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• Overview: Hereditary Similarity and Variation • Living organisms – Are distinguished by their ability to reproduce their own kind Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Heredity – Is the transmission of traits from one generation to the next • Variation – Shows that offspring differ somewhat in appearance from parents and siblings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Genetics – Is the scientific study of heredity and hereditary variation – Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes • Genes – Are the units of heredity – Are segments of DNA Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Each gene in an organism’s DNA – Has a specific locus on a certain chromosome • We inherit – One set of chromosomes from our mother and one set from our father Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Comparison of Asexual and Sexual Reproduction • In asexual reproduction – One parent produces genetically identical offspring by mitosis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In sexual reproduction – Two parents give rise to offspring that have unique combinations of genes inherited from the two parents Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Concept 13.2: Fertilization and meiosis alternate in sexual life cycles • A life cycle – Is the generation-to-generation sequence of stages in the reproductive history of an organism Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sets of Chromosomes in Human Cells • In humans – Each somatic cell has 46 chromosomes, made up of two sets – One set of chromosomes comes from each parent Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • A karyotype – Is an ordered, visual representation of the chromosomes in a cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Homologous chromosomes – Are the two chromosomes composing a pair – Have the same characteristics – May also be called autosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Sex chromosomes – Are distinct from each other in their characteristics – Are represented as X and Y – Determine the sex of the individual, XX being female, XY being male Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • A diploid cell – Has two sets of each of its chromosomes – In a human has 46 chromosomes (2n = 46) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In a cell in which DNA synthesis has occurred – All the chromosomes are duplicated and thus each consists of two identical sister chromatids Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Centromere Figure 13.4 Two nonsister chromatids in a homologous pair Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pair of homologous chromosomes (one from each set) Figure 13.3 APPLICATION TECHNIQUE Pair of homologous duplicated chromosomes Centromere Sister chromatids Metaphase chromosome Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 5 m Figure 13.3a Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 13.3b Pair of homologous duplicated chromosomes Centromere Sister chromatids Metaphase chromosome Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 5 m Figure 13.3c 5 m Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Unlike somatic cells – Gametes, sperm and egg cells are haploid cells, containing only one set of chromosomes • At sexual maturity – The ovaries and testes produce haploid gametes by meiosis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • During fertilization – These gametes, sperm and ovum, fuse, forming a diploid zygote • The zygote – Develops into an adult organism Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The human life cycle Key Haploid gametes (n = 23) Haploid (n) Diploid (2n) Ovum (n) Sperm Cell (n) FERTILIZATION MEIOSIS Ovary Testis Mitosis and development Figure 13.5 Multicellular diploid adults (2n = 46) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Diploid zygote (2n = 46) The Variety of Sexual Life Cycles • The three main types of sexual life cycles – Differ in the timing of meiosis and fertilization Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In animals – Meiosis occurs during gamete formation – Gametes are the only haploid cells Key Haploid Diploid n n Gametes n MEIOSIS FERTILIZATION Zygote 2n Figure 13.6 A Diploid multicellular organism 2n Mitosis (a) Animals Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Plants and some algae – Exhibit an alternation of generations – The life cycle includes both diploid and haploid multicellular stages Haploid multicellular organism (gametophyte) n Mitosis n Mitosis n n n Spores Gametes MEIOSIS Diploid multicellular organism (sporophyte) Figure 13.6 B 2n (b) Plants and some algae Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings FERTILIZATION 2n Mitosis Zygote • In most fungi and some protists – Meiosis produces haploid cells that give rise to a haploid multicellular adult organism – The haploid adult carries out mitosis, producing cells that will become gametes Haploid multicellular organism Mitosis n Mitosis n n n Gametes MEIOSIS FERTILIZATION 2n Figure 13.6 C Zygote (c) Most fungi and some protists Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings n • Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid • Meiosis – Takes place in two sets of divisions, meiosis I and meiosis II Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Meiosis I – Reduces the number of chromosomes from diploid to haploid • Meiosis II – Produces four haploid daughter cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Division in meiosis I occurs in four phases – Prophase I – Metaphase I – Anaphase I – Telophase I and cytokinesis © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ©Copyright 2011 Pearson Education, Inc. Figure 13.8 MEIOSIS I: Separates sister chromatids MEIOSIS I: Separates homologous chromosomes Prophase I Metaphase I Centrosome (with centriole pair) Sister chromatids Chiasmata Telophase I and Cytokinesis Anaphase I Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n 6 in this example. Anaphase II Telophase II and Cytokinesis Centromere (with kinetochore) Metaphase plate Cleavage furrow Fragments of nuclear envelope Metaphase II Sister chromatids remain attached Spindle Homologous chromosomes Prophase II Homologous chromosomes separate Microtubule attached to kinetochore Chromosomes line up by homologous pairs. Each pair of homologous chromosomes separates. During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes. Sister chromatids separate Two haploid cells form; each chromosome still consists of two sister chromatids. