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Meiosis Honors Biology Introduction to Heredity Offspring acquire genes from parents by inheriting chromosomes Inheritance is possible because: – Sperm and ova carrying each parent’s genes are combined in the nucleus of the fertilized egg Actual transmission of genes depends on the behavior of chromosomes •Chromosomes-organizational unit of hereditary material in the nucleus of eukaryotic organisms •Contain hundreds of thousands of genes, each of which is a specific region of the DNA molecule, or locus Human Life Cycle Each somatic cell (body cell) has 46 chromosomes or 23 matching pairs (diploid) Karyotype: male Sex chromosomes: determine gender (XX; XY) Autosomes: nonsex chromosomes Human Life Cycle Gametes (sex cells) have a single set of 22 autosomes and a single sex chromosome, either X or Y With 23 chromosomes, they are haploid Haploid sperm + haploid ova n n haploid number: n = 23 diploid number: 2n = 46 fertilization zygote (2n) 2n Meiosis Reduces chromosome number from diploid to haploid Increases genetic variation among offspring Steps resemble steps in mitosis Single replication of DNA is followed by 2 consecutive cell divisions Meiosis I Meiosis II Produces 4 different daughter cells which have half the number of chromosomes as the original cell In the first division, meiosis I, homologous chromosomes are paired While they are paired, they cross over and exchange genetic information The homologous pairs are then separated, and two daughter cells are produced Interphase I Chromosomes replicate (still as chromatin) Duplicated chromosomes consist of 2 identical sister chromatids attached by centromere Centriole pairs replicate MEIOSIS I: Homologous chromosomes separate INTERPHASE Centrosomes (with centriole pairs) Nuclear envelope PROPHASE I METAPHASE I Microtubules attached to Spindle kinetochore Sites of crossing over Chromatin Sister chromatids Tetrad Figure 8.14, part 1 Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings Metaphase plate Centromere (with kinetochore) ANAPHASE I Sister chromatids remain attached Homologous chromosomes separate Meiosis I This cell division separates the 2 chromosomes of each homologous pair and reduce the chromosome number by one-half Prophase I Chromosomes condense Synapsis occurs (homologues pair) Chromosomes seen as distinct structures; each chromosome has 2 chromatids, so each synapsis forms a tetrad Prophase I Sister chromatids held together by centromeres; nonsister chromatids held together by chiasmata where crossing-over occurs (exchange of DNA) Late Prophase I Centriole pairs move apart and spindle fibers form Nuclear envelope disappears and nucleoli disperse Prophase I Metaphase I Homologous chromosomes line up along metaphase plate Metaphase I Anaphase I Homologous chromosomes separate, independently from others Anaphase I Telophase I and Cytokinesis Each pole now has a haploid set of chromosomes (each with 2 sister chromatids) Usually, cytokinesis occurs simultaneously with telophase I, forming 2 haploid daughter cells (cleavage furrow forms in animals; cell plate forms in plants) Telophase I Meiosis II is essentially the same as mitosis The sister chromatids of each chromosome separate The result is four haploid daughter cells MEIOSIS II: Sister chromatids separate TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS Cleavage furrow Sister chromatids separate Figure 8.14, part 2 Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings Haploid daughter cells forming Meiosis II This cell division separates the 2 sister chromatids of each chromosome Prophase II Spindle apparatus forms and chromosomes move toward metaphase II plate Prophase II Metaphase II Chromosomes align singly on the metaphase plate Metaphase II Anaphase II Sister chromatids of each pair (now individual chromosomes) separate and move toward opposite poles of the cell Anaphase II Anaphase II Telophase II and Cytokinesis Nuclei form at opposite poles of the cell Cytokinesis occurs producing 4 haploid daughter cells (each genetically different) Telophase II Telophase II Key Differences Between Mitosis and Meiosis Meiosis is a reduction division Mitotic cells produce clones (same xsome #) Meiosis produces haploid cells Meiosis creates genetic variation Mitosis produces 2 identical daughter cells Meiosis produces 4 genetically different daughter cells Meiosis is 2 successive nuclear divisions Mitosis has one division Copyright © 2001 Pearson Education, Inc. publishing Benjamin Cummings Spermatogenesis Process of sperm production Results in 4 viable sperm Oogenesis Process of egg (ova) production Results in 1 viable egg and 3 polar bodies that will not survive Polar bodies result from an uneven division of cytoplasm Mechanisms of Genetic Variation Independent assortment—each pair of homologous chromosomes separate independently Results in gametes with different gene combinations Crossing-over—exchange of genetic material between non-sister chromatids Results in genetic recombination Random fertilization—random joining of two gametes Independent Assortment POSSIBILITY 1 POSSIBILITY 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1 Combination 2 Figure 8.16 Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings Combination 3 Combination 4 Mechanisms of Genetic Variation Independent assortment—each pair of homologous chromosomes separate independently Results in gametes with different gene combinations Crossing-over—exchange of genetic material between non-sister chromatids Results in genetic recombination Random fertilization—random joining of two gametes Tetrad Chaisma Centromere Figure 8.18A Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings Coat-color genes Eye-color genes Brown Black C E c e White Pink Tetrad in parent cell (homologous pair of duplicated chromosomes) Figure 8.17A, B Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings C E C E c e c e Chromosomes of the four gametes Coat-color genes Eye-color genes Tetrad (homologous pair of chromosomes