Meiosis and Sexual Life Cycles Chapter 13: Meiosis and Sexual Life Cycles

Figure 13.1 Figure 13.1 What accounts for family resemblance?

Asexual vs. Sexual reproduction The Birds and the Bees Asexual vs. Sexual reproduction

0.5 mm Parent Bud (a) Hydra (b) Redwoods Figure 13.2 0.5 mm Parent Bud Figure 13.2 Asexual reproduction in two multicellular organisms. (a) Hydra (b) Redwoods

Chromosome arrangement Somatic cells – typical body cells 46 chromosomes in 23 pairs (humans) The chromosomes are not normally paired up Each pair is called homologous chromosomes Gametes – sex cells 23 chromosomes (humans) One member from each pair

Homologous chromosomes 22 of the pairs (autosomes) are “true homologues” One of each came from mom and dad Identical in length and type of genes carried Genes on each are slightly different Sex chromosomes (23rd pair) don’t match up exactly (X vs Y)

Figure 13.3a Figure 13.3 RESEARCH METHOD: Preparing a karyotype

Pair of homologous duplicated chromosomes Figure 13.3b 5 m Pair of homologous duplicated chromosomes Centromere Sister chromatids Figure 13.3 RESEARCH METHOD: Preparing a karyotype Metaphase chromosome

Maternal set of chromosomes (n  3) Figure 13.4 Key Key Maternal set of chromosomes (n  3) 2n  6 Paternal set of chromosomes (n  3) Sister chromatids of one duplicated chromosome Centromere Figure 13.4 Describing chromosomes. Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)

Mitosis and development Multicellular diploid adults (2n  46) Figure 13.5 Key Haploid (n) Diploid (2n) Egg (n) Haploid gametes (n  23) Sperm (n) Ovary Testis Mitosis and development Diploid zygote (2n  46) Multicellular diploid adults (2n  46) MEIOSIS FERTILIZATION The Human Life Cycle Figure 13.5 The human life cycle.

Sexual Life Cycles Figure 13.6 Key Haploid (n) Haploid multi- cellular organism (gametophyte) Haploid unicellular or multicellular organism Diploid (2n) n Gametes n n Mitosis n Mitosis Mitosis n Mitosis n n n n n MEIOSIS FERTILIZATION Spores n n Gametes n Gametes MEIOSIS FERTILIZATION Zygote MEIOSIS FERTILIZATION 2n 2n 2n Diploid multicellular organism (sporophyte) 2n Zygote Diploid multicellular organism 2n Figure 13.6 Three types of sexual life cycles. Mitosis Mitosis Zygote (a) Animals (Including Humans) (b) Plants and some algae (c) Most fungi and some protists

Meiosis Reduces chromsome number Produces haploid cells Results in four daughter cells All are haploid All are genetically unique from each other and from the parent DNA replicated before meiosis begins Two stages Meiosis I: separates homologous chromosomes Meiosis II: separates sister chromatids

How does meiosis reduce chromosome number? 2 divisions

Interphase Pair of homologous chromosomes in diploid parent cell Figure 13.7-1 Interphase Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Chromosomes duplicate Sister chromatids Diploid cell with duplicated chromosomes Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number.

Figure 13.7-2 Interphase Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Chromosomes duplicate Sister chromatids Diploid cell with duplicated chromosomes Meiosis I 1 Homologous chromosomes separate Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number. Haploid cells with duplicated chromosomes

Interphase Meiosis I Meiosis II Figure 13.7-3 Interphase Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Chromosomes duplicate Sister chromatids Diploid cell with duplicated chromosomes Meiosis I 1 Homologous chromosomes separate Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number. Haploid cells with duplicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unduplicated chromosomes

Figure 13.8 Exploring: Meiosis in an Animal Cell MEIOSIS I: Separates homologous chromosomes MEIOSIS I: Separates sister chromatids Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Centrosome (with centriole pair) Sister chromatids remain attached Sister chromatids Chiasmata Centromere (with kinetochore) Spindle Metaphase plate During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes. Cleavage furrow Homologous chromosomes separate Fragments of nuclear envelope Sister chromatids separate Haploid daughter cells forming Homologous chromosomes Microtubule attached to kinetochore Each pair of homologous chromosomes separates. Two haploid cells form; each chromosome still consists of two sister chromatids. Figure 13.8 Exploring: Meiosis in an Animal Cell Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n  6 in this example. Chromosomes line up by homologous pairs.

