Chapter 13 Meiosis and Sexual Life Cycles Power

Chapter 13 Meiosis and Sexual Life Cycles Power. Point Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• 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 Figure 13. 1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Genetics – Is the scientific study of heredity and hereditary variation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Concept 13. 1: Offspring acquire genes from parents by inheriting chromosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Inheritance of Genes • 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 Parent Bud Figure 13. 2 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 0. 5 mm

• 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 Pair of homologous chromosomes Centromere Sister chromatids Figure 13. 3 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 5 µm

• 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 (2 n = 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) 2 n = 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)

• Unlike somatic cells – Gametes, sperm and egg cells are haploid cells, containing only one set of chromosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Behavior of Chromosome Sets in the Human Life Cycle • 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 (2 n) Ovum (n) Sperm Cell (n) FERTILIZATION MEIOSIS Ovary Testis Mitosis and development Figure 13. 5 Multicellular diploid adults (2 n = 46) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Diploid zygote (2 n = 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 Zygote 2 n Figure 13. 6 A Diploid multicellular organism FERTILIZATION 2 n 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 n n Spores Gametes MEIOSIS Diploid multicellular organism (sporophyte) Figure 13. 6 B 2 n (b) Plants and some algae Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings FERTILIZATION 2 n 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 n n Gametes MEIOSIS FERTILIZATION 2 n 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

The Stages of Meiosis • An overview of meiosis Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Figure 13. 7 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Haploid cells with unreplicated chromosomes

• 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

• Interphase and meiosis I MEIOSIS I: Separates homologous chromosomes INTERPHASE PROPHASE I Centrosomes (with centriole pairs) Sister chromatids METAPHASE I Chiasmata ANAPHASE I Sister chromatids remain attached Centromere (with kinetochore) Spindle Nuclear envelope Metaphase plate Homologous Microtubule chromosomes Tetrad attached to Chromatin separate kinetochore Pairs of homologous Chromosomes duplicate Tertads line up chromosomes split up Homologous chromosomes (red and blue) pair and exchange Figure 13. 8 segments; 2 n = 6 in this example Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Telophase I, cytokinesis, and meiosis II MEIOSIS II: Separates sister chromatids TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II Cleavage furrow Figure 13. 8 Two haploid cells form; chromosomes are still double ANAPHASE II Sister chromatids separate TELOPHASE II AND CYTOKINESIS Haploid daughter cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

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

• Synapsis and crossing over – Homologous chromosomes physically connect and exchange genetic information Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Tetrads on the metaphase plate – At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Separation of homologues – At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell – In anaphase II of meiosis, the sister chromatids separate 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 Chromosome replication Duplicated chromosome (two sister chromatids) Chromosome replication Tetrad formed by synapsis of homologous chromosomes 2 n = 6 Metaphase Chromosomes positioned at the metaphase plate Anaphase Telophase Sister chromatids separate during anaphase 2 n 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 2 n 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• In independent assortment – Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs Key Maternal set of chromosomes Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Figure 13. 10 Combination 1 Combination 2 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Combination 3 Combination 4

Crossing Over • Crossing over – Produces recombinant chromosomes that carry genes derived from two different parents Prophase I of meiosis Nonsister chromatids Tetrad Metaphase I Chiasma, site of crossing over Metaphase II Daughter cells Figure 13. 11 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recombinant chromosomes

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