Meiosis

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

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

Synthesis (S stage) Chromatin strands duplicated

Prophase 1 Similar as mitosis, except… Two Events: Mom 1 Dad 1 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

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

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

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

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

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

No Interphase 2 The Point of Meiosis: If Interphase 2 were to happen… Reduce the amount of chromosomes by half If Interphase 2 were to happen… DNA duplicated during the S stage Dad #2 Dad #2 Mom #2 Mom #2 Mom #1 Mom #1 Dad #1 Dad #1 Mom #3 Mom #3 Dad #3 Dad #3

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

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

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

Sister chromatids are pulled to opposite ends Anaphase 2 Sister chromatids are pulled to opposite ends

Telophase 2 Cytokinesis divides the cells Nucleus reforms on the four new cells Chromatids uncoil End Result: Four haploid cells

Mitosis vs. Meiosis

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

Spermatogenesis Video Clip

Oogenesis Defined: Production of the egg cell (ovum) oocyte Oogenesis Meiosis 1 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 2 Oogenesis

Oogenesis Video Clip

Mitosis Meiosis Division of gametes Division of somatic cells Ex: Sperm and Egg cells End Result: 4 Haploid cells Importance: Reduces the gamete chromosome number by half 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

Recap 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?