Meiosis: Occurs at a particular point in the life cycle. Two successive divisions (meiosis I and meiosis II) of a diploid (2N) nucleus after one DNA replication (chromosome duplication) cycle. Cell division (cytokinesis) results in 4 haploid (1N) cells from a single parent cell. Animals: gametogenesis -> gametes Plants: sporogenesis -> meiospores

Meiosis I: Chromosomes are reduced from diploid (2N) to haploid (1N). Four stages Prophase I Similar to prophase of mitosis, except that homologous chromosomes pair and cross- over. Spindle apparatus begins to form, and nuclear envelope disappears. Metaphase I Chromosome pairs (bivalents) align across equatorial plane. Different from metaphase of mitosis because independent alignment of homologs does not occur. Anaphase I Homologous chromosome pairs separate and migrate toward opposite poles. Telophase I Chromosomes complete migration, and new nuclear envelopes form, followed by cell division.

Synaptonemal complex (SC) A proteinaceous structure that connects paired chromosomes. Lateral elements Transverse elements Recombination nodules

Are there specific regions pairing occurs? Pairing is generally near or in repetitive sequences. For example, a 240 bp repeat sequence between ribosomal RNA genes clustered on Drosophila X and Y chromosomes, when found in multiple copies it facilitates pairing during meiosis.

Synapsis Defined by the formation of synaptonemal complex (SC), which bridges the ~100 nm gap between homologs. SC is comprised of two lateral elements (LE) that are drived from axial cores or axial elements. LEs are connected to each other by transverse elements.

Lateral Elements (LEs) & Transverse fibers LEs are derived mainly from the cohesin complex, which mediates sister chromatid cohesion (e.g., REC8, SMC3). SCP1 in mammals form transverse fibers. These fibers, a coiled-coil rich segment is located in the center of the protein. These proteins form dimers through the coiled-coil rich segments.

Chiasmata Chiasmata is the physical manifestation of reciprocal meiotic exchange, lock the chromosomes together. Once the two homologous centromeres are attached to opposite poles of the meiotic spindle, chiasmata balances the poleward forces at the kinetochore. Sister chromatid cohesion maintains chiasmata until Anaphase I. Cleavage of cohesin by separase is necessary for chromosome segregation during anaphase I.

Meiosis II: Very similar to mitotic division. Also four stages: Prophase II Chromosomes condense. . Metaphase II Spindle forms and centromeres align on the equatorial plane. Anaphase II Centromeres split and chromatids are pulled to opposite poles of the spindle (one sister chromatid from each pair goes to each pole). Telophase II Chromatids complete migration, nuclear envelope forms, and cells divide, resulting in 4 haploid cells. Each progeny cell has has one chromosome from each homologous pair, but these are not exact copies due to crossing-over.

Fig. 1.20 The stages of meiosis in an animal cell Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.

PARENT CELL (before chromosome replication) Site of crossing over MITOSIS MEIOSIS PARENT CELL (before chromosome replication) Site of crossing over MEIOSIS I PROPHASE PROPHASE I Tetrad formed by synapsis of homologous chromosomes Duplicated chromosome (two sister chromatids) Chromosome replication Chromosome replication 2n = 4 Chromosomes align at the metaphase plate Tetrads align at the metaphase plate METAPHASE METAPHASE I ANAPHASE I TELOPHASE I ANAPHASE TELOPHASE Sister chromatids separate during anaphase Homologous chromosomes separate during anaphase I; sister chromatids remain together Haploid n = 2 Daughter cells of meiosis I 2n 2n No further chromosomal replication; sister chromatids separate during anaphase II MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II Figure 8.15