Meiosis Occurs in the Gonads: Produces Haploid Sex Cells Eggs Sperm

1. Pair of homologous chromosomes 5 µm Centromere Sister chromatids Figure 13.3 • Chapter 13: Meiosis and Sexual Life Cycles READ: p. 247 - 249 • Meiosis • Occurs in the Gonads: Produces Haploid Sex Cells • Eggs • Sperm • Provides A Key Means of Genetic Variation

2. Figure 13.4 • In a cell in which DNA synthesis has occurred • All the chromosomes are duplicated and thus each consists of two identical sister chromatids Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)

3. Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Figure 13.7 Haploid cells with unreplicated chromosomes The Stages of Meiosis • Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid • Meiosis • Takes place in two sets of divisions, meiosis I and meiosis II • Meiosis I • Reduces the number of chromosomes from diploid to haploid • Meiosis II • Produces four haploid daughter cells

4. MEIOSIS I: Separates homologous chromosomes INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Sister chromatids remain attached Centromere (with kinetochore) Centrosomes (with centriole pairs) Chiasmata Metaphase plate Sister chromatids Spindle Nuclear envelope Homologous chromosomes separate Microtubule attached to kinetochore Tetrad Chromatin Pairs of homologous chromosomes split up Chromosomes duplicate Tertads line up Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example Red Arrows = Provides Genetic Variation Blue Arrow = A difference from mitosis • Interphase and meiosis I Crossing over will occur This arrangement could be different (red on top) Figure 13.8 Sister chromatids do NOT separate

5. MEIOSIS II: Separates sister chromatids TELOPHASE II AND CYTOKINESIS TELOPHASE I AND CYTOKINESIS METAPHASE II ANAPHASE II PROPHASE II Cleavage furrow Haploid daughter cells forming Sister chromatids separate Two haploid cells form; chromosomes are still double During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes Figure 13.8 • Telophase I, cytokinesis, and meiosis II Sister chromatids DO separate

6. A Comparison of Mitosis and Meiosis • Meiosis can be distinguished from mitosis • By three events in Meiosis • 1. Tetrads on the metaphase plate: At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates. In Mitosis, there are no Tetrads. • Synapsis and crossing over: Homologous chromosomes physically connect and exchange genetic information in PROPHASE 1. In Mitosis, there is no crossing over. • 3. Separation of homologues • At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell. • In anaphase II of meiosis, the sister chromatids separate

7. MITOSIS MEIOSIS Chiasma (site of crossing over) Parent cell (before chromosome replication) MEIOSIS I Prophase I Prophase Chromosome replication Chromosome replication Tetrad formed by synapsis of homologous chromosomes Duplicated chromosome (two sister chromatids) 2n = 6 Tetrads positioned at the metaphase plate Chromosomes positioned at the metaphase plate Metaphase I Metaphase Sister chromatids separate during anaphase Anaphase Telophase Homologues separate during anaphase I; sister chromatids remain together Anaphase I Telophase I Haploid n = 3 Daughter cells of meiosis I 2n 2n MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II Sister chromatids separate during anaphase II Figure 13.9 • A comparison of mitosis and meiosis

8. Origins of Genetic Variation Among Offspring • In species that produce sexually • The behavior of chromosomes during meiosis and fertilization ARE responsible for most of the variation that arises each generation. Mutation is also a key source of genetic variation. • Meiosis provides 2 means of genetic variation 1. Crossing Over in Prophase I 2. Independent Assortment at Metaphase I

9. Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes Figure 13.11 Crossing Over • Produces recombinant chromosomes that carry genes derived from two different parents

10. Key Maternal set of chromosomes Possibility 1 Possibility 2 Paternal set of chromosomes Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 • 2. Independent Assortment: Homologous pairs of chromosomes • Orient randomly at metaphase I of meiosis BioFlix: Meiosis Figure 13.10

11. A Third and Major source of Genetic Variation is Random Fertilization • The fusion of gametes • Will produce a zygote with any of about 64 trillion diploid combinations • Origins of Genetic Variation Among Offspring: • Independent Assortment of homologous pairs in Metaphase I • Crossing Over of tetrads during synapsis of Prophase I • Random Fertilization • Mutation is the “original” source of genetic variation

12. Evolutionary Significance of Genetic Variation Within Populations • Genetic variation • Is the raw material for evolution by natural selection • Millions of years ago there were 2 types of giraffes: • Short Neck • Long Neck • The environment changed. The variation that already existed (long neck) had an advantage survived long enough to reproduce more genetically long necked giraffes. • Mutations • Are the original source of genetic variation • Sexual reproduction • Produces new combinations of variant genes, adding more genetic diversity