Meiosis

Chapter 10

Chromosome Number

Homologous: chromosomes that are passed on from parents 1 from mom/1 from dad

Diploid: cell that contains both sets of homologous chromosomes 2 complete sets of genes # chromosomes in diploid cells written

as “2N” (Human 2N = 46)

Chromosome Number (con’t)

Haploid: cell that contains only 1 set of chromosomes (gamete) Written as “N” (Human N=23) Gametes: reproductive cells (sperm &

eggs) When gametes are formed, contain

only 1 set of chromosomes

Meiosis

Def: The process by which the # chromosomes in a cell is cut in half, and homologous chromosomes are separated

2 Stages: Meiosis I Meiosis IICreates 4 haploid cells (gametes), all

genetically different

Meiosis I

Interphase I (before Meiosis): DNA replicates during S phase Makes 2 new chromatids (copies)

connected by a centromere Occurs in 4 steps: 1) Prophase I:

Each chromosome pairs with homologous chromosome & forms a tetrad

Meiosis I (con’t)

Sometimes when tetrads form, crossing-over can occur

Crossing Over: when homologous chromosomes exchange portions of themselves

Results in new combinations of genes, different from parents

Prophase I

Meiosis I (con’t)

2) Metaphase I: Spindle fibers attach and

chromosomes line up 3) Anaphase I:

Fibers pull homologous chromosomes apart

4) Telophase I: Cytokinesis occurs Results in 2 new diploid (2N)

“daughter” cells

Meiosis II Chromosomes DO NOT DUPLICATE again

(no Interphase) Occurs in 4 steps: 1) Prophase II:

From Meiosis I, have 2 diploid (2N) daughter cells

Meiosis II 2) Metaphase II: Chromosomes line up in center of

cell (just like Metaphase I)

3) Anaphase II: Fibers pull chromatids apart

Meiosis II

4) Telophase II: Nucleus forms around each set

chromatids Cytokinesis occurs Results in 4 haploid (N) daughter cells

(gametes)

Mitosis vs. Meiosis

Mitosis → 2 genetically identical diploid (2N) cells Used for cell growth and replacement

Meiosis → 4 genetically different haploid (N) cells Used for reproduction & formation on

gametes