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Mitosis Recap Small part of the cell cycle During mitosis, the nucleus of the cell divides (nucleus contains DNA) Purpose to create two identical daughter cells that: grow and develop repair damaged tissue Mitosis Phases Prophase Nuclear envelope disintegrates chromatin coil and chromosomes become visible Metaphase Chromosomes (sister chromatids) line up in middle of cell Anaphase Sister chromatids (chromosomes) are pulled apart by spindle fibers Telophase Nuclear membrane forms again Cell contains two nuclei Remember! The result of mitosis is the creation of two identical daughter cells Mitosis is Asexual Reproduction Asexual Reproduction – one parent cell or organism makes an exact copy of itself Chromosomes Remember: All eukaryotic cells have a nucleus The nucleus contains DNA DNA coils up to form chromatin then chromosomes Chromosomes contain genes Genes are made up of DNA Gene: segment of DNA that codes for proteins Autosomes and Sex Chromosomes Chromosomes autosomes #1-22 are chromosomes that do not control gender Chromosome #23 are the sex chromosomes (males XY, females XX) Ponder This… If a new human is created from the union of two cells (one from the father and mother)… How then, do you avoid doubling your chromosome number each time? sperm (father) 46 egg 92 (mother) zygote (cell) Ponder This… If each of your cells contains two alleles (alternative forms of genes – like brown and blond hair) for each trait… How then, do you only inherit one from each parent? two alleles Bb two alleles BbBb four alleles??? Ponder This… If all the cells made by your mother are the same, & If all the cells made by your father are the same… Why don’t you and your siblings look exactly the same? Explanation The cells used to pass on genes from parent to offspring are significantly different than every other cell in your body. They contain half the normal number of chromosomes than a normal cell they are called sex cells (gametes) they are produced by meiosis, not mitosis. Somatic Cells vs. Gametes Two kinds of cells in your body: Somatic Cell – non-sex cells; all cells except egg and sperm cells Gametes – sex cells Females have egg cells Males have sperm cells Different #s of chromosomes in these cells: Somatic cells – diploid (2N) – 46 chromosomes Gametes – haploid (1N) – 23 chromosomes Diploid Cells that contain both sets of homologous chromosomes are said to be diploid (2n). The diploid number in… Drosophilia melanogaster is 8. (2n=8) Homo sapiens is 46. (2n=46) Cambarus clarkii (crayfish) is 200! (2n=200) All the cell types in your body (except for one type) are diploid cells. Haploid Cells that contain one set of homologous chromosomes are said to be haploid (n). Therefore, the haploid number in… Drosophilia melanogaster is 4. (n=4) Homo sapiens is 23. (n=23) Cambarus clarkii (crayfish) is 100! (n=100) Notice the haploid numbers are exactly ½ the diploid number Only one type of cells are haploid cells: the sex cells or gametes (sperm & egg) Ponder This… How can you avoid doubling your chromosome number each time? Each sperm & egg contain half the number of chromosomes as before. sperm (father) 23 egg 46 (mother) zygote (cell) Ponder This… How can you only inherit one from each parent? Each gamete only contains 1 allele from each parent. Bb b it works! Ponder This… Why don’t you and your siblings look exactly the same? Because not all of the cells produced are identical. Meiosis Haploid cells are produced through a process called meiosis Meiosis is the process of cell division in which the number of chromosomes per cell is cut in half. Chromosomes in a sex cell. Notice there are ½ as many as in a body cell Meiosis vs. Mitosis Meiosis Occurs in two phases: Mitosis Occurs in one phase. Meiosis I Meiosis II Genetic information is shuffled around. Results in four different daughter cells Genetic information remains the same. Results in two identical daughter cells. Phases of Meiosis Meiosis I: prophase I metaphase I anaphase I telophase I cytokinesis Phases of Meiosis Meiosis I: prophase I metaphase I anaphase I telophase I cytokinesis Meiosis II: prophase II metaphase II anaphase II telophase II cytokinesis Prophase I Chromosomes become visible (they coil) Nuclear membrane breaks down (disintegrates) Homologous chromosomes pair up in what is called a tetrad They then exchange parts in a process is called crossing over Metaphase I Homologous chromosomes line up with each other in the middle of the cell. Notice the difference between this stage in meiosis & mitosis. What do you notice? Meiosis: Mitosis: Anaphase I Homologous chromosomes move to opposite poles of the cell Telophase I Chromosomes gather at the poles of the cell. Cytokinesis The cytoplasm divides Both cells contain one chromosome from each pair of homologous chromosomes. * Chromosomes do not replicate between meiosis I and meiosis II. Anaphase I, Telophase I & Cytokinesis For the most part these processes are the same as mitosis. However, in Meiosis, because of the way the chromosomes were lined up during metaphase, neither of the daughter cells are identical. This is good! Meiosis: Mitosis: Meiosis II The second stage of meiosis is exactly like mitosis. Chromosomes line up, travel to opposite sides of the cell, and the cell splits. Prophase II A new spindle forms around the chromosomes. Metaphase II Chromosomes line up at the equator (middle) Chromosomes are attached at their centromere to the spindle fibers Anaphase II Centromeres divide Chromatids move to opposite poles of the cell Telophase II Nuclear envelope forms around each set of chromatids Spindle breaks down Cytokinesis Cell undergoes cytokinesis Result of meiosis is four haploid cells Each cell is genetically different from parent cell and other daughter cells! Importance of Meiosis The products of these two processes are very different: Mitosis: 2 identical daughter cells. Meiosis: 4 different daughter cells. Why are these differences in the cells important? Increases genetic variation in the offspring. THIS is the big idea. Remember this!! Recap! Meiosis Prophase II Prophase I Metaphase I Metaphase II Anaphase I Anaphase II Telophase I Telophase II
