1. Genetics Lecture II Meiosis The formation of gametes aka. Gametogenesis

2. Biology Standards Covered  2a~ students know meiosis is an early step in sexual reproduction in which the pairs of chromosomes separate and segregate randomly during cell division to produce gametes containing one chromosome of each type  2b~ students know only certain cells in a multi-cellular organism undergo meiosis

3. Biology Standards Covered  2c~ students know how random chromosome segregation explains the probability that a particular allele will be in a gamete  2d~ students know new combinations of alleles may be generated in a zygote through the fusion of male and female gametes (fertilization)  2e~ students know why approximately half of an individual’s DNA sequence comes from each parent

4. Vocabulary  Homologous Chromosomes – two chromosomes that are similar in appearance carrying the same genes but may have different alleles for those genes  Haploid – a cell having only one “set” of chromosomes  Diploid – a cell that contains both “sets” of chromosomes (one from each parent)

5. Meiosis I and II  Gamete production happens in two stages and produce haploid gametes  Meiosis I –Prophase I –Metaphase I –Anaphase I –Telophase I  Meiosis II –Prophase II –Metaphase II –Anaphase II –Telophase II

6. of Meiosis I Prophase I   The nucleus breaks down and the chromosomes duplicate   Homologous chromosomes pair up and “crossing over” happens Diploid (2N)

7. Chromosome Duplication  Chromosome duplication occurs during the S- phase of the Cell Cycle  This takes the genetic information and makes an exact copy “bonded” by a centromere  The two copies of the chromosome are now called “sister chromatids”

8. Crossing Over  This is the process by which pairs of homologous chromosomes exchange alleles by allowing their “inside sister chromatids” to ‘touch’  This process increases genetic variety by 50%!  Genetic variation – differences in genetic information within a population allowing for maximum survival in a rapidly changing world

9. of Meiosis I Spindle fibers arrange the chromosomes along the center of the cell in homologous pairs This prepares them for separation Diploid (2N) Metaphase I

10.   Spindle fibers attached to the centromeres of the duplicated chromosomes “pull” the entire chromosome to opposite ends of the cell   This process separates the homologous pairs of Meiosis I Diploid (2N) Anaphase I

11. Telophase I of Meiosis I   The cell divides into two haploid (n) cells having ½ of the original chromosomes   Each cell has a unique combination of 23 duplicated chromosomes Haploid (n)

12. of Meiosis II Prophase II   The nuclear membrane breaks down and the spindle fibers reform from the centrioles on opposite sides of the cell   Both cells are still haploid (n) containing a unique ½ of the organism’s complete chromosomes Haploid (n)

13. Metaphase II of Meiosis II * The duplicated chromosomes are arranged along the middle of the cell by the spindle fibers Each chromosome still has 2 sister chromatids at this stage Spindle fibers are attached to the centromere of each duplicated chromosome Haploid (n)

14. of Meiosis II Anaphase II   The sister chromatids are “pulled” apart by the spindle fibers bringing each ½ to opposite sides of the cell   Because crossing over occurred, each sister chromatid carries “unique” genetic information Haploid (n)

15. of Meiosis II Telophase II   The final stage of Meiosis II is Telophase II   Meiosis II results in four haploid (n) cells that are genetically unique!   All four have a unique combination of genes from both parents – –In Males, each of the cells becomes a unique sperm cell

16. Gametogenesis in Animals