Remember! Homologous Chromosomes

1. Remember! Homologous Chromosomes • Paired chromosomes; both chromosomes in a pair carry the same genes! • Control the same inherited characteristics • Homologous – same information Diploid (2n) 2n=4 1 copy from mom & 1 copy from dad! Human cells are 2n = 46! (23 from mom & 23 from dad)

3. Cell Division & Asexual Reproduction • Mitosis produces cells with the same information! • Identical daughter cells • Clones • Same amount of DNA

4. Asexual Reproduction • Binary Fission • Prokaryotes (bacteria) • Budding • Mostly single-celled eukaryotes (yeasts, amoebas, etc.)

5. What are the advantages of asexual reproduction? Are there disadvantages?

6. What about us? • What if a complex, multicellular organism (like us) wants to reproduce? • Joining of egg & sperm Are egg & sperm cells made by mitosis? If they were, then… 46 46 92 Egg Sperm Zygote It doesn’t work!

7. So, how do we make sperm & egg cells? • Must reduce the number of chromosomes in the cell from 46 chromosomes  23 chromosomes! • Must reduce the number of chromosomes by half Egg Zygote Sperm Fertilization

8. Gametogenesis – Gamete Formation • In male animals, the haploid gametes are called sperm. • In the female animals, the haploid gametes are called eggs. • The process in which male & female gametes join is called fertilization. • This produces a fertilized egg called a zygote!

9. Meiosis & Sexual Reproduction

10. Meiosis • The process that divides 1 diploid (2n) cell to form 4 haploid (n) cells • How gametes (sex cells) are formed • Meiosis has 2 divisions: Meiosis I and Meiosis II

11. Meiosis – Production of Gametes (Sperm & Egg) • Chromosome number must be reduced • Diploid  Haploid • 2n  n • In humans, 46  23 • Fertilization restores the chromosome number • Haploid  Diploid • n  2n

12. Sexual Reproduction Life Cycle • 2 copies • Diploid (2n) Fertilization Meiosis • 1 copy • Haploid (n) • 1 copy • Haploid (n) Gametes Gametes

13. Meiosis • Before meiosis begins, cells go through interphase! • DNA Replication occurs in interphase, forming a duplicate copy of each chromosome.

14. Meiosis • After DNA Replication, each chromosome is made of 2 sister chromatids!

15. Meiosis – 2 Cell Divisions • Meiosis I • Reduces Diploid (2n)  Haploid (n) • Separates homologous pairs • Meiosis II • Sperm & egg formed • Separates sister chromatids Warning! Meiosis evolved from mitosis, so the stages & “machinery” are similar, but the processes are different! Do NOT confuse them!

16. Overview of Meiosis (I.P.M.A.T.P.M.A.T.)

17. 2n = 4 1 copy of each homologous pair Overview of Meiosis I 2n = 4 Now sister chromatids Reduction 1n = 2 Homologous pairs separated, sister chromatids remain

18. Meiosis I – similar to mitosis! Prophase I • Centrioles move to opposite sides of the nucleus • Nuclear envelope breaks down.

19. Prophase I 3. Homologous chromosomes pair up. This is different than mitosis! • Each pair of chromosomes align & a tetrad is formed.

20. Prophase I • Chromosomes exchange portions of their chromatids in a process called crossing over. • Crossing over produces new combinations of DNA!

21. Here is a tetrad formed from 2 sets of sister chromatids. Gene A – Height (“A” is tall & “a” is short) Gene B – Hair Color (“B” is brown hair & “b” is blonde) The gametes produced can carry several varieties of genes; they’re not identical to the parent cell! This person can pass on tall/brown traits, tall/blonde traits, short/brown traits, or short/blonde traits!

22. Metaphase I • Spindle fibers attach to the chromosomes at the centromere.

23. Anaphase I • Spindle fibers pull the homologous chromosomes toward opposite ends of the cell. • Chromosomes move & separate into 2 groups near the spindle. • Anaphase ends when the chromosomes stop moving.

24. Telophase I • Nuclear membranes form around chromosomes. Remember! Cytokinesis is when the cytoplasm divides!

25. Meiosis I End Results • Produces 2 haploid (n) daughter cells that have only 1 set of each chromosome • Each set is made of 2 sister chromatids!

26. Meiosis I

27. Meiosis II • After Meiosis I, cells enter Meiosis II: • No DNA Replication occurs between Meiosis I & Meiosis II • Meiosis II separates the sister chromatids.

28. Prophase II • Spindle fibers appear.

29. Metaphase II • Chromosomes align like they do in mitosis & attach to the spindle at the centromere.

30. Anaphase II • Sister chromatids separate & move towards opposite ends of the cell.

31. Telophase II • Nuclear membranes form around chromosomes. • Then, cytokinesis!

32. Meiosis II End Results • Produces 4 haploid (n) daughter cells

34. What are the advantages to sexual reproduction? • Increased diversity • Independent assortment of chromosomes: random alignment of homologous chromosomes in Metaphase 1 • Crossing over: mixing of alleles across homologous chromosomes • Random fertilization – which sperm fertilizes which egg? • Driving evolution • Providing variation for natural selection

35. Variation from Independent Assortment of Chromosomes Random assortment in humans produces 223 (8,388,608) different combinations in gametes!

36. Variation from Crossing Over • Creates completely new combinations of traits on each chromosome

37. Variation from Random Fertilization • Sperm + Egg = ? • Any 2 parents will produce a zygote with over 70 trillion possible diploid combinations!

38. Sexual reproduction allows us to maintain both genetic similarity & differences. Jonas Brothers

39. Are there any disadvantages to sexual reproduction?