Fertilization and Meiosis Alternate in Sexual Life Cycles

Fertilization and Meiosis alternate in sexual life cycles. A life cycle is the generation to generation sequence of stages in the reproductive history of an organism, from conception Gametes Meiosis Fertilization Zygote 2n Diploid multicellular organism (a) Animals Mitosis 2n Haploid multicellular organism Mitosis Mitosis Meiosis 2n Zygote (b) Most fungi and some algae Haploid multicellular organism (gametophyte) Mitosis Mitosis n n n Spores Gametes Gametes (n Melosis Fertilization Fertilization 2n 2n Zygote Mitosis Diploid multicellular organism (sporophyte) (c) Plants and some algae Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Sister chromatids remain attached Homologous chromosomes Fragments of nuclear envelope Microtubule attached to kinetochore Homologous chromosomes separate Cleavage furrow Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming Meiosis Reduces the number of chromosome sets from diploid to haploid. XX Crossing Over and Synapsis Sister chromatids 1 Synaptonemal complex Homologous chromosomes Transverse filament (~100 nm) B A Homologous chromosome pair in synapsis Crossover- Lateral element (LE) Central element (CE) Lateral element (LE) 50 Chromatin loops SYCP1 SYCP1 Chromatin loops 2 3 Crossover- Paternal chromosome Maternal chromosome Transverse filaments (TF) Lateral element: SYCP2 SYCP3 Central element: SYCE1-3 TEX12 4 Lateral element SYCP2 SYCP3 2016 Pearson Education, Inc Crossing Over homologous chromosomes replicate and synapse to form a bivalent 1 A C bivalent A C a C a C homologous chromosomes undergo crossing-over A C homologous chromosomes contain heterogeneous regions A C A C a C a C Two of the gametes are parental types a C A C a C Two of the gametes are recombinant types a C a C A C Independent Assortment Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II X Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 Mitosis (diploid and haploid) Meiosis (diploid only) Property DNA replication Number of divisions Synapsis of homologous chromosomes Number of daughter cells and genetic composition Role in the animal or plant body Genetic variation produced in sexual life cycles contributes to evolution.