Quick review: identify this stage of the diploid cell cycle.

Defined: Process where the chromosome number is reduced by half Gametes created (egg & sperm) End Result: Four Haploid Cells

Same process –G1: Growth –S: Chromatin duplicated –G2: Growth

Synthesis (S stage) Chromatin strands duplicated

Similar as mitosis, except… Two Events: –1) Synapsis: Pairing of homologous chromosomes –Dad’s #1 chromosome pairs with Mom’s #1, etc… –The four paired chromatids equals a TETRAD –2) Crossing Over: Exchange of genes Mom 1 Dad 1

Early Prophase Dad #2 Mom #2 Mom #1 Dad #1 Dad #3 Mom #3 Chromosomes form Nucleus dissolves

Synapsis Dad #2 Mom #2 Mom #1 Dad #1 Dad #3 Mom #3 Spindle fibers pull homologous chromosomes together tetrad

Homologous chromosomes (tetrads) align along cell equator Dad #2 Dad #3 Dad #1 Mom #2 Mom #3 Mom #1

Tetrads separated Homologous chromosomes are pulled to opposite ends Mom #2 Dad #2 Mom #1 Dad #3 Dad #1 Mom #3

Cytokinesis divides the cells Each cell contains 1 of the homologous chromosomes –Humans: 23 full chromosomes each Mom #2 Dad #2 Mom #1 Dad #3 Dad #1 Mom #3

Same as mitosis Note: No new DNA was created (no 2 nd interphase) Mom #2 Dad #2 Mom #1 Dad #3 Dad #1 Mom #3

Remaining chromosomes are aligned at cell equator Mom #2 Dad #2 Mom #1 Dad #3 Dad #1 Mom #3

Sister chromatids are pulled to opposite ends

Cytokinesis divides the cells Nucleus reforms on the four new cells Chromatids uncoil End Result: Four haploid cells

Mitosis vs. Meiosis

Defined: Production of sperm cells Location: male testes Makes final changes to sperm cell Four haploid sperm cells at a time Meiosis 1 Meiosis 2 Spermatogenesis spermatogonium

Spermatogenesis Video Clip

Defined: Production of the egg cell (ovum) –In female ovaries During meiosis I: Cytoplasm distributed unequally After meiosis II: –1 cell matures –3 “Polar Bodies” dissolve Meiosis 1 Meiosis 2 Oogenesis oocyte

Oogenesis Video Clip

Division of somatic cells Ex: Blood cells, skin cells, nerve cells… End Result: 2 Diploid Cells Importance: Replaces aging cells Importance: Form of asexual reproduction Division of gametes Ex: Sperm and Egg cells End Result: 4 Haploid cells Importance: Reduces the gamete chromosome number by half

Review 1) Name three ways that meiosis and mitosis differ. 2) Pick a stage of meiosis…any stage. Describe what happens in it. 3) What type of cells does meiosis create? 4) What are homologous chromosomes? 5) When do homologous chromosomes move to opposite ends of the cell? 6) When do chromatids move to opposite ends of the cell? 7) Why is no DNA created in meiosis 2? 8) How does meiosis differ in males vs. females?