3.1 Meiosis

3.1 Meiosis Multicellular organisms that reproduce sexually have two types of cells: Somatic & Sex cells Somatic Cells (AKA body cells) reproduce by cell division Make up the majority of the cells in the organism Have 23 pairs of chromosomes (total of 46) Sex cells Called gametes ½ the chromosomes of the parent cell Makeup tiny fraction of organisms cells Male gametes = sperm, female gametes = eggs or ova

Individuals of the same species have the same number of chromosomes. Ex. Goldfish have 94, fruit flies 8, humans 46 To maintain the same number of chromosomes from generation to generation a parent creates gametes through meiosis. Only happens in reproductive cells

Diploid (2n) have a complete set of chromosomes Haploid (n) are cells with half the number of chromosomes of the parent Diploid (2n) have a complete set of chromosomes Homologous Chromosomes are pairs of chromosomes Each parent gives one copy during meiosis to form a gamete. Ex. Humans have 23 pairs

Homologous Chromosomes

Meiosis There are two stages: Meiosis I Meiosis II They produce 4 haploid (23 chromosomes) gametes Each stage is divided into four parts: Prophase I & II Metaphase I & II Anaphase I & II Telophase I & II

Meiosis Diagram Meiosis I separates the homologous chromosomes and meiosis II separates the pairs of chromatids.

Meiosis I Homologous chromosomes separate & move to opposite poles along with their sister chromatid. The sister chromatids do not separate. Results in two daughter cells, with only one of the homologous chromosomes

Crossing over Exchange of genetic material Genetically different daughter cells

Meiosis II Similar to mitosis Sister chromatids line up at the equator The chromatids are pulled to the opposite pole Nuclear membranes reform & cytokinesis occurs Results in 4 haploid daughter cells called gametes (sex cells)

Meiosis Animations How Meiosis Works Comparison of Mitosis & Meiosis http://highered.mcgraw- hill.com/sites/0072495855/student_view0/chapter28 /animation__how_meiosis_works.html Comparison of Mitosis & Meiosis http://highered.mcgraw- hill.com/sites/0072495855/student_view0/chapter28 /animation__comparison_of_meiosis_and_mitosis__quiz _1_.html

Atypical Meiosis Once in a while errors do occur , the most common being nondisjunction. Nondisjunction occurs when the homologous chromosomes do not separate during Meiosis The result is gametes with the wrong number of chromosomes Can occur in meiosis I or meiosis II E.g.: Down syndrome results in a zygote with 47 chromosomes (3 copies of Chromosome #21)