11.4 Meiosis KEY CONCEPT During meiosis, diploid cells undergo two cell divisions that result in haploid cells.

Cells go through two rounds of division in meiosis. Meiosis I (first division) and Meiosis II (second division) occurs in gametes (sperm and egg) diploid  haploid (2n  1n) creates genetic diversity (variations) Diploid Meiosis I Meiosis II Haploid

11.4 Meiosis Meiosis I and meiosis II each have four phases, similar to those in mitosis Interphase  Meiosis 1 (P1-M1-A1-T1-Cytokinesis)  Meiosis 2 (P2-M2-A2-T2-Cytoknesis) Pairs of homologous chromosomes separate in meiosis I. Sister chromatids divide in meiosis II. homologous chromosomes sister chromatids sister chromatids

11.4 Meiosis Meiosis I occurs after DNA has been replicated (interphase). Meiosis I divides homologous chromosomes in four phases.

11.4 Meiosis Prophase I - similar to Prophase in mitosis except homologous chromosomes pair up (one from Dad and one from Mom)

11.4 Meiosis Prophase I - When homologous chromosomes pair up, crossing over may occur. - Crossing over is the exchange of DNA between two homologous chromosome. - Crossing over results in genetic variations. (this is why siblings may look similar, but still different)

11.4 Meiosis How does meiosis explain family resemblance and differences…………. Let’s look at some Celebrity Brothers and Sisters !!

11.4 Meiosis Metaphase 1, Anaphase 1, Telophase 1, Cytokinesis - homologous chromosomes line up, split up, divide into two cells, and ready to proceed to Meiosis 2

11.4 Meiosis Meiosis II divides sister chromatids in four phases. DNA is not replicated between Meiosis I and Meiosis II. Two cells divide into four haploid cells.

Haploid cells develop into mature gametes. 11.4 Meiosis Haploid cells develop into mature gametes. Gametogenesis is the production of gametes. Gametogenesis differs between females and males. Spermatogenesis = sperm production 1 germ cell  4 sperm cells Oogenesis = egg production 1 germ cell  1 egg cell + 3 polar bodies

11.4 Meiosis Meiosis differs from mitosis in significant ways.

Comparing Mitosis and Meiosis # of divisions 1 2 # of daughter cells 4 Genetically identical? Yes No Chromosome # Same as parent (2n) Half of parent (n) Where Somatic cells Sex cells When Throughout life At sexual maturity Role Growth and repair Sexual reproduction