How do we maintain 46 Chromosomes? Produce gametes – half the number of chromosomes per cell Humans have 23 chromosomes per gamete Cell with ½ the number of regular chromosomes (n) is called haploid Cell with all chromosome (2n) is called diploid

6 Stages of Cell Division Interphase Prophase Metaphase Anaphase Telophase Cytokinesis

Meiosis I Interphase – Chromosomes replicate Prophase I homologous chromosomes pair 1 from mom, 1 from dad Nuclear envelope breaks down, spindles form Crossing over!

Meiosis I Metaphase I Anaphase I Telophase I Homologous chromosomes line up at center Anaphase I Homologous pairs move to opposite sides of cell Telophase I Chromosomes uncoil Cell divides

Meiosis II Prophase II - no DNA replication!!! Metaphase II Chromosomes condense Spindle fibers appear Metaphase II Chromosomes line up at center Anaphase II Chromosomes split and move to opposite sides Telophase II Nuclei form around chromosomes Cells divide

  Meiosis Mitosis Two cell divisions Centrioles appear Half the original chromosomes Homologous Chromosomes pair up Spindle fibers form Cytokinesis Four daughter cells 46 Chromosomes at end X X X X X X X X X X X

Meiosis Mitosis Chromosome Number: Reduced by half (homologous pairs separate) Remains the same Pairing of Homologues: Yes No Function: sexual reproduction Cellular (asexual) Reproduction (cell division during which the cell nucleus divides); general growth and repair of the body Number of Divisions: 2 nuclear and 2 cytoplasmic divisions 1 division of the nucleus and 1 Cytokinesis Number of Cells produced: 4 2 Creates: Sex cells only: Female egg cells or Male sperm cells Makes everything other than sex cells (Somatic Cells) Occurrence of Crossing Over: Produces: four haploid daughter cells two diploid daughter cells Genetically: different identical

Drawing Meiosis

2. Metaphase I 1. Prophase I 3. Anaphase I 4. Telophase/Cytokinesis I

3. Anaphase II Prophase II Cells from Meiosis I 2. Metaphase II 4. Telophase II /Cytokinesis II

Mitosis/Meiosis Review 1. Number of Cells Mitosis creates 2 identical cells Meiosis creates 4 individual cells 2. Number of chromosomes Mitosis creates cells with 46 chromosomes (2n – diploid) Meiosis creates cells with 23 chromosomes (n – haploid) 3. Locations Mitosis – somatic (“normal”) cells Meiosis – gametes (sex) cells

What is crossing over? Agenda for Friday Dec 9th Finish meiosis notes Meiosis internet lesson Quiz Monday

Importance of Meiosis Genetic Variation Random distribution of chromosomes occurring during meiosis Different chromosomes are passed to offspring independently of each other

Sexual vs. Asexual Reproduction Organisms inherit all chromosomes from single parent Genetically identical Chromosomes from 2 parents Genetic variation Why reproduce sexually? Greater chance of beneficial mutations Crossing over, random assortment

Check the correct box for each description   Meiosis Mitosis Two cell divisions Creates gametes Half the original chromosomes Homologous Chromosomes pair up Creates identical cells Cytokinesis Four daughter cells Same number of chromosomes at end