Meiosis and Sexual Reproduction

1. Meiosis and Sexual Reproduction

2. Meiosis • Meiosis • specialized cell division process • produces haploid gametes • Each gamete receives 1 member of each pair of homologous chromosomes

3. Homologous Chromosomes hopes.stanford.edu

4. Meiosis Separates Homologues • Meiosis consists of 1 round of DNA replication, followed by 2 rounds of nuclear divisions • These events occur in 2 stages: • Meiosis I • Meiosis II

5. Replication Before Meiosis • Both members of each homologous chromosome pair are replicated prior to meiosis • After replication, each chromosome consists of 2 sister chromatids

6. Meiosis I • Each daughter cell receives one member of each pair of homologous chromosomes

7. Meiosis II • Sister chromatids separate into independent chromosomes • Each daughter cell receives one of these independent chromosomes

8. Fusion of Haploid Gametes • Meiosis reduces chromosome number by half, producing 1n gametes (eggs and sperm) • Fusion of gametes (fertilization) combines two chromosome sets to produce diploid (2N) zygote

10. Overview of Meiosis I & II • Phases of meiosis have same names as the phases in mitosis, followed by I or II to distinguish the two nuclear divisions that occur in meiosis

11. Overview of Meiosis I • Meiosis I separates homologous chromosomes into 2 haploid daughter nuclei

13. Overview of Meiosis II • Meiosis II separates sister chromatids into 4 gametes

15. Meiotic Prophase I 1. Homologous chromosomes pair up 2. Crossing over(genetic recombination) occurs between homologues • Enzymes facilitate exchange of DNA between arms of adjacent chromatids, producing chiasmata 3. Spindle microtubules assemble, nuclear envelope breaks down, and microtubules capture chromosomes

19. Meiotic Metaphase I • Duplicated homologous chromosomes are pulled into a line perpendicular to the spindle • Chromosomes line up as pairs of replicated homologous chromosomes

22. Meiotic Anaphase I • Meiotic Anaphase I • Homologous chromosome pairs separate • Each homologous chromosome pair moves to a pole, pulled by microtubules

23. Meiotic Telophase I • Meiotic Telophase I • Spindle microtubules disappear • Cytokinesis occurs • Nuclear envelopes may reappear • Chromosomes usually remain condensed

24. Meiosis II • Meiotic Prophase II • Spindle microtubules reform and capture duplicated chromosomes • Each chromatid contains a kinetochore • Meiotic Metaphase II • Duplicated chromosomes line up singly, perpendicular to the spindle • Meiotic Anaphase II • Chromatids separate • Meiotic Telophase II • Cytokinesis occurs, nuclear membranes reform, chromosomes relax

27. When Do Mitotic and Meiotic Cell Divisions Occur in the Life Cycles of Eukaryotes?

29. Haploid Life Cycles • Fungi and unicellular algae • Most of life cycle is haploid • Asexual reproduction by mitotic cell division produces a population of identical, haploid cells • Life cycle - Chlamydomonas

30. Diploid Life Cycles • Most animals • Most of cycle is in diploid state • Haploid gametes are formed by meiosis • Gametes fuse to form a diploid zygote • Zygote develops into adult through mitotic cell divisions

31. Alternation-of-Generation Cycles • Plants • Includes both multicellular diploid and multicellular haploid body forms • Multicellular diploid body gives rise to haploid spores, through meiosis • Spores undergo mitosis to produce a multicellular haploid generation

32. Alternation-of-Generation Cycles • Eventually, certain haploid cells differentiate into haploid gametes • 2 gametes fuse to form a diploid zygote • Zygote grows by mitotic cell division into a diploid multicellular diploid generation

33. How Do Meiosis and Sexual Reproduction Produce Genetic Variability?

34. Novel Chromosome Combinations • Genetic variability among organisms is essential in a changing environment • Mutations produce new variation but are relatively rare occurrences

35. Novel Chromosome Combinations • Randomized line up and separation of homologous chromosomes in Meiotic Metaphase I and Anaphase I increase variation • # of possible combinations is 2n, where n = number of homologous pairs Metaphase I Anaphase I

36. Crossing Over • Variation enhanced by genetic recombination • Crossing over creates chromosomes with new allele combinations • Combined with homologue shuffling in Metaphase/Anaphase I, each gamete produced in meiosis is virtually unique

37. Fusion of Gametes • Fusion of gametes from 2 individuals further increases possible 2n combinations • Gametes from 2 humans could produce about 64 trillion different 2n combinations • Taken together with crossing over, each human individual is absolutely genetically unique

38. The End