Meiosis Formation of Sex Cells to Transfer Genetic Information to the Next Generation

Mitosis and Meiosis in the Human Life Cycle Diploid number of chromosomes, two full sets Mitosis and Meiosis in the Human Life Cycle Nuclear division retaining the original chromosome number Nuclear division reducing the chromosome number, leading to sperm or eggs Haploid number of chromosomes, one full set

Mitosis and Meiosis Human Cell with 46 chromosomes (DIPLOID) Interphase Mitosis Cytokinesis Interphase Meiosis Cytokinesis 2 Genetically-Identical Human Cells each with 46 chromosomes (DIPLOID) 4 Genetically-Different Human Cells each with 23 chromosomes (HAPLOID)

Homologous Chromosome Pairs Diploid cells have pairs of chromosomes Each homologous chromosome pair is genetically matched so that the genes are arranged in the same order Meiosis distributes members of each pair to separate nuclei Figure: 11.UN06b Title: Formation of daughter chromosomes Caption: Humans have 1 pair of sex chromosomes and 22 pairs of autosomes

Cell Cycle With Meiosis Interphase includes G1 = growth phase 1 S = synthesis phase G2 = growth phase 2 Meiosis cytokinesis G2 synthesis of proteins and assembly of components for division Meiosis I Meiosis II cytokinesis Meiosis includes two chromosomal divisions Meiosis I Meiosis II Cytokinesis = division of cytoplasmic contents S chromosomes duplicate G1 cytoplasm doubles, including organelles Interphase

Applying Your Knowledge G1 phase S phase G2 phase Meiosis I and II When does chromosomal division occur? When do the cytoplasmic contents double? When does chromosome duplication occur?

Meiosis Involves Two Chromosomal Divisions Equational Division: Separation of sister chromatids, no change in chromosome number Reduction Division: Separation of homologous chromosomes, reducing the chromosome number to haploid Meiosis Involves Two Chromosomal Divisions

Duplicated Chromosomes in Early Meiosis Homologous Chromosome Pair Duplicated chromosomes joined at their centromeres Nonsister Chromatids are joined to different centromeres Centromere = constricted region Duplicated Chromosomes in Early Meiosis

Prophase I of Meiosis I Chromosomes thicken Genetic exchange occurs between nonsister chromatids through crossing-over

Metaphase I of Meiosis I Spindle fiber Homologous pairs line up at the cell equator Spindle fibers attached to centromeres cause chromosomal movement

Anaphase I and Telophase I of Meiosis I Members of homologous pair separate and are pulled to opposite poles of the cell Telophase I Chromosomes reach opposite poles + cytokinesis Cytokinesis Cytoplasm divides into two separate cells Daughter cells are haploid with duplicated chromosomes

Prophase II and Metaphase II of Meiosis II Chromosomes thicken Metaphase II Chromosomes line up at cell equator

Anaphase II and Telophase II of Meiosis II Sister Chromatids separate and move to opposite poles Telophase II Chromosomes have reached opposite poles + cytokinesis Cytokinesis Cytoplasm divides into separate cells Daughter cells are haploid and have genetic differences

Applying Your Knowledge Prophase I Metaphase I Anaphase I Metaphase II Anaphase II When do sister chromatids separate? When does crossing-over occur? When do homologous pairs line up at the cell equator?

Equal divisions of cytoplasm Four functional products per meiosis Sperm formation Equal divisions of cytoplasm Four functional products per meiosis Fertilization occurs after Meiosis I and stimulates Meiosis II Egg formation Unequal divisions of cytoplasm Small polar bodies formed One functional product per meiosis

Comparing Mitosis and Meiosis Cell type at start Cell type at end Are products identical? (Y/N) Number of cells produced Number of chromosome duplications Pairing of homologues (Y/N) Crossing-over (Y/N) Number of chromosome divisions

Comparing Mitosis and Meiosis Cell type at start Cell type at end Are products identical? (Y/N) Number of cells produced Number of chromosome duplications Pairing of homologues (Y/N) Crossing-over (Y/N) Number of chromosome divisions Diploid Diploid Diploid Haploid Yes No 2 4 1 1 No Yes No Yes 1 2