in synapsis) How crossing over leads to genetic recombination 1 Breakage of homologous chromatids 2 Joining of homologous chromatids Chiasma 3 Separation of homologous chromosomes at anaphase I 4 Separation of chromatids at anaphase II and completion of meiosis Parental type of chromosome Recombinant chromosome Recombinant chromosome Parental type of chromosome Figure 8.18B Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings Gametes of four genetic types Crossing Over In Prophase I of Meiosis I, synapsis occurs allowing the crossing over of genetic material between non-sister chromatids Creates new combinations of genes not seen in either parent Mechanisms of Genetic Variation Independent assortment—each pair of homologous chromosomes separate independently Results in gametes with different gene combinations Crossing-over—exchange of genetic material between non-sister chromatids Results in genetic recombination Random fertilization—random joining of two gametes Random Fertilization Random as to which gametes join and form a gamete Importance of Genetic Variation Essential to evolution (change over time) Variation can cause changes that leads to different traits Some favorable Some unfavorable Errors and Exceptions in Chromosomal Inheritance Alterations in chromosome number or structure causes some genetic disorders Physical and chemical disturbances Errors during meiosis ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE To study human chromosomes microscopically, researchers stain and display them as a karyotype A karyotype usually shows 22 pairs of autosomes and one pair of sex chromosomes Preparation of a karyotype Blood culture Packed red And white blood cells Hypotonic solution Stain White Blood cells Centrifuge 3 2 1 Fixative Fluid Centromere Sister chromatids Pair of homologous chromosomes 4 Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings 5 Figure 8.19 Human female bands Figure 8.19x1 Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings Human female karyotype Figure 8.19x2 Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings Human male bands Figure 8.19x3 Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings Human male karyotype Figure 8.19x4 Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings Alterations of Chromosome Numbers Nondisjunction—pair of homologues do not move apart during Meiosis I, or sister chromatids do not separate during Meiosis II Results in uneven distribution of chromosomes to daughter cells Alterations of Chromosome Numbers Aneuploidy: abnormal chromosome number Trisomy: three copies of chromosomes Monosomy: one copy of a chromosome Trisomy and monosomy are usually lethal Accidents during meiosis can alter chromosome number Abnormal chromosome count is a result of nondisjunction Either homologous pairs fail to separate during meiosis I Copyright © 2003Pearson Education, Inc. publishing Benjamin Cummings Nondisjunction in meiosis I Normal meiosis II Gametes n+1 n+1 n–1 n–1 Number of chromosomes Figure 8.21A Or sister chromatids fail to separate during meiosis II Normal meiosis I Nondisjunction in meiosis II Gametes n–1 n+1 n Number of chromosomes Copyright © 2003Pearson Education, Inc. publishing Benjamin Cummings n Figure 8.21B Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome Egg cell n+1 Zygote 2n + 1 Sperm cell n (normal) Figure 8.21C Copyright © 2003Pearson Education, Inc. publishing Benjamin Cummings Trisomy 21 (Down Syndrome) *Short stature, characteristic facial features, and heart defects (varying severity) *Most common serious birth defect (1 out of 700 births) *Mothers 35+ years of age have higher chance of having a Down baby The chance of having a Down syndrome child goes up with maternal age Figure 8.20C Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings Down syndrome karyotype Figure 8.20Ax Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings Nondisjunction with Sex Chromosomes Table 8.22 Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings Klinefelter’s karyotype Figure 8.22Ax Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings XYY karyotype Figure 8.22x Alterations of Chromosome Structure Breakage of a chromosome can lead to four types of changes in chromosome structure Deletion: chromosomal fragment is lost during cell division Duplication: fragment may join to the homologous chromosome Inversion: fragment may reattach to the original chromosome but in the reverse orientation Translocation: fragment joins a nonhomologous chromosome Chromosome Mutation: Deletion Deleted region Before Deletion After Deletion Cri du Chat Syndrome: Partial deletion 5p Chromosome Mutation: Inversion Inverted region Before inversion After inversion Chromosome Mutation: Translocation Before Translocation Region being moved After Translocation Chromosome 20 Chromosome 20 Chromosome 4 Chromosome 4 Chromosomal changes in a somatic cell can cause cancer A chromosomal translocation in the bone marrow is associated with chronic myelogenous leukemia Chromosome 9 Chromosome 22 Reciprocal translocation “Philadelphia chromosome” Activated cancer-causing gene Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings Figure 8.23C Philadelphia Chromosome t(9,22) Translocation Figure 8.23Bx Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings Acknowledgements Unless otherwise noted, illustrations are credited to Pearson Education which have been borrowed from BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by Campbell, Reece, Mitchell, and Taylor, ©2003. These images have been produced from the originals by permission of the publisher. These illustrations may not be reproduced in any format for any purpose without express written permission from the publisher. BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by Campbell, Reece, Mitchell, and Taylor, ©2001. These images have been produced from the originals by permission of the publisher. These illustrations may not be reproduced in any format for any purpose without express written permission from the publisher.