BIO 105

1. BIO 105 Meiosis

2. Overview of Meiosis • Consists of 2 rounds of division unlike mitosis in which there is only one round of division • It produces haploid (1n) cells from diploid (2n) cells • Gametes are produced from somatic cells. • It involves 2 chromosome separations with only one chromosome replication. • There are three unique features of meiosis: • Synapsis • Homologous recombination • Reduction division

3. Overview (cont.) • Chromosomes were first discovered in 1882. • In 1887 it was observed that gametes had only half the number of chromosomes that somatic cells had. • Fertilization creates the first somatic cell called the zygote which is diploid (2n). • Reduction division proposed

4. Reduction Division • Meiosis reduces the number of chromosome to half that of somatic cells. • If fertilization occurred without RD, what would be result in humans? • In 10 generations, the # of chromosomes in a somatic cell would be 46 X 102 or ~47,000

5. Sexual Life Cycle

6. Stages of Meiosis • Meiosis I and II • Each stage is divided into subdivions • Prophase • Metaphase • Anaphase • Telophase • Prophase I is much more complex than prophase in mitosis.

7. Prophase I • We start with a pair of homologous chromosomes (homologs). • Each homologue has 2 sister chromatids. • The pair of homologues  have 4 chromatids.

8. 2 Homologues

9. Stages of Prophase I • Leptotene – chromosomes condense. • Zygotene • Pachytene • Diplotene • Diakinesis

10. 2. Zygotene • Just after chromosome replication and condensation, the homologous chromosomes become parallel to each other. • They then pair all along their length. • This process is called synapsis. • Synapsis is unique to Meiosis I (doesn’t occur in Meiosis II nor mitosis). • Synapsis occurs in zygotene.

11. Synapsis

12. Stages of Prophase I

13. 2. Zygotene (cont.) • A framework of proteins is formed between the 2 homologues. • This structure that spans and connects the homologues is the synaptonemal complex.

14. 3. Pachytene • Begins just after synaptonemal complex is formed (synapsis)….about 100nm across • This stage lasts for several days • Complex holds the homologue chromosomes in exact register….gene #1 on homologue #1 is in register with gene #1 on homologue #2 • The DNA within the complex begins to unwind. • Base pairings occur between complimentary strands of the homologues. • This structural interaction allows for exchange of genetic material ….crossing over.

15. 3. Pachytene

16. Crossing Over • Exchange of genetic material between paired homologues. • W/in synaptonemal complex are very large complexes of proteins called recombinant nodules….cut and paste machines. • ~ 2-3 exchanges occur per chromosome pair • When crossing over is complete, the synaptonemal complex begins to break down.

17. Crossing Over (cont.) • After complex breaks down, homologues are still held together where crossing over has occurred • This attachment is called a chiasma (X-shaped). • They are also held to gether by the centrosome. chiasma

18. Crossing Over Two crossovers

19. 4. Diplotene • Synaptonemal complex disassembles • DNA unwinds • Transcription begins again

20. 4. Diplotene

21. 5. Diakinesis • Chromosomes recondense • Transcription stops • Prelude to entering Metaphase I

22. Metaphase I • Nuclear envelope has disintegrated • Microtubules assemble into spindle apparatus (same as w/ mitosis) • Homologues line up on the metaphase plate. • The orientation of each pair is random….paternal homologue on right, female on left….or visa versa

23. Metaphase I • # of possible combinations = 2 raised to the power of the # of chromosomes in the genome • 23 = 8 • 223 =8,388,608 • This leads to independent assortment of maternal and paternal chromo- somes into the gametes.

24. Metaphase I • Chiasmata hold homologues together.

25. Metaphase I • Only one side of the centrosome (kinetochore) faces outwards toward microtubules

26. Metaphase I • This one sided attachment is different from mitosis where kinetochores on both sides of centromere bind to microtubules.

27. Anaphase I • Prophase I + Metaphase I take up about 90% of the total time of Meiosis I • Microtubules begin to shorten….breaking the chiasmata • Microtubule shortening pulls centromeres toward the poles….dragging chromosomes with them • Both sister chromatids are pulled because centromere fuses them together.

28. Telophase I • Homologues have clustered at opposite ends (poles) of the cell. • Nuclear membranes form around each cluster. • Remember….each chromosome in both nuclei replicated before entering Meiosis I. •  each chromosome contains 2 sister chromatids • BUT…sister chromatids are not identical because of crossing over…..different from mitosis. • Cytokinesis may or may not occur prior to entering Meiosis II.

29. Overview of Meiosis

30. Meiosis II • During interphase between I and II, there is no replication of DNA….unlike mitosis. • Prophase II – nuclear envelope breaks down and new spindles form • Metaphase II – microtubules attach to both sides of centromere • Anaphase II -- Microtubules shorten, splitting the centromere and moving sister chromatids in opposite directions. • Telophase II – nuclear envelopes reform around 4 sets of daughter chromosomes.

31. Meiosis Summary • Starts with diploid cell • 2 rounds of division = 4 haploid cells • 1n different from 2n because of crossover and independent assortment.