Meiosis Chapter 10, Section 2

The Basics Genes are located on the chromosomes. Each organism must inherit one copy of every gene from both parents. Some organisms have 2 complete sets of genes. Those two sets must be separated so that each gamete produced contains just one set of genes.

Chromosome Number Humans have 46 chromosomes. 23 came from your mother 23 came from your father These chromosomes are homologous (same gene). This means that one complete set of genes came from each parent. All 46 chromosomes are present in every human body cell (a.k.a. somatic cell). This is the diploid chromosome number (2 sets).

Gametes Gametes are sex cells—sperm or egg. Gametes contain only 1 copy of each chromosome. These cells are haploid (1 set). Humans have 23 chromosomes in every gamete.

Diploid & Haploid Diploid = a cell with two sets of chromosomes, 2n Haploid = a cell with one set of chromosomes, gametes, 1n Meiosis forms haploid gametes required for sexual reproduction Meiosis has 2 stages Meiosis I Meiosis II

 MEIOSIS I Separation of homologous pairs Homologous Pairs: Sets of same chromosome from different parents

 1. Prophase I DNA coils into chromosomes Nuclear envelope & nucleolus break down Synapsis = homologous chromosomes pair up into tetrads Crossing-over = homologous pairs switch pieces of their chromatids Genetic Recombination = maternal & paternal genetic info get shuffled during crossing-over

 2. Metaphase I Tetrads line up in the middle of the cell Spindle fibers attach at the centromeres of each homologous chromosome (one member of the pair)

 3. Anaphase I Each homologous chromosome moves to the opposite end of the cell (one member of the pair) Independent Assortment = random separation of chromosomes, allows for genetic variation

 4. Telophase I & Cytokinesis I Two newly formed cells are haploid Each chromosome is still made up of sister chromatids (two copies of chromosome)

Separation of sister chromatids  MEIOSIS II Separation of sister chromatids Sister Chromatids = identical halves of a chromosome that has been duplicated

 5. Prophase II 2 newly created cells from Meiosis I start to divide

 6. Metaphase II Chromosomes line up in the middle

 7. Anaphase II Chromatids separate

 8. Telophase II & Cytokinesis II 4 new cells, each with half the number of chromosomes of the original cell (genetically different than parent cell)

Development of Gametes Spermatogenesis = production of sperm cells Oogenesis = production of mature egg cells

Nondisjunction Nondisjunction = failure of homologous pairs to separate in Meiosis I If an organism survives, it may have a genetic disorder

Polyploidy Polyploid = organisms with more than the usual number of chromosome sets Occurs frequently in plants, results in plants that are larger than normal

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Mitosis vs. Meiosis Mitosis Body cells 2 cells produced Diploid (2 of each chromosome) Genetically identical to each other AND to the original cell Meiosis Sex cells (sperm/egg) 4 cells produced Haploid (1 of each chromosome) Genetically different