Meiosis

meiosis Nuclear division Sexual reproduction, genetic variation Reduces the chromosome number Diploid-2n, haploid-n, 46, 23 Gamete – sex cell, haploid Zygote – fused gametes, diploid

Karyotype 46 chromosomes 44 autosomes 2 sex chromosomes

Homologous chromosomes Look alike, have similar banding pattern, same length and centromere position One from mom, one from dad Alleles – alternate forms of a gene, have genes for same traits, just code for differences.

Meiosis I and II 2 nuclear divisions, Produce 4 haploid cells Not genetically identical Differences from mitosis Homologous chromosomes line up together during meiosis 1 Called synapsis, results in a bivalent (aka tetrad) No interphase between phases

Genetic Variation Crossing over – exchange of genetic material between non-sister chromatids of a bivalent (2-3 times) Nucleoprotein lattice forms Chiasmata – regions where non-sister chromatids are held together due to crossing over. Results in different sequence of alleles and are different from their parents.

Independent assortment Homologous chromosomes separate in a random order. (8,388,608 possible chromosomal combinations)

Fertilization Random… Significance of genetic variation: In changing environments, sexual reproduction is an advantage

Phases of meiosis Prophase 1 Spindle fibers form, centrosomes migrate Nuclear envelopes fragments, nucleolus gone Chromosomes condense Homologous chromosomes line up, synapsis forming bivalents Crossing over occurs

Metaphase 1 Homologous chromosomes move to equator Kinetochores are seen outside of centromere and connected to spindles Bivalents independently align in middle

Anaphase 1 Homologous chromosomes separate from each other and move to opposite poles

Telophase 1 Spindle disappears Nuclear envelope appears (not necessary by end of telophase to move on) Cytokinesis may or may not happen during this phase

interkinesis Between divisions Like interphase with no “s” phase

Meiosis II Similar to Mitosis in division except ends with 4 haploid daughter cells Haploid cells become gametes, in plants = spores

Comparison of Mitosis and Meiosis 1 nuclear division Results in 2 daughter cells Daughter cells genetically identical to parent Daughter cells have same # of chromosomes as parent (diploid) 2 nuclear divisions Results in 4 daughter cells Daughter cells not genetically identical to parent Daughter cells have half the # of chromosomes as parent (haploid)

Comparison of Mitosis and Meiosis Occurs in all tissues for growth and repair, common Occurs only in reproductive organs and produces gametes

Human life cycle Gametogenesis – formation of gametes Spermatogenesis – formation of sperm in testes, male meiosis Oogenesis – egg production in ovary, female meiosis

spermatogenesis Testes contain stem cells – spermatogonia which keep the testes supplied with primary spermatocytes (2n - 46) that undergo spermatogenesis Primary spermatocytes form secondary spermatocytes (n- 23), which form spermatids (n-23), which differentiate into 4 viable sperm

oogenesis Ovaries have stem cells called oogonia Produce primary oocytes (2n-46) during fetal development and undergoes oogenesis during puberty Forms 1 secondary oocyte (n-23), which receives the most cytoplasm, and a polar body Secondary oocyte begin meiosis II but stops at metaphase II. It leaves ovary and moves to oviduct where it awaits sperm. If no sperm enters, it disintergrates. If sperm enter oocyte, Meiosis II continues and another polar body is formed.