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  1. 1. Meiosis Chapter 12
  2. 2. Genetics <ul><li>“Like begets like” </li></ul><ul><li>Heredity: </li></ul><ul><ul><li>The continuity of traits from one generation to the next </li></ul></ul><ul><ul><li>This is why organisms resemble their parents </li></ul></ul><ul><li>Variation: </li></ul><ul><ul><li>Offspring exhibit individuality, different somewhat from their parents and their siblings </li></ul></ul>
  3. 3. Genes <ul><li>Gene: </li></ul><ul><ul><li>A hereditary unit that codes for genetic information </li></ul></ul><ul><ul><li>A segment of DNA </li></ul></ul><ul><li>Genome: </li></ul><ul><ul><li>The tens of thousands of genes we inherit from our parents make up our genome </li></ul></ul><ul><ul><li>All of genes put together make up our GENOME </li></ul></ul><ul><li>The DNA of a eukaryotic cell is subdivided into chromosomes, located in the nucleus of every cell </li></ul><ul><li>A gene’s specific location along the length of a chromosome is called the gene’s locus </li></ul><ul><ul><li>Like an address </li></ul></ul>
  4. 4. Asexual Reproduction <ul><li>A single individual is the sole parent and passes copies of all its genes on to its offspring </li></ul><ul><li>Since the offspring has the same genetic info as its parent, it is known as a clone </li></ul><ul><li>The only genetic variation is due to mutations </li></ul>
  5. 5. Sexual Reproduction <ul><li>Usually results in greater variation </li></ul><ul><ul><li>Two parents give rise to offspring that have unique combinations of genes inherited from both parents. </li></ul></ul><ul><ul><li>Offspring of sexual reproduction vary genetically from their siblings and both parents. </li></ul></ul><ul><ul><li>HOW?? </li></ul></ul><ul><ul><ul><li>MEIOSIS!!  </li></ul></ul></ul>
  6. 6. The Human Life Cycle <ul><li>Gametes vs. Somatic Cells </li></ul><ul><ul><li>Gametes: Sperm or ova cells </li></ul></ul><ul><ul><li>Somatic Cells: Any other cell besides sperm or ova (muscle, nerve, etc.) </li></ul></ul><ul><li>Every human somatic cell contains 46 chromosomes </li></ul><ul><li>Human gametes contain 23 chromosomes </li></ul>
  7. 7. Chromosomes <ul><li>A chromosome is a long coiled-up strand of DNA </li></ul><ul><li>Humans have 23 pairs of homologous chromosomes in each somatic cell </li></ul><ul><ul><li>Homologous Chromosomes: </li></ul></ul><ul><ul><ul><li>Chromosomes that have the same length, centromere position, and staining pattern </li></ul></ul></ul><ul><ul><ul><li>Carry genes controlling the same inherited characteristics (ie. Eye color, attached ear lobes) </li></ul></ul></ul><ul><ul><ul><li>One chromosome of each pair comes from each parent </li></ul></ul></ul><ul><li>Only exception… </li></ul><ul><ul><li>Male sex chromosomes (XY) </li></ul></ul><ul><ul><li>Female sex chromosomes (XX) </li></ul></ul>
  8. 8. Haploid vs. Diploid Cells <ul><li>Haploid cells contain a single set of chromosomes </li></ul><ul><ul><li>Abbreviated n </li></ul></ul><ul><ul><li>In humans, there are 23 chromosomes in a haploid cell </li></ul></ul><ul><ul><li>Gametes are the only haploid cells </li></ul></ul><ul><ul><li>Produced by meiosis </li></ul></ul><ul><li>Diploid cells contain a double set of chromosomes </li></ul><ul><ul><li>Abbreviated 2n </li></ul></ul><ul><ul><li>In humans, there are 46 chromosomes in a diploid cell </li></ul></ul><ul><ul><li>All cells other than gametes are diploid cells </li></ul></ul><ul><ul><li>Produced by mitosis </li></ul></ul>
