Meiosis Produces cells for sexual reproduction. The cells where meiosis occurs are called gametes (sex cells)

Two kinds of cells: somatic cell – any body cell besides egg and sperm (46 chromosomes) Human sex cells, or gametes, need to have 23 chromosomes (sperm and egg) *Why is this?

Therefore.. Somatic cells are diploid, represented by 2n. Gametes are haploid, or n

Homologous chromosomes In a diploid cell, there are 2 copies of each chromosome, homologous chromosomes. These chromosomes are exactly alike in size, location of centromere, and the banding pattern seen after staining with dyes.

Autosomes – chromosomes 1-22 Sex chromosomes differ in males, having an X and a Y chromosome while women have two X chromosomes.

Karyotyping

How is meiosis different from mitosis? Consists of 2 cycles Results in a reduction of the chromosome # Is a division of sex cells Results in 4 daughter cells

Steps of meiosis Before meiosis: chromosomes replicate to get 2 sister chromatids joined at centromere. Meiosis I: Prophase I  homologous pairs “find each other” to form a tetrad.

Crossing over, which occurs in Prophase I (Meiosis I), is the exchange of genetic material between homologous chromosomes. X

Metaphase I, Anaphase I, and Telophase I movements of chromosomes are similar to mitosis, except the tetrad gets separated, not the sister chromatids.

After Meiosis I… Each daughter cell has one member of each homologous pair (2 sister chromatids) Each haploid daughter cell then undergoes Meiosis II, a second division. Sister chromatids move to center of cell and then the centromeres split, separating the chromatids. Result: four daughter cells, containing one copy of one complete set of chromosomes

Spermatogenesis & Oogenesis Human males: 4 haploid sperm cells are produced Human females: 1 of the four haploid nuclei forms an egg cell

What is the biological advantage to reproducing sexually? Random assortment of parental genes and crossing over during meiosis I greatly increase variation in the genetic code of a species. This is important for a population’s survival. => greater the diversity, the stronger a population/ecosystem Asexual reproduction does not provide variance and any change in the environment that would kill an organism could destroy the entire population. Variation is thought to be the reason that sexual reproduction is the dominant form of reproduction.

Fertilization

Mitosis vs. Meiosis