Cell Division Meiosis Diploid l A cell with

Cell Division Meiosis

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

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

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

Interphase l All the same steps as mitosis

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

Crossing Over

Prophase I

Metaphase I l Tetrads line up at equator

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

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

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

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

Male vs. Female Gamete Formation