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 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 Mitosis vs. Meiosis