11-4 Meiosis

1. 11-4 Meiosis Describe the process of meiosis. Compare meiosis and mitosis.

2. Introduction • Meiosis – • A Process by which the number of chromosomes per cell is cut in HALF through the separation of homologous chromosomes in a diploid cell • Meiosis has TWO Phases: Meiosis I and Meiosis II • Starts: with ONE DIPLOID (2n) CELL • Ends: with FOUR HAPLOID (n) CELLS – that are NOT identical to each other or to the parent cell

3. Why do Cells Undergo Meiosis? • Gametes (aka sex cells: egg and sperm) need to be HAPLOID (n) so that when they combine, the new organism is DIPLOID (2n) instead of TETRAPLOID(4n)

4. Chromosome Number • Each parent has a full number of chromosomes, 46 in humans • During meiosis this number has to be cut in half or the human offspring would have 92 chromosomes • When the sperm (male sex cell) fertilizes the egg (female sex cell) the result is a zygote that contains the diploid number of chromosomes

5. Haploid: (1n) a cell with half the number of chromosomes • 23 chromosomes in human sex cells aka gametes • Diploid: (2n) a cell with the full number of chromosomes • 46 in humans in somatic human cells • Homologous chromosomes: two chromosomes, one from mom and one from dad. Each one has genes for the same trait. • Ex. B and b could be the dominant and recessive forms of hair color. One allele on each chromosomes located in the gene. One chromosome from each parent. This offspring is heterozygous for hair color Bb and heterozygous Aa.

6. The Phases of Meiosis • Consists of two separate cell divisions named Meiosis I and Meiosis II • Starts with one diploid cell and ends with four haploid cells called gametes (sex cells) • Meiosis I is known as REDUCTION

7. Meiosis I: REDUCTION: homologous chromosomes are separated • Interphase I: DNA is duplicated, chromatin & centrioles appear • Prep for cell division • DNA duplicates S phase • Cell grows • Organelle duplication • Very similar to interphase of mitosis

8. Meiosis I - Prophase I • Prophase I: Nuclear envelope and nucleolus disappears, chromatin condenses into chromosomes (sister chromatids), homologous chromosomes pair up (TETRADS), crossing over may occur • Synapsis- formation of tetrads - when homologous pairs find one another (this is the main difference between meiosis and mitosis) • Tetrad – two homologous chromosomes grouped together (total of FOUR chromatids) • Crossing over – process by which homologous chromosomes exchange pieces of chromatids forming new combinations

9. Meiosis I - Prophase I • Crossing Over - Chromatids of homologues exchange equal portions (usually) • Chiasma – cross-over exchange point • New chromatids are called recombinants

10. Meiosis I – Metaphase Iand Anaphase I • Metaphase I: Tetrads - Homologous chromosomes line up in the middle (the metaphase plate) • Anaphase I: Homologous chromosomes separate, are pulled apart towards the poles • Tetrads split – sister chromatids are still together, attached at centromere

11. Meiosis I: Telophase I and Cytokinesis • Telophase I: Homologous chromosomes gather at opposite poles • Nuclear membrane and nucleolus begin to reform • Chromosomes begin to unwind  chromatin • Followed by cytokinesis I • Cytokinesis: Cytoplasm divides • TWO new cells – NON IDENTICALS (crossing over) • Sister chromatids are still together

12. Meiosis II: Sister chromatids are separated (just like mitosis) • Just like mitosis – may be preceded by brief period of rest (Interkinesis or Interphase II) • NO DNAREPLICATION OCCURS

13. Meiosis II – SAME AS MITOSIS • Prophase II: same as mitosis (nuclear membrane and nculeolus disappear, centrioles move to opposite poles, spindle fibers stretch across cell , chromatin condenses  chromosomes • Metaphase II: sister chromatids line up in the middle/equator • Anaphase II: Centromeres divide, sister chromatids separate and move apart to the poles • Telophase II: reverse of prophase - nuclear envelope and nucleolus reform, chromosomes uncoil  chromatin, cytokinesis begins • Result: 4 haploid cells, genetically different

14. Meiosis – End Result • Start  1 diploid (2n) parent cell • End  4 haploid (n) daughter cells • Each cell contains HALF as much DNA as the original parent cell • Each cell is genetically different (they are NOT all identical)

15. Meiosis and Genetics • Genetic Reassortment – leads to greater genetic variation (why you have characteristics from both your parents) • Sources • Crossing Over (synapsis) – new combinations of genes on chromosomes created by crossing over during prophase I • Results in creation of chromosomes with genes from both parents on them “hybrid” mom-dad chromosome • Humans average crossing over 2-3X per homologous pair • Independent Assortment – new assortment of chromosomes, when chromosomes line up at the equator during metaphase I – it is completely random and lead to LOTS of different combinations • Fertilization – random meeting of sperm and egg

16. Gamete Formation • Males = • Spermatogenesis – “birth” of sperm • 4 equal size sperm produces for every meiotic division • Females = • Oogenesis – “birth” of egg (largest cell in female body) • 1 egg and 3 polar bodies produced for every meiotic division

17. Comparing Mitosis and Meiosis

18. 1 1 1 2 2 4 2n n Sister chromatids Homologous then sister chromatids identical different Production of somatic cells Production of gametes