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Cecie Starr Christine Evers Lisa Starr www.cengage.com/chemistry/starr Chapter 12 Meiosis and Sexual Reproduction (Sections 12.1 - 12.3) Albia Dugger • Miami Dade College 12.1 Why Sex? • Sex mixes up the genes of two parents, so offspring of sexual reproducers have unique combinations of traits • Diversity offers sexual reproducers as a group a better chance of surviving environmental change than clones • sexual reproduction • Reproductive mode by which offspring arise from two parents and inherit genes from both Sex and Adaptation • In asexual reproduction, new combinations of traits arise only by mutation • Sexual reproduction generates new combinations of traits in far fewer generations than does mutation alone 12.2 Sexual Reproduction and Meiosis • Asexual reproduction produces clones • Sexual reproduction mixes up alleles from two parents • Meiosis, the basis of sexual reproduction, is a nuclear division mechanism that occurs in reproductive cells of eukaryotes Introducing Alleles • Somatic cells of sexually-reproducing multicelled organisms contain pairs of chromosomes: one from the mother and one from the father • Except for non-identical (male) sex chromosomes, the two chromosomes of every pair carry the same set of genes • somatic • Relating to the body Introducing Alleles (cont.) • The two genes of a pair are often not identical: Maternal and paternal genes can encode slightly different forms (alleles) of the same gene’s product • alleles • Forms of a gene that encode slightly different versions of the gene’s product (ex: gene for rat eye color: one allele from mom encodes for red and the other allele from dad encodes for black) Introducing Alleles (cont.) • Most genes have multiple alleles (EX: blood type in humans has alleles for A, B, and O) – one reason individuals of a sexually reproducing species do not look exactly the same • Offspring of sexual reproducers inherit new combinations of alleles, which is the basis of new combinations of traits Homologous Chromosomes • One chromosome in a homologous pair is inherited from the mother, one from the father – different forms are alleles What Meiosis Does • Sexual reproduction involves fusion of reproductive cells from two parents • Meiosis halves (cuts in half) the chromosome number in reproductive cells so offspring have the same number of chromosomes as the parents • meiosis • Nuclear division process that halves the chromosome number • Basis of sexual reproduction Sexual Reproduction • The process of sexual reproduction begins with meiosis in germ cells, which produces gametes • germ cell • Diploid reproductive cell that gives rise to haploid gametes by meiosis • gamete • Mature, haploid reproductive cell (egg or sperm) • Gametes usually form inside special male and female reproductive structures Gamete Formation in Humans Gamete Formation in Humans Fig. 12.3a, p. 176 Gamete Formation in Humans Reproductive organs of a human male testis (where sperm originate) Fig. 12.3a, p. 176 Gamete Formation in Humans Fig. 12.3b, p. 176 Gamete Formation in Humans Reproductive organs of a human female ovary (where eggs develop) Fig. 12.3b, p. 176 Gamete Formation in Flowers Gamete Formation in Humans Reproductive organs of a flowering plant anther (where sexual spores that give rise sperm cells form) ovary (where sexual spores that give rise to to egg cells form) Fig. 12.3c, p. 176 Maintaining Chromosome Number • Gametes have a single set of chromosomes, so they are haploid (n): Their chromosome number is half of the diploid (2n) number • Diploid number is restored at fertilization, when two haploid gametes fuse to form a zygote, the first cell of a new individual Key Terms • haploid • Having one of each type of chromosome characteristic of the species • fertilization • Fusion of two gametes to form a zygote • zygote • Cell formed by fusion of two gametes • The first cell of a new individual Meiosis I and Meiosis II • Meiosis starts like mitosis, but sorts chromosomes into new nuclei twice (2 divisions instead of 1), forming four haploid nuclei How Meiosis Halves the Chromosome Number How Meiosis Halves the Chromosome Number B Homologous partners separate. The still-duplicated chromosomes are packaged into two new nuclei. A In meiosis I, each duplicated chromosome in the nucleus pairs with its homologous partner. C Sister chromatids separate in meiosis II. The now unduplicated chromosomes are packaged into four new nuclei. Fig. 12.4, p. 177 How Meiosis Halves the Chromosome Number B Homologous partners separate. The still-duplicated chromosomes