Meiosis

1. Meiosis • Overview of meiosis • Homologous chromosomes • Stages of meiosis • Spermatogenesis & oogenesis

2. A. Overview of Meiosis • Meiosis: • Specialized nuclear division in which the number of chromosomes is reduced by half • Purpose of meiosis: the formation of gametes • Occurs only in germ-line tissue • Diploid number: the number of chromosomes in a germ-line cell • Haploid number: the number of chromosomes in a gamete; ½ The diploid number

3. A. Overview of Meiosis • Zygote: • During the process of fertilization, a haploid gamete from one parent fuses with the haploid gamete from the other parent • The resulting diploid cell is called a zygote

4. A. Overview of Meiosis • Gametes: • In most sexually reproducing species, there are two distinct types of gametes • Spermatozoa (or pollen in plants) are compact, highly motile gametes that contribute their chromosomes to the zygote; “Male” gametes • Ova (or ovules in plants) are much larger and contribute both chromosomes and cytoplasm (the bulk of the cell mass) to the zygote; “Female” gametes

5. A. Overview of Meiosis • Gonads • In most multicellular species, germ-line tissue is found in organs called gonads • Spermatozoa are produced in gonads called testes • Ova are produced in gonads called ovaries • Sexually dimorphic species: two separate genders, with each individual having either male or female gonads • Sexually monomorphic species (hermaphroditic species): each individual contains both male and female gonads

6. B. Homologous Chromosomes • Homologous chromosome pairs: • For each chromosome in a diploid nucleus, there is another very similar chromosome in the same nucleus • This pair of very similar chromosomes is called a homologous chromosome pair • One chromosome in each pair comes from one parent, and the other chromosome comes from the other parent

7. B. Homologous Chromosomes • Homologous chromosomes are similar in: • Size • Position of the centromere • Banding patterns in staining procedures • The type of genetic information they contain • During meiosis, the homologous chromosomes are separated, so that a gamete receives only one member of each homologous chromosome pair

8. C. Stages of Meiosis • Prior to meiosis: • The diploid germ-line cell goes through a complete interphase, including an S phase • Therefore, at the start of meiosis, each of the chromosomes is in a replicated state (consisting of sister chromatids connected at the centromere) • Meiosis is accomplished in two divisions: meiosis I and meiosis II

9. C. Stages of Meiosis • Meiosis I: • At the start of meiosis I, the two chromosomes in each homologous chromosome pair line up along their lengths • During meiosis I the homologous chromosomes separate and move to opposite sides of the cell. (Note that the chromatids stay together at this point.) • At the end of meiosis I, the cell divides into two • Note that each daughter cell formed by meiosis I is haploid, but the chromosomes are still in their replicated state

10. C. Stages of Meiosis • Meiosis II: • Each of the cells from meiosis I can undergo meiosis II • During meiosis II, the centromeres split, the sister chromatids separate and become daughter chromosomes, and the daughter chromosomes move to opposite sides of the cell • New nuclei form, and the cell divides • Therefore, meiosis (I and II together) has the potential of forming four haploid cells, with the chromosomes in an unreplicated state at the end of the process

11. C. Stages of Meiosis • Meiosis I is divided into four stages: prophase I, metaphase I, anaphase I, and telophase I • Prophase I: during prophase I, the nuclear membrane & nucleolus disperse, and a spindle forms. The homologous chromosomes condense and pair in five steps: • Leptonema: the chromosomes begin to condense and have the appearance of slender threads • Zygonema: the homologous chromosomes align completely along their lengths, forming paired chromosomes called bivalents. The connection between the chromosomes is called the synaptonemal complex

12. C. Stages of Meiosis • Prophase I (continued): • Pachynema: the bivalent chromosomes continue to condense, becoming very short & thick • Diplonema: the chromosomes in each bivalent begin to partially separate. The two chromosomes in the bivalent remain connected at X-shaped regions called chiasmata (singular: chiasma) • Diakinesis: the chiasmata migrate to the ends of the bivalents

13. C. Stages of Meiosis • Metaphase I: • The bivalents align at the equator of the spindle • Anaphase I: • The homologous chromosomes separate and move to opposite poles of the spindle • Note that the the chromatids do not separate at this time • Telophase I: • The chromosomes at each pole may decondense, and new nuclei form • Cytokinesis takes place, resulting in two cells

14. C. Stages of Meiosis • Meiosis II is divided into four stages: prophase II, metaphase II, anaphase II, and telophase II • Prophase II • Chromosomes condense; Membrane disperses; Spindle forms

15. C. Stages of Meiosis • Metaphase II • Chromosomes align at equator of spindle • Anaphase II • The sister chromatids separate and move to opposite poles of the spindle • Telophase II • Chromosomes decondense; New nuclei form • Cytokinesis takes place

16. D. Gametogenesis • Spermatogenesis • The cytokinesis divisions (cell divisions) after meiosis I and meiosis II are equal • This means that one germ line cell in the testes divides by meiosis to produce four cells of equal size • Each of these four cells develops into a spermatozoan

17. D. Gametogenesis • Spermatogenesis • Stages • Spermatogonium • Primary spermatocyte • Secondary spermatocyte • Spermatid • Mature spermatozoan

18. D. Gametogenesis • Oogenesis • When the primary oocyte undergoes meiosis I, the cytokinesis is unequal, resulting in one very large cell (the secondary oocyte) and one much smaller cell (the first polar body) • When the secondary oocyte divides in meiosis II, again the division is unequal. The result is one very large gamete (the ovum) and a second polar body • Therefore, a single diploid germ-line cell in an ovary will produce only one gamete

19. D. Gametogenesis • Oogenesis • Stages • Oogonium • Primary oocyte • Secondary oocyte • Ootid • Ovum