Meiosis• The form of cell division by which gametes, with half the number of chromosomes, are produced.• Diploid (2n) → haploid (n)• Meiosis is sexual reproduction.• Two divisions (meiosis I and meiosis II). II
Fertilization• The fusion of a sperm and egg to form a zygote. zygote• A zygote is a fertilized egg sperm n=23 n=23 egg 2n=46 zygote
Meiosis• Sex cells divide to produce gametes (sperm or egg). egg)• Gametes have half the # of chromosomes. chromosomes• Occurs only in gonads (testes or ovaries). Male: spermatogenesis Female: oogenesis• Meiosis is similar to mitosis with some chromosomal differences.
Spermatogenesis n=23 human sex cell sperm n=23 n=23 2n=46 haploid (n) n=23diploid (2n) n=23 n=23 meiosis I meiosis II
Interphase I• Similar to mitosis interphase.• Chromosomes replicate (S phase).• Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres. centromeres• Centriole pairs also replicate.
Interphase I • Nucleus and nucleolus visible. chromatin nuclear membranecell membrane nucleolus
Meiosis I (four phases)• Cell division that reduces the chromosome number by one-half.• four phases: phases a. prophase I b. metaphase I c. anaphase I d. telophase I
Prophase I• Longest and most complex phase (90%).• Chromosomes condense.• Synapsis occurs: homologous chromosomes come together to form a tetrad. tetrad• Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids).
Homologous Chromosomes• Pair of chromosomes (maternal and paternal) that are paternal similar in shape and size.• Homologous pairs (tetrads) carry genes controlling the same inherited traits.• Each locus (position of a gene) is in the same position on homologues.• Humans have 23 pairs of homologous chromosomes. a. 22 pairs of autosomes b. 01 pair of sex chromosomes
Karyotype• A method of organizing the chromosomes of a cell in relation to number, size, and type.
Homologous Chromosomes eye color eye color locus locus hair color hair color locus locus Paternal Maternal
Humans have 23 Sets of Homologous ChromosomesEach Homologous set is made up of 2 Homologues.HomologueHomologue
Autosomes (The Autosomes code for most of the offspring’s traits)In Humans the “Autosomes”are sets 1 - 22
21 trisomy – Downs Syndrome Can you see the extra 21st chromosome? Is this person male or female?
Sex Chromosomes The Sex Chromosomes code for the sex of the offspring. ** If the offspring has two “X” chromosomes it will be a female. ** If the offspring has one “X” chromosome and one “Y” chromosome it will be a male. In Humans the “Sex Chromosomes” are the 23rd setXX chromosome - female XY chromosome - male
Boy or Girl? The Y Chromosome “Decides” Y chromosome X chromosome
Crossing Over• Crossing over (variation) may occur between nonsister chromatids at the chiasmata. chiasmata• Crossing over: segments of nonsister over chromatids break and reattach to the other chromatid. chromatid• Chiasmata (chiasma) are the sites of crossing over. over
Crossing Over - variationnonsister chromatids Tetrad chiasmata: site variation of crossing over
Another Way Meiosis Makes Lots ofDifferent Sex Cells – Crossing-OverCrossing-over multiplies the already huge number of different gamete types produced by independent
Sex ChromosomesXX chromosome - female XY chromosome - male
Prophase I spindle fiber centriolesasterfibers
Metaphase I• Shortest phase• Tetrads align on the metaphase plate. plate• INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation 3. Formula: 2n Example: 2n = 4 then n = 2 thus 22 = 4 combinations