Telophase I and Cytokinesis Prophase I Metaphase I Anaphase I Figure 13.8a Telophase I and Cytokinesis Prophase I Metaphase I Anaphase I Centrosome (with centriole pair) Sister chromatids remain attached Sister chromatids Chiasmata Centromere (with kinetochore) Spindle Metaphase plate Cleavage furrow Homologous chromosomes separate Homologous chromosomes Fragments of nuclear envelope Figure 13.8 Exploring: Meiosis in an Animal Cell Microtubule attached to kinetochore Each pair of homologous chromosomes separates. Two haploid cells form; each chromosome still consists of two sister chromatids. Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n  6 in this example. Chromosomes line up by homologous pairs.

Telophase II and Cytokinesis Prophase II Metaphase II Anaphase II Figure 13.8b Telophase II and Cytokinesis Prophase II Metaphase II Anaphase II During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes. Sister chromatids separate Haploid daughter cells forming Figure 13.8 Exploring: Meiosis in an Animal Cell

Chromosome duplication Chromosome duplication Duplicated chromosome Figure 13.9a MITOSIS MEIOSIS Parent cell Chiasma MEIOSIS I Prophase Prophase I Chromosome duplication Chromosome duplication Duplicated chromosome Homologous chromosome pair 2n  6 Metaphase Metaphase I Anaphase Telophase Anaphase I Telophase I Figure 13.9 A comparison of mitosis and meiosis in diploid cells. Daughter cells of meiosis I Haploid n  3 2n 2n MEIOSIS II Daughter cells of mitosis n n n n Daughter cells of meiosis II

SUMMARY Property Mitosis Meiosis Figure 13.9b SUMMARY Property Mitosis Meiosis DNA replication Occurs during interphase before mitosis begins Occurs during interphase before meiosis I begins Number of divisions One, including prophase, metaphase, anaphase, and telophase Two, each including prophase, metaphase, anaphase, and telophase Synapsis of homologous chromosomes Does not occur Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion Number of daughter cells and genetic composition Two, each diploid (2n) and genetically identical to the parent cell Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other Figure 13.9 A comparison of mitosis and meiosis in diploid cells. Role in the animal body Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes

Sexual Reproduction leads to genetic variation! 3 processes that contribute to variation Independent assortment of chromosomes Crossing over Random fertilization

Two equally probable arrangements of chromosomes at metaphase I Figure 13.10-1 1. Independent Assortment Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Each homologous pair lines up independently of the other pairs at the Metaphase Plate. Figure 13.10 The independent assortment of homologous chromosomes in meiosis.

Two equally probable arrangements of chromosomes at metaphase I Figure 13.10-2 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Figure 13.10 The independent assortment of homologous chromosomes in meiosis.

Two equally probable arrangements of chromosomes at metaphase I Figure 13.10-3 Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 Figure 13.10 The independent assortment of homologous chromosomes in meiosis.

2. Crossing Over

Nonsister chromatids held together during synapsis Figure 13.11-1 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Figure 13.11 The results of crossing over during meiosis.

Nonsister chromatids held together during synapsis Figure 13.11-2 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Figure 13.11 The results of crossing over during meiosis.

Nonsister chromatids held together during synapsis Figure 13.11-3 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Figure 13.11 The results of crossing over during meiosis.

Nonsister chromatids held together during synapsis Figure 13.11-4 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Figure 13.11 The results of crossing over during meiosis. Anaphase II

Nonsister chromatids held together during synapsis Figure 13.11-5 Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Figure 13.11 The results of crossing over during meiosis. Anaphase II Daughter cells Recombinant chromosomes

Which sperm ends up fertilizing which egg is “random” 3. Random fertilization Which sperm ends up fertilizing which egg is “random” In humans: There are 23 pairs of chromosomes, so there are 223 combinations during independent assortment 223 =

Variation in Humans Independent Assortment There are 23 pairs of chromosomes, so there are 223 possible combinations during independent assortment 223 = 8,388,608 combinations of chromosomes from one meiotic event Random fertilization 8,388,608 x 8,388,608 = 70,368,744,177,664 That’s 70.3 TRILLION possible combinations of sperm and eggs Crossing over Makes the variation essentially limitless

Darwin vs. Mendel