Meiosis: Formation of gametes Utilizing BioCoach Activity “Meiosis”

Introduction to Meiosis Meiosis occurs only in sexually reproducing organisms. In animals, where the somatic (body) cells are diploid, the products of meiosis are the gametes. Some plants, algae and fungi have more complex cycles involving both mitosis and meiosis.

Introduction to Meiosis A two-step cell-division process in sexually reproducing eukaryotes. Results in cells (typically gametes) with one-half the chromosome number of the original parental cell. One diploid cell (2N) results in 4 haploid cells (N).

Introduction to Meiosis Meiosis consists of two successive nuclear divisions: meiosis I meiosis II Each division consists of :  Prophase  Metaphase  Anaphase  Telophase

Premeiotic Interphase in Animal Cells All chromosomes are duplicated The two centriole are produced by the duplication of a single centriole

MeiosisMeiosis I: Prophase I Chromosomes become visible as tetrads  A tetrad is a pair of homologous chromosomes (ex. both copies of chr #3) Crossing-over occurs  Homologous chromosomes exchange portions of their chromatids  Produces new combinations of alleles

MeiosisMeiosis I: Prophase I Nucleolus disappears Meiotic spindle forms Nuclear envelope disappears Prophase I consumes 90% of the time for meiosis

Crossing Over Crossing Over During Meiosis crossing-over results in a physical exchange of equivalent segments of maternal and paternal homologous chromosomes

Crossing Over Crossing Over During Meiosis

Prophase I Meiotic prophase I

Meiosis I: Metaphase IMetaphase I The centrioles are at opposite poles of the cell The pairs of chromosomes (tetrads) are arranged along the center of cell Spindle fibers are attached to one chromosome of each pair

Metaphase I Meiotic metaphase I

Meiosis I: Anaphase IAnaphase I Chromosomes of each tetrad separate and move to opposite poles Note: Sister chromatids remain attached and move together to the pole

Anaphase I Meiotic anaphase I

Meiosis I: Telophase ITelophase I Homologous chromosome pairs reach the poles of the cell Nuclear envelopes form around them Cytokinesis follows to produce two cells

Telophase I Meiotic telophase I

Product of Meiosis I Two cells with a haploid number of chromosomes is produced. Each cell has two copies of each chromatid. The two cells have sets of chromosomes that are different from each other and from the parent cell that entered meiosis. Animation

Meiosis II There is no interphase between the two stages (chromosomes are not duplicated).

Meiosis II: Prophase IIProphase II Centrioles duplicate Nuclear envelope breaks down Spindle apparatus forms

Prophase II Meiotic prophase II

Comparing Prophase I and Prophase II

Meiosis II: Metaphase IIMetaphase II Spindle apparatus attaches to each chromosome Single chromosomes align along center of each cell

Meiotic metaphase II

Comparing Metaphase I and Metaphase II

Meiosis II: Anaphase IIAnaphase II Centromeres separate Sister chromatids — now individual chromosomes — move toward the opposite poles of the cell.

Anaphase II Meiotic anaphase II

Comparing Anaphase I and Anaphase II

Meiosis II: Telophase IITelophase II Nuclear envelope forms around each set of chromosomes Cytokinesis occurs Four daughter cells are produced, each with a haploid set of chromosomes.

Telophase II Meiotic telophase II

Comparing Telophase I and Telophase II

Product of Meiosis II Four cells with a haploid number of chromosomes is produced. Each cell has a single copy of each chromatid. The four cells have sets of chromosomes that are genetically different from each other and from the parent cell that entered meiosis. Animation

Mitosis and Meiosis Reviewed

Compare meiosisCompare meiosis to mitosis In interphase prior to both, replication of DNA occurs Both are methods of cell division Both include Prophase, Metaphase, Anaphase and Telophase Cytokinesis occurs in both How are they alike?

Compare meiosisCompare meiosis to mitosis Meiosis Two cell divisions Sexual Reproduction --makes gametes Results in 4 haploid cells Mitosis One cell division Asexual reproduction --makes body cells Also for growth and regeneration Results in 2 diploid cells How are they different?

Compare meiosisCompare meiosis to mitosis Meiosis Two parts - eight phases DNA in daughter cells differs from parent cell Crossing over occurs Mitosis One part – four phases DNA in daughter cells is same as parent cell Crossing over doesn’t occur How are they different?

Compare anaphase in mitosis to anaphase I in meiosis Mitotic AnaphaseMeiotic Anaphase I How are they different?