Meiosis

Chapter 10

Chromosomes

Genes are located on chromosomes inside the cell nucleus

When offspring are formed, 1 set of chromosomes from each parent is passed on

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)

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 genes

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: chromosomes replicate , making 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

Sometimes when tetrads form, crossing-over can occur

Crossing Over: when homologous chromosomes exchange portions of themselves

Results in new combinations of genes

Meiosis I

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 2) Metaphase II: Chromosomes

line up in center of cell (just like Metaphase I)

3) Anaphase II: fibers pull chromatids apart

4) Telophase II: cytokinesis occurs; results in 4 haploid (N) daughter cells

Gamete Formation

Male Animals: 4 haploid gametes all become sperm

Female Animals: 4 haploid gametes become 1 egg and 3 polar bodies (not used)

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

Linkage and Gene Maps

Thomas Morgan (1910): discovered many traits inherited together i.e.: fruit fly’s red eyes and mini wings Genes can be placed in “linkage

groups” “Linkage Group” = chromosome

Mendel just happened to study genes on different chromosomes

T. Morgan: genes on same chromosome should be inherited together Chromosomes = group of linked genes

Chromosomes assort independently; not individual genes

However, crossing-over may “unlink” some genes Farther apart = more likely to separate

This action creates new combinations of alleles → more genetic diversity

Gene Mapping

Rate which genes become separated and recombine is measured

Describes the relative location of each known gene on a chromosome

Created by “mapping” genes relative locations