Cell Division Meiosis

Meiosis Meiosis occurs in sexual reproduction when a diploid germ cell produces four haploid daughter cells that can mature to become gametes (sperm or egg).

Meiosis Goal: reduce genetic material by half Why? n (mom) + n (dad) = 2n (offspring) from mom from dad child too much! meiosis reduces genetic content Just right!

Meiosis: cell division in two parts Sister chromatids separate Haploid Diploid Meiosis I (reduction division) Meiosis II Chromatids separate (equational division) 2n n n Result: one copy of each chromosome in a gamete.

Meiosis I : the reduction division Prophase I (early) (diploid) (late) Metaphase I Anaphase I Telophase I Nucleus Spindle fibers Nuclear envelope

What are Homologous chromosomes? sister chromatids sister chromatids

Chromosomes condense forming Tetrads. -Nuclear envelope fragments. Prophase I 1 2 Late prophase -Homologous Chromosomes condense forming Tetrads. -Spindle forms. -Nuclear envelope fragments. Early prophase Chromatids pair up. Crossing over occurs.

Metaphase I (Homologous) Chromatid pairs align along the equator of the cell.

Anaphase I Chromosomes separate and move to opposite poles. Sister chromatids remain Attached at their centromeres.

Telophase I Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into 2

Meiosis II Gene X Sister chromatids carry identical genetic Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene. Sister chromatids carry identical genetic information.

Meiosis II : the equational division Prophase II (haploid) Metaphase II Anaphase II Telophase II Four Non-identical haploid daughter cells

Prophase II Nuclear envelope fragments. Spindle forms.

Metaphase II Chromosomes align along equator of cell.

Anaphase II Individual chromatids separate and move to opposite poles.

Telophase II Nuclear envelope assembles. Chromosomes unravel. Spindle disappears. Cytokinesis divides cell into 2.

Results of meiosis Four haploid cells One copy of each chromosome

“Putting It All Together” - Fertilization

What Meiosis is About Meiosis allows the creation of unique individuals through sexual reproduction.

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 Sex cells When Throughout life At sexual maturity Role Growth and repair Sexual reproduction

MITOSIS MEIOSIS Occurs in somatic (body) cells Occurs only in gonads (sex organs: ovary/testes) Produces cells for repair, maintenance, growth, asexual reproduction Only produces gametes (sex cells: egg/sperm) Results in identical diploid (2n) daughter cells Reduction division results in haploid (n) cells

Haploid cells develop into mature gametes. Gametogenesis is the production of gametes. Gametogenesis differs between females and males. Sperm become streamlined and motile. Sperm primarily contribute DNA to an embryo. Eggs contribute DNA, cytoplasm, and organelles to an embryo. During meiosis, the egg gets most of the contents; the other cells form polar bodies.