Meiosis and Sexual Reproduction

1. Meiosis and Sexual Reproduction Production of Haploid Cells Variation in Genetic Material

2. Sperm and Egg • Sperm trying to enter egg and leave packet of genetic information – i.e. chromosomes containing DNA. • Only one sperm will be successful • For zygote to have same number chromosomes as parents, chromosome number must be halved when egg and sperm made • Halving chromosome number done by MEIOSIS (produces haploid cells) • Diploid number restored when sperm fertilizes egg

3. Review What are the following? Answers Change in genetic makeup of species over time. Individuals with favorable traits are more likely to survive and reproduce Chromosomes similar in size, shape and genetic makeup Copied chromosomes still attached by centromere • Evolution • Natural selection • Homologous chromosomes • Chromatids

4. Review What are the following? Answers Cells with one set of chromosomes Cells with two sets of chromosomes A structure made of microtubules - helps organize and separate chromosomes Prophase, Metaphase, Anaphase, Telophase • Haploid Cells • Diploid Cells • Spindle Fibers • The steps of mitosis

5. What would happen if the number of chromosomes weren’t reduced

6. Formation of Haploid Cells MEIOSIS

7. Meiosis I 1st Meiotic Division

8. Before Meiosis 1, DNA replicated 1st step of Meiosis 1 Prophase 1 • Chromosomes condense • Watch it coil! • Nuclear membrane breaks down • Homologous chromosomes pair up • Crossing over (parts of chromatid on one homologous chromosome exchanged with corresponding part on other homologous chromosome) • Animation of Prophase 1

9. Meiosis 1, continued 2nd phase of Meiosis 1 Metaphase 1 • Pairs of homologous chromosomes are moved by spindle fibers to cell equator • Homologous chromosomes remain together • This is when independent assortment occurs • Animation of Metaphase 1

10. Meiosis 1, continued 3rd phase of Meiosis 1 Anaphase 1 • Homologous chromosomes separate • Chromosomes of each pair are pulled to opposite poles by spindle fibers • CHROMATIDS DO NOT SEPARATE AT THEIR CENTROMERES! • Each chromosome made of two RECOMBINED chromatids • Animation of Anaphase 1

11. Meiosis 1, continued 4th stage of Meiosis 1 Telophase 1 and cytokinesis • Individual chromosomes gather at each of the poles • Cytoplasm divides • Both cells contain 1 chromosome from each pair of homologous chromosomes • Animation of Telophase 1 and cytokinesis

12. Meiosis II 2nd Meiotic Division

13. Meiosis II 1st Phase of Meiosis II Prophase II • New spindle fibers form around the chromosomes • Animation of Prophase II

14. Meiosis II, continued 2nd phase of Meiosis II Metaphase II • Chromosomes line up along the equator and are attached at their centromeres to spindle fibers • Animation of Metaphase II

15. Meiosis II, continued 3rd phase of Meiosis II Anaphase II • Centromeres divide • Chromatids (now called chromosomes) move to opposite poles of cell • Animation of Anaphase II

16. Meiosis II, continued 4th phase of Meiosis II Telophase II and cytokinesis • Nuclear membrane forms around each set of chromosomes. • Spindle fibers break down • Cells undergo cytokinesis • There are now 4 haploid cells • Animation of Telophase II and cytokinesis

17. IMPORTANT! • The purpose of meiosis is to produce cells with half the genetic material as the parent cell

18. IMPORTANT! • The purpose of mitosis is to produce cells with same amount of genetic material as the parent cell

19. Meiosis and Genetic Variation Independent Assortment & Crossing Over  Genetic Variation

20. Meiosis = rapid generation of new genetic combinations How genetic variation arises Why don’t they all look alike? • Independent Assortment • Crossing Over • Random Fertilization – Random combination of 2 gametes

21. Why is Genetic Variation Important? • Evolution! • Pace of evolution increases as genetic variation increases • Genetic recombination increases evolution • 3rd type is produced – not identical to either parent • Genetic variation can bring out good traits or not so successful traits

22. Gamete Formation • Sperm production = spermatogenesis – meiosis results in production of 4 sperm • Egg production = oogenesis – meiosis results in production of 1 egg cell and 3 polar bodies because cytoplasm divides unequally • Polar bodies die • Egg lives • Egg = Ovum

23. Modeling Crossing Over Each person in group needs Procedure On two strips of the same color, write “A” , “B”, and “C” –one of the two homologous chromosomes On two strips of another color, write “a” ,“b” , “c” this is another the two homologous chromosomes Use scissors and tape to demonstrate crossing over between chromatids of homologous chromosomes • 4 paper strips – 2 of one color, 2 of another color – taped together (loosely) to represent chromatids joined by centromeres • Pen or pencil • Scissors • Tape • A textbook (you should end up with 4 textbooks in a group)

24. Modeling Crossing Over • What do the letters “A”, “B” and “C” and “a” , “b” , and “c” represent? • Why are the chromosomes you made homologous? • Compare the number of types of different chromatids (combinations of “A”, “B”, “C”, “a”, “b”, and “c”) before crossing over and after crossing over. • How does crossing over relate to genetic recombination? • During what phase of meiosis does crossing over occur?

25. Modeling Independent Assortment • In your groups of 4, line up homologous pairs as the chromosomes would line up during Metaphase I. Do this on top of two textbooks • How many different arrangements can you make? • What will happen during Anaphase 1? Will the 2 cells which are the result of Meiosis one contain the same genetic information? • Pull books apart to represent cytokinesis • How does independent assortment relate to genetic recombination

26. Modeling Meiosis II • You should now have 2 cells – each with 4 double chromosomes in them (there will be more or less chromosomes, depending on how many people are in your group) • Line up double chromosomes as in Metaphase II. Use another textbook to add to your “cytoplasm”. • Snip tape between the centromeres so that the single chromosomes can travel to opposite poles. Pull books apart to model cytokinesis • Do the same thing with your second cell • How many cells do you have as a result of Meiosis? Compare the chromosomes of your gametes. What do you notice?