Meiosis

Meiosis Defined: Process where the chromosome number is reduced by half Cells that undergo meiosis will start diploid, but… end haploid.

Meiosis Defined: Process where the chromosome number is reduced by half Gametes created (egg & sperm) End Result: Four Haploid Cells 2 Sperm cell 1 3 4 Egg cell (ovum)

Interphase Same process G1: Growth S: Chromatin duplicated G2: Growth 6 strands of DNA to start Interphase Chromatin strands duplicated Same process G1: Growth S: Chromatin duplicated G2: Growth

Prophase I Similar to mitosis, except… Chromosomes form 6 strands of DNA to start Similar to mitosis, except… Chromosomes form Dad 1 Mom 3 Mom 1 Dad 2 Dad 3 Mom 2 Nucleus dissolves

Prophase I Similar to mitosis, except… Two Events: 6 strands of DNA to start Similar to 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 Dad 1 tetrad Mom 3 Mom 1 Dad 2 Dad 3 Mom 2 tetrad tetrad

Prophase I Similar to mitosis, except… Two Events: 6 strands of DNA to start Prophase I Similar to 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 Lets zoom into one tetrad Dad #1 Mom #1 tetrad Dad 1 Mom 1 Dad 3 Mom 3 tetrad Mom 2 Dad 2 Lets zoom back out tetrad

Crossing over Importance: Prophase I 6 strands of DNA to start Crossing over Importance: Creates genetic diversity. Each sperm and egg cell are genetically unique. Similar to 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 tetrad Mom 2 Dad 2 tetrad Mom 2 Dad 2 tetrad

Metaphase I Homologous chromosomes (tetrads) align along cell equator 6 strands of DNA to start Homologous chromosomes (tetrads) align along cell equator

Anaphase I Tetrads separated 6 strands of DNA to start Tetrads separated Homologous chromosomes are pulled to opposite ends

Telophase I Cytokinesis divides the cells 6 strands of DNA to start Telophase I Cytokinesis divides the cells Each cell contains 1 of the homologous chromosomes Humans: 23 full chromosomes each Sometimes the nucleus will regrow… sometimes it doesn’t

Prophase II Same as mitosis 6 strands of DNA to start Prophase II Same as mitosis Note: No new DNA was created (no 2nd interphase) Nucleus dissolves; Spindle fibers form

Metaphase II Remaining chromosomes are aligned at cell equator 6 strands of DNA to start Remaining chromosomes are aligned at cell equator

Anaphase II Sister chromatids are pulled to opposite ends 6 strands of DNA to start Sister chromatids are pulled to opposite ends

Telophase II Cytokinesis divides the cells 6 strands of DNA to start Cytokinesis divides the cells Nucleus reforms on the four new cells Chromatids uncoil End Result: Four genetically unique haploid cells 3 strands of DNA 3 strands of DNA 3 strands of DNA 3 strands of DNA

Mitosis vs. Meiosis

Spermatogonium (diploid) 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 Mature sperm cells

Spermatogenesis Video Clip

Oogenesis Defined: Production of the egg cell (ovum) In female ovaries Oocyte (diploid) 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 Mature ovum

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

Kobe Kuiz 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?