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
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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

9. FIGURE 11-20 (part 2) Meiotic cell division is essential for sexual reproduction

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

12. FIGURE 11-21 (part 1) Meiotic cell division in an animal cell
In meiotic cell division (meiosis and cytokinesis), the homologous chromosomes of a diploid cell are separated, producing four haploid daughter cells. Each daughter cell contains one member of each pair of parental homologous chromosomes. In these diagrams, two pairs of homologous chromosomes are shown, large and small. The yellow chromosomes are from one parent (for example, the father), and the violet chromosomes are from the other parent (for example, the mother).

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

14. FIGURE 11-21 (part 2) Meiotic cell division in an animal cell
In meiotic cell division (meiosis and cytokinesis), the homologous chromosomes of a diploid cell are separated, producing four haploid daughter cells. Each daughter cell contains one member of each pair of parental homologous chromosomes. In these diagrams, two pairs of homologous chromosomes are shown, large and small. The yellow chromosomes are from one parent (for example, the father), and the violet chromosomes are from the other parent (for example, the mother).

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

16. FIGURE 11-21a Meiotic cell division in an animal cell
In meiotic cell division (meiosis and cytokinesis), the homologous chromosomes of a diploid cell are separated, producing four haploid daughter cells. Each daughter cell contains one member of each pair of parental homologous chromosomes. In these diagrams, two pairs of homologous chromosomes are shown, large and small. The yellow chromosomes are from one parent (for example, the father), and the violet chromosomes are from the other parent (for example, the mother).

17. FIGURE 11-22a The mechanism of crossing over

18. FIGURE 11-22c,d,e The mechanism of crossing over

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

20. FIGURE 11-21b Meiotic cell division in an animal cell
In meiotic cell division (meiosis and cytokinesis), the homologous chromosomes of a diploid cell are separated, producing four haploid daughter cells. Each daughter cell contains one member of each pair of parental homologous chromosomes. In these diagrams, two pairs of homologous chromosomes are shown, large and small. The yellow chromosomes are from one parent (for example, the father), and the violet chromosomes are from the other parent (for example, the mother).

21. FIGURE 11-23 Chromosome attachment to the spindle in mitosis

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 Meiotic Metaphase 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

25. FIGURE 11-21 (part 2) Meiotic cell division in an animal cell
In meiotic cell division (meiosis and cytokinesis), the homologous chromosomes of a diploid cell are separated, producing four haploid daughter cells. Each daughter cell contains one member of each pair of parental homologous chromosomes. In these diagrams, two pairs of homologous chromosomes are shown, large and small. The yellow chromosomes are from one parent (for example, the father), and the violet chromosomes are from the other parent (for example, the mother).

26. Table 11-1a A Comparison of Mitotic and Meiotic Cell Divisions in Animal Cells

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

28. FIGURE 11-25 The three major types of eukaryotic life cycles
The lengths of the arrows correspond roughly to the proportion of the life cycle spent in each stage.

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