Meiosis

Homologous Chromosomes Square 1 Homologous Chromosomes 1 chromosome from parent 1 chromosome from parent Tetrad

Square 2 Diploid 2 sets (chromosomes) 2n Somatic cells Body cells ex.: liver, skin, etc. Human 2n = 46

Square 3 Haploid 1 set (chromosomes) 1n Sex cells Sperm/Egg Gamete Human 1n = 23

Square 4 WHY? maintain chromosome # from gen. 2 gen. 2n -> 1n # of chromosomes cut in ½ by separating homologous chromosomes Creates genetic variation Used in sexual reproduction

Do not write “Putting It All Together” Fertilization

Do not write Goal: reduce genetic material by half Why? n (mom) + n (dad) = 2n (offspring) from mom from dad child too much! meiosis reduces genetic content Just right!

2 rounds of cell division Do not write Meiosis Overview 2 rounds of cell division Meiosis 1 called reduction division. (diploid cell becomes 2 haploid cells.) Meiosis 2 divides 2 cells into 4 cells.

Meiosis I : the reduction division Do not write Meiosis I : the reduction division Prophase I (early) (diploid) (late) Metaphase I Anaphase I Telophase I Nucleus Spindle fibers Nuclear envelope

Square 5 MEIOSIS I: PROPHASE I Prophase I Interphase 2n: Homologs pair up Interphase

MEIOSIS I: PROPHASE I Crossing over Top 1/3 of squares 6/7 MEIOSIS I: PROPHASE I Crossing over Portions of the homologous chromosomes exchange – more genetic variation

Square 6 METAPHASE I 2n: Homologs line up at Metaphase Plate Law #1

ANAPHASE I Square 7 Homologs separate; move to opposite poles. Sister chromatids remain attached

TELOPHASE I & CYTOKINESIS Square 8 TELOPHASE I & CYTOKINESIS Cytokinesis divides cell in two (Reduction Division…2n 1n)

Meiosis II Do not write Gene X Sister chromatids carry Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene. Sister chromatids carry identical genetic information.

Meiosis II Do not write Prophase II (haploid) Metaphase II Anaphase II Telophase II Four Non-identical haploid daughter cells

Square 9 PROPHASE II Similar to mitosis… 1n

Square 10 METAPHASE II Sister chromatids line up 1n

Square 11 ANAPHASE II Sister chromatids separate 1n Law #2

TELOPHASE II & CYTOKINESIS Square 12 TELOPHASE II & CYTOKINESIS Four haploid cells One copy of each gene

Meiosis: cell division in two parts Do not write Meiosis: cell division in two parts Sister chromatids separate Haploid Diploid Meiosis I (reduction division) Meiosis II Chromatids separate (equational division) 2n n n Result: one copy of each chromosome in a gamete.

#1 REPRODUCTION #2 # OF DIVISIONS Square 13 MITOSIS MEIOSIS Mitosis – Asexual Meiosis - Sexual #2 # OF DIVISIONS Mitosis = 1 2 identical 2n cells Meiosis = 2 4 unique 1n cells VS.

#3 change in CHROMOSOME # Square 14 MITOSIS MEIOSIS #3 change in CHROMOSOME # Mitosis – no change – 1 diploid :2 diploid Meiosis - change occurs (1/2 of original) – 1 diploid: 4 haploid #4 SEPERATION OF GENETIC MATERIAL Mitosis – each daughter cell gets complete set (2n) Meiosis – each gamete gets 1 copy (1n) VS.

LAW #1: INDEPENDENT ASSORTMENT Square 15 LAW #1: INDEPENDENT ASSORTMENT Homologous chromosomes do not affect how others sort Gives even more genetic variation

LAW #2: SEGREGATION Square 16 Separation of alleles Needed so gametes have only 1 set of each gene