Meiosis Differs from mitosis The process that halves the number of chromosomes in a cell Occurs in Ovaries or Testes

A Variety of Sexual Lifecycles Human Life cycle Most fungi and some protists (including some algae) Plants and some other species of algae

Figure 13.4 The human life cycle

Alternation of generations Figure 13.5 Three sexual life cycles differing in the timing of meiosis and fertilization (syngamy) Alternation of generations

Figure 13.6 Overview of meiosis: how meiosis reduces chromosome number Four daughter chromosomes IMPORTANT: Homologous chromosomes are different than sister chromatids 4 Haploid (n) cells instead of 2 diploid cells (2n)

Meiosis I : Separates Homologous Chromosomes Interphase Each of the chromosomes replicate The result is two genetically identical sister chromatids which remain attached at their centromeres

Prophase I Lasts longer and is more complex than prophase in mitosis Chromosomes begin to condense and homologues, each consisting of two sister chromatids, pair up During Synapsis: A protein structure attaches the homologous chromosomes tightly together (synaptonemal complex)

Prophase I Later in prophase, when the synaptonemal complex disappears, each chromosome pair becomes visible in the microscope as a tetrad A cluster of four chromatids At various places along their length, chromatids of homologous chromosomes are crisscrossed Occur at chiasmata Hold the homologous pairs together until anaphase I

Prophase I Other cellular components prepare for division of the nucleus in a manner similar to that of mitosis Centrosomes move away from each other and spindle microtubules form between them The nuclear envelope and nucleoli disperse The spindle microtubules capture the kinetochores that form on the chromosomes The chromosomes begin moving to the metaphase plate Can last for days or longer (over 90% of meiosis)

Metaphase I The chromosomes are now arranged on the metaphase plate Still in homologous pairs Kinetochore microtubles from one pole of the cell are attached to one chromosome of each pair while microtubules from the opposite pole are attached to the homologue

Anaphase I The spindle apparatus guides the movement of the chromosomes toward the poles Sister chromatids remain attached Move as a unit towards the same pole The homologous chromosome moves toward the opposite pole Contrasts mitosis – chromosomes appear as individuals instead of pairs (meiosis)

Telophase I The members of each pair of homologous chromosomes continue to move apart until they reach the poles of the cell Each pole now has a haploid chromosome set but each chromosome still has two sister chromatids

Cytokinesis Occurs simultaneously with telophase I Forms 2 daughter cells Plant cells – cell plate Animal cells – cleavage furrows NO FURTHER REPLICATION OF GENETIC MATERIAL PRIOR TO THE SECOND DIVISION OF MEIOSIS

Figure 13.7 The stages of meiotic cell division: Meiosis I

Figure 13.7 The stages of meiotic cell division: Meiosis II

Meiosis II : Separates sister chromatids Proceeds similar to mitosis THERE IS NO INTERPHASE II !

Prophase II A spindle apparatus forms and the chromosomes progress toward the metaphase II plate

Metaphase II The chromosomes are positioned on the metaphase plate in a mitosis-like fashion Kinetochores of sister chromatids of each chromosome pointing toward opposite poles

Anaphase II The centromers of sister chromatids finally separate The sister chromatids of each pair move toward opposite poles Now individual chromosomes

Telophase II and Cytokinesis Nuclei form at opposite poles of the cell and cytokinesis occurs After completion of cytokinesis there are four daughter cells All are haploid (n)

Figure 13.7 The stages of meiotic cell division: Meiosis II

Figure 13.8 A comparison of mitosis and meiosis

Figure 13.8 A comparison of mitosis and meiosis: summary

Origins of Genetic Variation As mentioned earlier, in species that reproduce sexually, the behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation Independent assortment of chromosomes Crossing Over Random Fertilization

Figure 13.9 Independent Assortment

Figure 13.10 Crossing Over

Random Fertilization A human ovum plus a human sperm 1 of 8 million combinations possible for each ovum and sperm 223 X 223 = over 70 billion combinations 70 trillion possible combinations with out considering crossing over YOU REALLY ARE UNIQUE!

Gametogenesis See handout for details

THE END OF MEIOSIS