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MEIOSIS Modified with permission from Robert Goodman, Biology Department,Citrus Community College 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 & meiosis II) Meiosis • Sex cells undergo meiosis to produce gametes – Gametes have half the # of chromosomes. • Male: sperm • Female: egg – Meiosis only occurs in gonads • Male Testes: spermatogenesis • Female Ovaries: oogenesis • Meiosis is similar to mitosis with some chromosomal differences. Spermatogenesis Head Mid piece n=23 human sex cell Tail sperm n=23 n=23 2n=46 haploid (n) n=23 diploid (2n) n=23 n=23 meiosis I meiosis II Interphase I • Similar to mitosis interphase – Chromosomes replicate (S phase) – Each duplicated chromosome consists of two identical sister chromatids attached at their centromeres – Centriole pairs also replicate Interphase I • Nucleus and nucleolus visible. chromatin nuclear membrane cell membrane nucleolus Meiosis I (four phases) • Cell division reduces the chromosome number by one-half. • Four 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 –Two homologous chromosomes which is four chromatids (sister and nonsister chromatids) Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad sister chromatids Homologous Chromosomes • Pair of chromosomes (maternal and paternal) that are 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. – 22 pairs of autosomes – 1 pair of sex chromosomes Homologous Chromosomes eye color locus eye color locus hair color locus hair color locus Paternal Maternal Sex Chromosomes XX chromosome - female XY chromosome - male Crossing Over • Crossing over (variation) may occur between nonsister chromatids at the certain areas. –Segments of nonsister chromatids break and reattach to the other chromatid. Crossing Over - Variation nonsister chromatids chiasmata: site of crossing over Tetrad VARIATION Prophase I spindle fiber aster fibers centrioles Metaphase I • Shortest phase • Tetrads align on the metaphase plate • Independent assortment occurs: 1.Orientation of homologous pair to poles is random 2.Variation 3.Formula: 2n 1. Example: 2. Then 3. Thus 2n = 4 n=2 22 = 4 combinations Metaphase I OR metaphase plate metaphase plate Question: In terms of Independent Assortment How many different combinations of sperm could a human male produce? Answer Formula: 2n Human chromosomes: 2n = 46 n = 23 223 ~8 million combinations Anaphase I Homologous chromosomes separate and move towards the poles Sister chromatids remain attached at their centromeres Anaphase I Telophase I • Each pole now has haploid set of chromosomes. • Cytokinesis occurs and two haploid daughter cells are formed. Telophase I Meiosis II • No interphase II (or very short - no more DNA replication) • Remember: Meiosis II is similar to mitosis Prophase II • Same as prophase in mitosis Metaphase II Same as metaphase in mitosis metaphase plate metaphase plate Anaphase II • Same as anaphase in mitosis • Sister chromatids separate Telophase II • Same as telophase in mitosis. • Nuclei form. • Cytokinesis occurs. • Remember: four haploid daughter cells produced. gametes = sperm or egg Telophase II Meiosis n=2 sex cell sperm n=2 n=2 2n=4 haploid (n) n=2 diploid (2n) n=2 n=2 meiosis I meiosis II Variation • Important to population as the raw material for Natural Selection. • Question: What are the three sexual sources of genetic variation? Answer: 1. Crossing over (prophase I) 2. Independent assortment (metaphase I) 3. Random fertilization Remember: variation is good! Question: • A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? Answer: 10 chromosomes (haploid) Karyotype • A method of organizing the chromosomes of a cell in relation to number, size, and type. Fertilization • The fusion of a sperm and egg to form a zygote. • A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote Question: A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? Answer: 10 chromosomes