Meiosis; Chapter 6.2 I. Purpose of meiosis is to create a gamete that is haploid (half the normal number of chromosomes), from a diploid cell (complete set of chromosomes). Upon fertilization, a male gamete (sperm) will combine with a female gamete (egg), to produce a zygote (single fertilized cell).

II. Phases; Divided into Meiosis I and Meiosis II A. Prophase I; Almost like prophase of mitosis. Homologous chromosomes line up with each other, gene by gene to form a tetrad.

During this phase, crossing over occurs. During crossing over homologous chromosomes can actually break apart and exchange genetic material.

Crossing over results in new allele combinations, and produces genetic variation.

B. Metaphase I; Spindle fibers attach to centromeres. Homologous chromosomes line up as tetrads (side by side) across the middle of the cell.

C. Anaphase I; Homologous chromosomes separate and C. Anaphase I; Homologous chromosomes separate and move to opposite ends of the cell. Centromeres, holding sister chromatids together, do not split.

D. Telophase I; Spindle fibers break down, and chromosomes uncoil. Cytoplasm divides to form two new cells. Each new cell has half the genetic info. of the original cell.

E. Prophase II; Spindle fibers form and attach to the E. Prophase II; Spindle fibers form and attach to the chromosomes in each new cell.

F. Metaphase II; Chromosomes (still made of sister F. Metaphase II; Chromosomes (still made of sister chromatids) line up along the center of the cell.

G. Anaphase II; Centromeres of each chromosome split G. Anaphase II; Centromeres of each chromosome split. Sister chromatides separate, and move to opposite poles.

H. Telophase II; Nuclei reform, spindle fibers break H. Telophase II; Nuclei reform, spindle fibers break down, and cytoplasm divides. New cells are haploid (n).

III. Nondisjunction; The failure of homologous chromosomes III. Nondisjunction; The failure of homologous chromosomes to separate properly during meiosis.

A. Autosomal Trisomy- Somatic cell ends up with an extra chromosome. 1. Trisomy 21- Down Syndrome 1 in 700 live births Increased risk with older mother. By age 45, risk increases to 1 in 30. 2. Trisomy 13 - Patau Syndrome 1 in 5000 live births 3. Trisomy 18- Edwards Syndrome 1 in 10,000 live births B. Trisomy in gametes- 1. Trisomy X- metafemales 3 X chromosomes 2. Klinefelter’s Syndrome- Male with XXY combination. 3. Super males- XYY combination. C. Monosomy- zygote is missing a chromosome 1. Turner Syndrome- XO affects females 2. OY lethal to males