chromosomes do not replicate In other organisms, however, the cells go from late anaphase of meiosis I directly to metaphase of meiosis II, skipping telophase I, interphase & prophase II. Prophase II A spindle forms in each of the 2 new cells and the spindle fibers attach to the chromosomes Prophase II Metaphase II The chromosomes, still made up of sister chromatids, are pulled to the center of the cell and line up randomly at the equator. Metaphase II Anaphase II Centromere of each chromosome splits, allowing the sister chromatids to separate and move to opposite poles Anaphase II Telophase II Nuclei reform Spindle breaks down Cytoplasm divides Each haploid cell contains one chromosome from each homologous pair These haploid cells will become gametes, transmitting the genes they contain to offspring Telophase II http://highered.mcgrawhill.com/sites/9834092339/student_view0/chapter11/meiosis_ii.html Cells that are formed by mitosis are identical to each other and to the parent cell Meiosis however, provides a mechanism for shuffling the chromosomes and the genetic info they carry. By shuffling the chromosomes, genetic variation is produced