Meiosis In Mitosis Parent and daughter cells have

Meiosis

In Mitosis • Parent and daughter cells have the same number of chromosomes • These chromosomes occur in two sets (♀, ♂) and the two sets are called homologous chromosomes • The total number of chromosomes is referred to as the diploid number (2 n)

Meiosis • Is similar to mitosis in that it is just a single replication of nuclear material BUT: • This replication is followed by TWO cell divisions, creating 4 gametes with haploid number of chromosomes (n) • Ex. each cell only has ONE copy of each chromosome

Meiosis: • Is the division process that forms sexual gametes (sex cells sperm and eggs in humans). These gametes can then unite to form a zygote. • Reduces chromosome number from diploid (2 n) to haploid (n).

Who Cares? § No Meiosis, No Fertilization. §No Fertilization, No Offspring. §No Offspring, No Family, No Future

Occurs in two major divisions: • Meiosis II Each of these divisions occurs in 4 stages: • Prophase I & II • Metaphase I & II • Anaphase I & II • Telophase I & II

Interphase – DNA replicates at end of stage Prophase I: • Chromosomes coil & condense • Sister chromatids become visible and bivalent pairs are called tetrads • Non-sister chromatids cross over at chiasma site • Crossing over allows for genetic variety Metaphase I: • Tetrads align along equator and attach to spindle fibres

Anaphase I: • One pair of chromatids moves along spindle fibres towards pole. The other pair moves to the opposite pole. • Each chromatid pair has some of the genetic info from other pair due to crossing over • *** Differs from mitosis in that chromatids remain joined. In Anaphase of Mitosis, they are pulled apart***

• Telophase I: • Cytokinesis occurs – Cleavage furrow forms, cell pinched into two – Nuclear membrane reforms around chromosomes – Each cell has nucleus with haploid set of replicated chromosomes

Meiosis I In summary: • TWO cells are created • Each cell contains one pair of homologous chromatids • These new cells are not identical (because of crossing over)

Meiosis I

Meiosis II Interphase (no replication occurs!) Prophase II: • Chromatids condense and become visible

Metaphase II: • Paired chromatids line up at the equatorial plate (metaphase plate in diagram) and become visible

Anaphase II: • Chromatids separate and move to opposite poles

Telophase II: • Cytokinesis occurs – nuclear envelope forms around each set of chromosomes – 4 daughter cells (gametes) produced, each with a haploid set of chromosomes

Meiosis II In summary: • 4 non-identical haploid cells are created! • Because of crossing over, some chromosomes are seen to have recombined segments of original parental chromosomes = genetic variety

Meiosis II