Meiosis Chapter 11-4

The Basics Genes are located on the chromosomes. Each organism must inherit one copy of every gene from both parents. Each organism has 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.

Meiosis Meiosis is a process of reduction division. The number of chromosomes per cell is cut in half through the separation of homologous chromosomes. 2 distinct stages: Meiosis I Meiosis II By the end of Meiosis II, 4 haploid cells are produced.

Meiosis I Chromosomes are duplicated in interphase. Prophase I Homologous chromosomes pair up Metaphase I Spindle fibers attach to chromosomes to line them up in the middle Crossing over occurs (genes are swapped between homologous chromosomes to increase genetic diversity) Anaphase I Homologous chromosomes are separated

Meiosis I Results Because of crossing over, the cells that are produced have chromosomes that are different from each other and from the original cell. The cells produced are NOT identical copies like in mitosis. Meiosis I results in 2 haploid daughter cells that are NOT genetically identical.

Meiosis II The chromosomes are NOT replicated in between Meiosis I and Meiosis II. Prophase II—the 2 haploid cells from Meiosis I prepare for another division Metaphase II—chromosomes line up in the middle of the spindle Anaphase II—sister chromatids are pulled apart Telophase—the nuclear membranes reform and the cytoplasm is divided

Gamete Formation Males produce 4 sperm cells each time meiosis takes place. Females produce 1 egg cell and 3 polar bodies (non-functional cells) because the cytoplasm is not divided equally.

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