  9. 9. Meiosis <ul><li>Big Picture: </li></ul><ul><ul><li>Replication of chromosomes (DNA) followed by two consecutive cell divisions </li></ul></ul><ul><ul><li>Results in 4 daughter cells, each with only half as many chromosomes as the parent </li></ul></ul>
  10. 10. Meiosis I: Separating Homologous Chromosomes <ul><li>Interphase I: </li></ul><ul><ul><li>Chromosome (DNA) replication occurs </li></ul></ul>
  11. 11. Meiosis I: Separating Homologous Chromosomes <ul><li>Prophase I: </li></ul><ul><ul><li>Chromosomes begin to condense (you can see them!) </li></ul></ul><ul><ul><li>Synapsis occurs: </li></ul></ul><ul><ul><ul><li>Homologous chromosomes come together (4 sister chromatids total) </li></ul></ul></ul><ul><ul><li>Chiasmata </li></ul></ul><ul><ul><ul><li>Chromatids cross over at various places, known as chiasmata </li></ul></ul></ul><ul><ul><ul><li>An exchange of genetic information occurs </li></ul></ul></ul><ul><ul><li>Nucleoli and nuclear envelope disappear </li></ul></ul><ul><ul><li>Spindle microtubules begin to form </li></ul></ul>
  12. 12. Meiosis I: Separating Homologous Chromosomes <ul><li>Metaphase I: </li></ul><ul><ul><li>Chromosomes are arranged on the metaphase plate in homologous pairs </li></ul></ul>
  13. 13. Meiosis I: Separating Homologous Chromosomes <ul><li>Anaphase I: </li></ul><ul><ul><li>The spindle fibers move the chromosomes toward the poles </li></ul></ul><ul><ul><li>Each homologous chromosome of a tetrad goes to opposite poles </li></ul></ul>
  14. 14. Meiosis I: Separating Homologous Chromosomes <ul><li>Telophase I: </li></ul><ul><ul><li>The chromosomes reach the poles of the cell </li></ul></ul><ul><ul><li>Cytokinesis occurs </li></ul></ul><ul><ul><ul><li>Animals: cleavage furrow </li></ul></ul></ul><ul><ul><ul><li>Plants: cell plate </li></ul></ul></ul>
  15. 15. Meiosis II: Separating Sister Chromatids <ul><li>Prophase II: </li></ul><ul><ul><li>Spindle apparatus re-forms (in some species) </li></ul></ul><ul><ul><li>Chromosomes move towards middle </li></ul></ul><ul><li>Metaphase II: </li></ul><ul><ul><li>Chromosomes line up on the metaphase plate </li></ul></ul><ul><li>Anaphase II: </li></ul><ul><ul><li>Centromeres of sister chromatids separate </li></ul></ul><ul><ul><li>Sister chromatids move toward opposite poles of the cell </li></ul></ul><ul><li>Telophase II/Cytokinesis: </li></ul><ul><ul><li>Nuclei form at opposite poles of the cell </li></ul></ul><ul><ul><li>Cytokinesis occurs </li></ul></ul><ul><ul><li>4 haploid daughter cells </li></ul></ul>
  16. 16. Mitosis vs. Meiosis <ul><li>Three key differences that occur during Meiosis: </li></ul><ul><ul><li>1. Prophase I </li></ul></ul><ul><ul><ul><li>Crossing over of nonsister chromosomes occurs, resulting in the exchange of genetic information </li></ul></ul></ul><ul><ul><li>2. Metaphase I </li></ul></ul><ul><ul><ul><li>Homologous pairs of chromosomes line up at the metaphase plate, instead of individual chromsomes </li></ul></ul></ul><ul><ul><li>3. Anaphase I </li></ul></ul><ul><ul><ul><li>Sister chromatids of a chromosome remain attached at the centromere and go to opposite poles </li></ul></ul></ul>
  17. 17. Crossing Over Results in Genetic Variation
  18. 18. Random Fertilization <ul><li>Random nature of fertilization adds to the genetic variation arising from meiosis </li></ul><ul><li>A zygote is made up of an ovum and a sperm cell </li></ul><ul><ul><li>8 million different possibilities for sperm </li></ul></ul><ul><ul><li>8 million different possibilities for ovum </li></ul></ul><ul><ul><li>64 trillion different diploid possibilities </li></ul></ul>