Meiosis
- 1. Sexual Life Cycle • Made up of meiosis and fertilization • Diploid cells – Somatic cells of adults have 2 sets of chromosomes • Haploid cells – Gametes have only 1 set of chromosomes • Offspring inherit genetic material from 2 parents
- 3. • Life cycles of sexually reproducing organisms involve the alternation of haploid and diploid stages • Some life cycles include longer diploid phases, some include longer haploid phases • In most animals, diploid state dominates – Zygote first undergoes mitosis to produce diploid cells – Later in the life cycle, some of these diploid cells undergo meiosis to produce haploid gametes Sexual Life Cycle Stages
- 5. Features of Meiosis • Meiosis includes two rounds of division – Meiosis I and meiosis II – Each has prophase, metaphase, anaphase, and telophase stages • Synapsis – During early prophase I – Homologous chromosomes become closely associated – Includes formation of synaptonemal complexes • Formation also called tetrad or bivalents
- 6. • First meiotic division is termed the “reduction division” – Results in daughter cells that contain one homologue from each chromosome pair • No DNA replication between meiotic divisions • Second meiotic division does not further reduce the number of chromosomes – Separates the sister chromatids for each homologue Meiosis I vs Meiosis II
- 8. Prophase I • Chromosomes coil tighter and become visible, nuclear envelope disappears, spindle forms • Each chromosome composed of 2 sister chromatids • Synapsis – Homologues become closely associated – Crossing over occurs between nonsister chromatids
- 9. Crossing Over • Genetic recombination between nonsister chromatids • Allows the homologues to exchange chromosomal material • Alleles of genes that were formerly on separate homologues can now be found on the same homologue • Chiasmata – site of crossing over – Contact maintained until anaphase I
- 11. Metaphase I • Terminal chiasmata hold homologues together following crossing over • Microtubules from opposite poles attach to each homologue – Not each sister chromatid as in mitosis • Homologues are aligned at the metaphase plate side-by-side • Orientation of each pair of homologues on the spindle is random
- 12. Random Orientation of Chromosomes During Metaphase I
- 13. Anaphase I • Microtubules of the spindle shorten – Chiasmata break • Homologues are separated from each other and move to opposite poles – Sister chromatids remain attached to each other at their centromeres • Each pole has a complete haploid set of chromosomes consisting of one member of each homologous pair • Independent assortment of maternal and paternal chromosomes
- 14. Telophase I • Nuclear envelope re-forms around each daughter nucleus • Sister chromatids are no longer identical because of crossing over (prophase I) • Cytokinesis may or may not occur after telophase I • Meiosis II occurs after an interval of variable length
- 15. Meiosis II • Resembles a mitotic division • Prophase II: nuclear envelopes dissolve and new spindle apparatus forms • Metaphase II: chromosomes align on metaphase plate • Anaphase II: sister chromatids are separated from each other • Telophase II: nuclear envelope re-forms around 4 sets of daughter chromosomes; cytokinesis follows
- 17. Final Result • Four cells containing haploid sets of chromosomes • In animals, develop directly into gametes – sperm and eggs • In plants, fungi, and many protists, divide mitotically to produce greater number of gametes
- 18. Errors in Meiosis • Nondisjunction – failure of chromosomes to separate from one another as they move to opposite poles during either meiotic division • Aneuploid gametes – gametes with missing or extra chromosomes • Most common cause of spontaneous abortion in humans
- 19. Meiosis vs. Mitosis Meiosis is characterized by 4 features: 1. Synapsis and crossing over 2. Sister chromatids remain joined at their centromeres throughout meiosis I 3. Kinetochores of sister chromatids attach to the same pole in meiosis I 4. DNA replication is suppressed between meiosis I and meiosis II