Making Haploid Gametes

 Both chromosomal problems (monosomy/trisomy & breakage) are result of improper haploid cell formation/division in process known as meiosis Two phases: meiosis I & meiosis II Goal: 2n (diploid)  n (haploid)

 Body or somatic cell division is called mitosis, which generates the EXACT same cell Mitosis: cell division which produces cells containing SAME number of chromosomes as parent’s cell ○ Start: One 2n skin cell ○ End: Two 2n skin cells

 Gamete or sex cell division is called meiosis, which generates cell with HALF number of chromosomes as original Start: One 2n cell End: Four n sperm or egg cells n n nnn nnn Video of Mitosis vs. Meiosis 2n2n

MITOSISMEIOSIS Growth & repairMaking gametes In body/somatic cellsIn sex cells/gametes One 2n cell  Two 2n cellsOne 2n cell  Four n cells One diploid  two diploids One diploid  four haploids Exact copy of parentHalf copy of parent 2n n n n n

 Two repeating stages with precursor stage: Steps of meiosis Meiosis I Meiosis II Interphase

 Interphase – DNA replication (2n  4n) 1. Prophase I – homologous chromosomes visible; crossing over occurs to add genetic variety 2. Metaphase I – homologs move to equator 3. Anaphase I – homologs move to opposite poles 4. Telophase I – 2n sets move to poles; cytokinesis 5. Prophase II – new spindle forms around chromosomes 6. Metaphase II – chromosomes move to equator 7. Anaphase II – centromeres divide; n chromatids to poles 8. Telophase II – n sets move to poles; cytokinesis MEIOSIS I = 4n  2n MEIOSIS II – 2n  n

Interphase  Doubling of chromosomes (2n  4n)  Cell looks same  Nucleus is darker because of chromosomal duplication A B C “tetrad” a b c a b c A B C

A A B B CC aa bb c c

Prophase I  Nuclear envelope disappears  Spindle fibers form from centrioles  Chromosomes in homologous pairs  Crossing-over occurs (genetic diversity!) Homologous pair A A B B CC aa bb c c A B B CC a bb c c a A Crossing over Same gene, different variety Nuclear envelope centrioles spindle fibers

Metaphase I  Homologs move to equator  How they line up varies (genetic diversity!) = law of independent assortment  Spindle fibers attach to centromeres

Anaphase I  Spindle fibers contract  Homologs pulled to poles

 What happens when anaphase goes wrong? If tetrads are not pulled apart evenly, chromosomal abnormalities result Trisomy (extra chromosome Monosomy (missing) Anaphase Mistakes

Telophase I  Chromosomes go to poles  Cytokinesis occurs; cells split in two  4n  2n (halfway to goal!)

Prophase II  New spindle fibers form  2n (2 sets of 23 = 46)

Metaphase II  Same as metaphase I  Homologs move to equator  Spindle fibers attach to centromeres

Anaphase II  Sister chromatids pulled apart at centromere  Chromatids move to opposite poles

 What happens when anaphase goes wrong? If chromatids are not pulled apart evenly, chromosomal abnormalities result Trisomy (extra chromosome Deletion/Monosomy (missing) Anaphase Mistakes – A Second Chance

Telophase II  Cytokinesis takes place, splitting two cells into four (sperm or eggs)  2n  n

Meiosis Overview Watch Meiosis Video Watch Meiosis Video

Overview 2n2n 4n4n 2n2n 2n2n n n n n Precursor cell INTERPHASE MEIOSIS I MEIOSIS II diploid haploid polyploid