Meiosis

What is an “n”?  Haploid (n): Number of chromosomes in a gamete. Diploid (2n): Number of chromosomes in a somatic cell One copy from mother, one copy from father. Homologous pairs of chromosomes

Two Types of Diploid Cells In Sexual Organisms  Two Types of Diploid Cells In Sexual Organisms Somatic GERM Body Cells Diploid Divide by mitosis to make exact copies In every tissue except reproductive tissue “Germinating” (reproductive) cells Diploid (Most) Divide by meiosis to make gametes (sex cells) (sperm & ova) In testes & ovaries

Mitosis vs. Meiosis Asexual Reproduction Sexual Reproduction Body cells (somatic) Sex cells (gamete) DNA replicates once # cell divisions: 1 # cell divisions: 2 2 identical daughter cells 4 different daughter cells Makes Diploids (2N) Makes Haploids (1N)

Meiosis is part of SEXUAL Reproduction (requiring 2 parents)

In Meiosis: ONE diploid, germ cell divides to become FOUR  In Meiosis: ONE diploid, germ cell divides to become FOUR (4) daughter cells (*circle*) … diploid / haploid (*circle*) … somatic / gamete

In order to go from diploid to haploid, Meiosis Involves Two Divisions  In order to go from diploid to haploid, Meiosis Involves Two Divisions Meiosis I Meiosis II Splits homologous pairs of chromosomes Splits sister chromatids apart

Time To Name The Phases PROPHASE I METAPHASE I ANAPHASE I TELOPHASE I PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II

TELOPHASE I & CYTOKINESIS Make 2 Tables PROPHASE I METAPHASE I ANAPHASE I TELOPHASE I & CYTOKINESIS WHAT IT LOOKS LIKE WHAT’S GOING ON TABLE I

TELOPHASE II & CYTOKINESIS Make 2 Tables PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II & CYTOKINESIS WHAT IT LOOKS LIKE WHAT’S GOING ON TABLE II

PROPHASE I  Nuclear envelope disintegrates Centrioles migrate, produce spindle fibers Homologous chromosomes pair up Homologous pairs CROSS OVER!

Exchange of segments of genes between homologous chromosomes  PROPHASE I CROSSING OVER Exchange of segments of genes between homologous chromosomes

 METAPHASE I Homologous pairs of chromosomes line up along metaphase plate

 ANAPHASE I Homologous pairs pulled apart to opposite poles

TELOPHASE I & CYTOKINESIS  TELOPHASE I & CYTOKINESIS Chromosomes (no longer homologous pairs!) prepare for meiosis II Cytoplasm begins to split into first 2 (of 4) daughter cells

TELOPHASE I & CYTOKINESIS  TELOPHASE I & CYTOKINESIS NOTE: At the end of meiosis I, daughter cells are already Haploid! (because homologous pairs were separated)

2 haploid daughter cells then go through MEIOSIS II In Meiosis II, 2 haploid cells will separate sister chromatids to make 4 gametes!

PROPHASE II  Centrioles migrate, produce spindle fibers Just like in mitosis!

METAPHASE II  Chromosomes line up along metaphase plate Just like in mitosis!

ANAPHASE II  Sister chromatids pulled apart to opposite poles Just like in mitosis!

TELOPHASE II & CYTOKINESIS  TELOPHASE II & CYTOKINESIS Nuclear envelopes reform around 4 daughter cells Cytoplasm splits to separate the 4 daughter cells Each of the 4 daughter cells is a new gamete with genetic information different from either parent!

Is This Meiosis I or Meiosis II? Why?

Is This Meiosis I or Meiosis II? Why?

Meiosis I Meiosis II Check Point: When do the cells become haploid?

Mitosis v. Meiosis

200 to 400 million sperm per ejaculate! But what happens to the other 3 eggs? The largest ovum absorbs the others! Only one wins per cycle, usually

Finally, Fertilization Male and female gametes unite Fusion of two haploid nuclei produces diploid nucleus Egg + Sperm = Zygote (new life in one diploid cell)

Meiosis and Sexual Reproduction Guarantee Variation and Diversity! In Three Ways Independent Assortment Homologous Chromosomes separate randomly Crossing Over Homologous pairs exchange gene segments Random Fertilization Which sperm meets which egg? (game of chance)