Cell Division Meiosis…page 263

Diploid A cell with two of each kind of chromosome 2n Example: human 2n=46

Haploid A cell containing one of each kind of chromosome n Example: human n=23

Why meiosis? Mitosis produces cells with the same number and kinds of chromosomes Bad for reproduction Meiosis produces cells with half the number of chromosomes Good for reproduction Meiosis produces cells called gametes

Interphase All the same steps as mitosis

Prophase I Same steps as mitosis Homologous chromosomes line up with each other Forms a tetrad Portions of tetrad break off and switch places – crossing over Crossing over makes new combinations of genes

Crossing Over…page 269

Prophase I

Metaphase I Tetrads line up at equator

Anaphase I Homologous chromosomes are pulled apart This reduces the number of chromosomes by half

Telophase I Same things happen as mitosis Nucleus usually does not return

Meiosis II Chromosomes DO NOT replicate This cycle is exactly like mitosis only with half the number of chromosomes Produces 4 genetically different haploid cells

Meiosis

Nondisjunction Failure of chromosomes to split properly End up with too many or too few chromosomes Down Syndrome – have an extra #21 chromosome

Male vs. Female Gamete Formation