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Haploid daughter cells forming Figure 13.8a Prophase I Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Telophase I and Cytokinesis Anaphase I Metaphase I Sister chromatids remain attached Centromere (with kinetochore) Metaphase plate Fragments Homologous chromosomes of nuclear envelope Homologous chromosomes separate Microtubule attached to kinetochore Each pair of homologous chromosomes separates. Chromosomes line up Duplicated homologous chromosomes (red and blue) by homologous pairs. pair and exchange segments; 2n 6 in this example. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cleavage furrow Two haploid cells form; each chromosome still consists of two sister chromatids. Prophase I • Prophase I typically occupies more than 90% of the time required for meiosis • Chromosomes begin to condense • In synapsis, homologous chromosomes loosely pair up, aligned gene by gene © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ©Copyright 2011 Pearson Education, Inc. • In crossing over, nonsister chromatids exchange DNA segments • Each pair of chromosomes forms a tetrad, a group of four chromatids • Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ©Copyright 2011 Pearson Education, Inc. Metaphase I • In metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole • Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad • Microtubules from the other pole are attached to the kinetochore of the other chromosome © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ©Copyright 2011 Pearson Education, Inc. Anaphase I • In anaphase I, pairs of homologous chromosomes separate • One chromosome moves toward each pole, guided by the spindle apparatus • Sister chromatids remain attached at the centromere and move as one unit toward the pole © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ©Copyright 2011 Pearson Education, Inc. Telophase I and Cytokinesis • In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids • Cytokinesis usually occurs simultaneously, forming two haploid daughter cells © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ©Copyright 2011 Pearson Education, Inc. • In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms • No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ©Copyright 2011 Pearson Education, Inc. • Division in meiosis II also occurs in four phases – Prophase II – Metaphase II – Anaphase II – Telophase II and cytokinesis • Meiosis II is very similar to mitosis © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ©Copyright 2011 Pearson Education, Inc. Figure 13.8b Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes. Sister chromatids separate Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Haploid daughter cells forming Prophase II • In prophase II, a spindle apparatus forms • In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ©Copyright 2011 Pearson Education, Inc. Metaphase II • In metaphase II, the sister chromatids are arranged at the metaphase plate • Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical • The kinetochores of sister chromatids attach to microtubules extending from opposite poles © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ©Copyright 2011 Pearson Education, Inc. Anaphase II • In anaphase II, the sister chromatids separate • The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles © 2005 Pearson Education, Inc. publishing as Benjamin Cummings ©Copyright 2011 Pearson Education, Inc. A Comparison of Mitosis and Meiosis • Meiosis and mitosis can be distinguished from mitosis – By three events in Meiosis l Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • A comparison of mitosis and meiosis MITOSIS MEIOSIS Chiasma (site of crossing over) Parent cell (before chromosome replication) MEIOSIS I Prophase I Prophase Chromosome replication Duplicated chromosome (two sister chromatids) Chromosome replication Tetrad formed by synapsis of homologous chromosomes 2n = 6 Metaphase Chromosomes positioned at the metaphase plate Anaphase Telophase Sister chromatids separate during anaphase 2n Tetrads positioned at the metaphase plate Homologues separate during anaphase I; sister chromatids remain together Metaphase I Anaphase I Telophase I Haploid n=3 Daughter cells of meiosis I 2n MEIOSIS II Daughter cells of mitosis n n n Daughter cells of meiosis II Figure 13.9 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sister chromatids separate during anaphase II n • Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution • Reshuffling of genetic material in meiosis – Produces genetic variation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Origins of Genetic Variation Among Offspring • In species that produce sexually – The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Independent Assortment of Chromosomes • Homologous pairs of chromosomes – Orient randomly at metaphase I of meiosis • In independent assortment – Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs • Crossing over – Produces recombinant chromosomes that carry genes derived from two different parents Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Random Fertilization • The fusion of gametes – Will produce a zygote with any of about 64 trillion diploid combinations Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Evolutionary Significance of Genetic Variation Within Populations • Genetic variation – Is the raw material for evolution by natural selection Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Mutations – Are the original source of genetic variation • Sexual reproduction – Produces new combinations of variant genes, adding more genetic diversity Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings