MEIOSIS Chapter 11

I. Meiosis A. Nuclear division that produces sex cells /gametes B. In REPRODUCTIVE ORGANS only C. 2 divisions: Meiosis I & Meiosis II

II. Stages A. PROPHASE I 1. centrioles & spindle fibers appear 2. nuclear mem. & nucleolus disappear

3. Crossing over= exchange of genetic material between Between homologous chromosomes Ensures genetic variation! Tetrad formed

B. METAPHASE I 1. chromosome PAIRS (homologous)line up in middle RANDOMLY 2. Ensures genetic variation! 3. spindle fibers attach to centromeres

C. ANAPHASE I 1. Chromosome PAIRS (homologous) SEPARATE to opposite sides

D. TELOPHASE I 1. nuclear mem. forms around chromosomes (temporarily) 2. spindle fibers & centrioles disappear 3. cytokinesis occurs

E. PROPHASE II 1.spindle fibers reappear 2. nuclear membrane disappears 3. starts with 46 chromosomes

F. METAPHASE II 1. chromosomes line up in middle 2. spindle fibers attach to centromeres

G. ANAPHASE II 1. chromosomes separate into chromatids & move to opposite ends

H. TELOPHASE II 1. spindle fibers & centrioles disappear 2. nuclear mem. & nucleolus reappear 3. cytokinesis follows

I. CYTOKINESIS 1. Four non-identical cells – Back in interphase

MitosisMeiosis 2 IDENTICAL cells 4 NON- IDENTICAL cells Start =46 chr. End = 46 chr. Start =46 chr. End = 23 chr. III. MITOSIS VS. MEIOSIS

MitosisMeiosis No crossing over Crossing over in Prophase I Makes BODY cells Makes SEX cells

Body CellsSex Cells Sci. name = SOMATIC cells Sci. name = GAMETES DIPLOIDHAPLOID 46 chr. (23 pairs) 23 chr. (unpaired)

IV. Misc. Meiosis A. Meiosis in males: SPERMATOGENESIS makes 4 sperm cells B. Meiosis in females: OOGENESIS makes only 1 mature egg (ovum)

1.Why only 1 egg? a. unequal division of cytoplasm in both telophase I & II

V. Asexual & Sexual Reproduction A. ASEXUAL reproduction = making of offspring from 1 parent B. SEXUAL reproduction = making of offspring by combining gametes from 2 parents