are packaged into two new nuclei. A In meiosis I, each duplicated chromosome in the nucleus pairs with its homologous partner. C Sister chromatids separate in meiosis II. The now unduplicated chromosomes are packaged into four new nuclei. Stepped Art Fig. 12.4, p. 177 Key Concepts • Sexual Versus Asexual Reproduction • In asexual reproduction, one parent transmits its genes to offspring • In sexual reproduction, offspring inherit genes from two parents who usually differ in some number of alleles • Differences in alleles are the basis of differences in traits ANIMATION: Reproductive Organs To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE ANIMATION: Meiosis I and II To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE 12.3 The Process of Meiosis • DNA replication occurs prior to meiosis – the nucleus is diploid (2n) with two sets of chromosomes, one from each parent • During meiosis, (duplicated) chromosomes of a diploid nucleus become distributed into four haploid nuclei Meiosis I: Prophase I • Homologous chromosomes condense, pair up, and swap segments (tetrad crossing over) • Spindle microtubules attach to chromosomes as the nuclear envelope breaks up Meiosis I: Metaphase I • The homologous chromosome pairs are aligned midway between spindle poles Meiosis I: Anaphase I • The homologous chromosomes separate and begin heading toward the spindle poles Meiosis I: Telophase I • Two clusters of chromosomes reach the spindle poles • A new nuclear envelope forms around each cluster, so two haploid (n) nuclei form Meiosis II: Prophase II • The chromosomes condense • Spindle microtubules attach to each sister chromatid as the nuclear envelope breaks up Meiosis II: Metaphase II • The (still duplicated) chromosomes are aligned midway between poles of the spindle Meiosis II: Anaphase II • All sister chromatids separate • The now unduplicated chromosomes head to the spindle poles Meiosis II: Telophase II • A cluster of chromosomes reaches each spindle pole • A new nuclear envelope encloses each cluster, four haploid (n) nuclei form Fig. 12.5a, p. 178 Meiosis I One diploid nucleus to two haploid nuclei 1 Prophase I. Homologous chromosomes condense, pair up, and swap segments. Spindle micro- tubules attach to them as the nuclear envelope breaks up. plasma membrane nuclear envelope breaking up 2 Metaphase I. The homologous chromosome pairs are aligned midway between spindle poles. 3 Anaphase I. The homologous chromosomes separate and begin heading toward the spindle poles. 4 Telophase I. Two clusters of chromosomes reach the spindle poles. A new nuclear envelope forms around each cluster, so two haploid (n) nuclei form. spindle centrosome one pair of homologous chromosomes Fig. 12.5a, p. 178 Meiosis I One diploid nucleus to two haploid nuclei 1 Prophase I. Homologous chromosomes condense, pair up, and swap segments. Spindle micro- tubules attach to them as the nuclear envelope breaks up. plasma membrane nuclear envelope breaking up 2 Metaphase I. The homologous chromosome pairs are aligned midway between spindle poles. 3 Anaphase I. The homologous chromosomes separate and begin heading toward the spindle poles. 4 Telophase I. Two clusters of chromosomes reach the spindle poles. A new nuclear envelope forms around each cluster, so two haploid (n) nuclei form. spindle centrosome one pair of homologous chromosomes Stepped Art Fig. 12.5a, p. 178 Fig. 12.5b, p. 178 Meiosis II Two haploid nuclei to four haploid nuclei 5 Prophase II. The chromosomes condense. Spindle microtubules attach to each sister chromatid as the nuclear envelope breaks up. 6 Metaphase II. The (still duplicated) chromosomes are aligned midway between poles of the spindle. 7 Anaphase II. All sister chromatids separate. The now unduplicated chromo somes head to the spindle poles. 8 Telophase II. A cluster of chromosomes reaches each spindle pole. A new nuclear envelope encloses each cluster, so four haploid (n) nuclei form. No DNA replication Fig. 12.5b, p. 178 Meiosis II Two haploid nuclei to four haploid nuclei 5 Prophase II. The chromosomes condense. Spindle microtubules attach to each sister chromatid as the nuclear envelope breaks up. 6 Metaphase II. The (still duplicated) chromosomes are aligned midway between poles of the spindle. 7 Anaphase II. All sister chromatids separate. The now unduplicated chromo somes head to the spindle poles. 8 Telophase II. A cluster of chromosomes reaches each spindle pole. A new nuclear envelope encloses each cluster, so four haploid (n) nuclei form. No DNA replication Stepped Art Fig. 12.5b, p. 178 Animation: Meiosis 3D ANIMATION: Meiosis