Meiosis The ability to pass on traits is called heredity. – This ability is one of the unifying themes of biology as individual units of heredity (genes) are passed from one generation to the next in reproductive cells called gametes. – Organisms can reproduce in 2 basic ways; sexually and asexually.

Genetic Variation Meiosis reduces the number of chromosomes and produces haploid (1n) cells from diploid cells (2n). – n = # of each chromosome

Stages of Meiosis - I Interphase – DNA is replicated and sister chromatids are joined at the centromere regions Prophase I – chromosomes condense and pair Forms a tetrad… complex of 4 sister chromatids crossing-over may occur as DNA segments are exchanged between homologous chromosomes – Increases genetic variability – the rest is the same as in mitosis

Stages of Meiosis - I Metaphase I – tetrads are now aligned at the metaphase plate – rest like mitosis Anaphase I – chromosome pairs (sister chromatids) move toward the poles – rest like mitosis Telophase I (cytokinesis) – each cell is now haploid but still consists of pairs of sister chromatids – cytokinesis splits the cell and cells are now haploid at 2n = 46

Stages of Meiosis - II Prophase II – spindle fibers again grow and sister chromatid pairs move toward the metaphase plate Metaphase II – chromosomes are now at the metaphase plate but are not genetically identical due to crossing over Anaphase II – sister chromatid pairs separate at the centromere region and move toward opposite poles Telophase II (cytokinesis) – nuclei begin to form and DNA begins to uncondense – cytokinesis separates divides the cell – there are now 4 haploid cells (n=23) – each daughter cell is genetically unique

Sexual Reproduction Fertilization - union of gametes from 2 different parents – gametes are produced via meiosis and each have 1/2 the DNA of the somatic cells from which they were derived

Genetic Variation Genetic variation is made possible through the following processes: – Independent assortment of chromosomes because the arrangement at the metaphase plate is random, each haploid has a 50% chance that it will receive a parental chromosome – Crossing over creates recombinant chromosomes that are a mix or both parental chromosomes – Random fertilization because of random fertilization each individual contains a genetically unique compliment of chromosomes and genes