Meiosis (How It Happens)

What’s the big picture? The purpose of meiosis is to produce GAMETES (sex cells-egg and sperm) that have only one set of chromosomes (haploid).

Phases of Meiosis Interphase Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II Meiosis I Meiosis II

Meiosis These cells begin Meiosis II

Interphase Interphase in meiosis occurs the same as in mitosis G1 S G2

Meiosis I (When Homologous Chromosomes Separate)

Prophase I Nuclear envelope disappears Spindle forms Chromosomes condense Homologous chromosomes pair up Crossing over occurs

What is Crossing Over? Homologous chromosomes join to form a tetrad and pieces of chromosomes are exchanged This produces genetic recombination (gametes are NOT identical to parent cell) Homologous chromosomes Join to form a TETRAD

Crossing Over

Homologous Chromosomes during crossing over height height Hair type Hair type Eye color (blue) Eye color (brown) Foot size Foot size earlobes earlobes Hair color Hair color MOM DAD

Metaphase I Homologous pairs of chromosomes line up along the equator of the cell

Anaphase I Homologous chromosomes separate and move to opposite sides. Sister chromatids remain attached at their centromeres.

Telophase I Homologous chromosomes reach opposite ends Nuclear envelopes reform Spindle disappears Chromosomes uncoil

Meiosis II (When Sister Chromatids Separate)

Prophase II Nuclear envelope disappears Chromosomes condense Spindle forms

Metaphase II Chromosomes line up along equator of cell

Anaphase II Sister chromatids separate and move to opposite sides Equator Sister chromatids separate and move to opposite sides

Telophase II Nuclear envelopes reform. Chromosomes uncoil. Spindle disappears. Cytokinesis divides the cells.

Results of Meiosis 4 haploid gametes (egg/sperm) that are each a different combination of the original cell

Meiosis: Two Part Cell Division Sister chromatids separate Meiosis I Meiosis II Homologous Chromosomes separate Diploid Haploid Haploid

Comparison of Divisions Mitosis Meiosis Number of divisions 1 2 Number of daughter cells 4 Genetically identical? Yes No Chromosome # Same as parent Half of parent Where Somatic cells Gametes When Throughout life At sexual maturity Role Growth and repair Sexual reproduction