Albia Dugger Miami Dade College Cecie Starr Christine Evers Lisa Starr Chapter 12 Meiosis and Sexual Reproduction (Sections )

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 An adaptive trait spreads more quickly through a sexually reproducing population than an asexually reproducing one 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

Sexual Reproduction in Plants In flowering plants, pollen tubes with male gametes grow down into the ovary, containing female gametes

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 nonidentical 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

Introducing Alleles (cont.) Most genes have multiple alleles – 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 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

Fig. 12.3a, p. 176 Gamete Formation in Humans

Fig. 12.3a, p. 176 testis (where sperm originate) Reproductive organs of a human male Gamete Formation in Humans

Fig. 12.3b, p. 176 Gamete Formation in Humans

Fig. 12.3b, p. 176 ovary (where eggs develop) Reproductive organs of a human female Gamete Formation in Humans

Gamete Formation in Flowers

Fig. 12.3c, p. 176 ovary (where sexual spores that give rise to to egg cells form) Reproductive organs of a flowering plant anther (where sexual spores that give rise sperm cells form) Gamete Formation in Humans

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, forming four haploid nuclei

How Meiosis Halves the Chromosome Number

Fig. 12.4, p. 177 A In meiosis I, each duplicated chromosome in the nucleus pairs with its homologous partner. B Homologous partners separate. The still-duplicated chromosomes are packaged into two new nuclei. C Sister chromatids separate in meiosis II. The now unduplicated chromosomes are packaged into four new nuclei. How Meiosis Halves the Chromosome Number

C Sister chromatids separate in meiosis II. The now unduplicated chromosomes are packaged into four new nuclei. Fig. 12.4, p. 177 A In meiosis I, each duplicated chromosome in the nucleus pairs with its homologous partner. B Homologous partners separate. The still-duplicated chromosomes are packaged into two new nuclei. Stepped Art How Meiosis Halves the Chromosome Number

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 HERECLICK 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 HERECLICK 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, chromosomes of a diploid nucleus become distributed into four haploid nuclei

Meiosis I: Prophase I Homologous chromosomes condense, pair up, and swap segments 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

Prophase I. Homologous chromosomes condense, pair up, and swap segments. Spindle micro- tubules attach to them as the nuclear envelope breaks up. Metaphase I. The homologous chromosome pairs are aligned midway between spindle poles. Anaphase I. The homologous chromosomes separate and begin heading toward the spindle poles. Telophase I. Two clusters of chromosomes reach the spindle poles. A new nuclear envelope forms around each cluster, so two haploid (n) nuclei form. plasma membrane spindle one pair of homologous chromosomes nuclear envelope breaking up centrosome Meiosis I One diploid nucleus to two haploid nuclei 1234

Fig. 12.5a, p. 178 Meiosis I One diploid nucleus to two haploid nuclei Prophase I. Homologous chromosomes condense, pair up, and swap segments. Spindle micro- tubules attach to them as the nuclear envelope breaks up. plasma membrane spindle nuclear envelope breaking up centrosome 1 Metaphase I. The homologous chromosome pairs are aligned midway between spindle poles. one pair of homologous chromosomes 2 Anaphase I. The homologous chromosomes separate and begin heading toward the spindle poles. 3 Telophase I. Two clusters of chromosomes reach the spindle poles. A new nuclear envelope forms around each cluster, so two haploid (n) nuclei form. 4 Stepped Art

Fig. 12.5b, p. 178

Prophase II. The chromosomes condense. Spindle microtubules attach to each sister chromatid as the nuclear envelope breaks up. Metaphase II. The (still duplicated) chromosomes are aligned midway between poles of the spindle. Anaphase II. All sister chromatids separate. The now unduplicated chromo - somes head to the spindle poles. 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 Meiosis II Two haploid nuclei to four haploid nuclei 5678

Fig. 12.5b, p. 178 Meiosis II Two haploid nuclei to four haploid nuclei Prophase II. The chromosomes condense. Spindle microtubules attach to each sister chromatid as the nuclear envelope breaks up. No DNA replication 5 Metaphase II. The (still duplicated) chromosomes are aligned midway between poles of the spindle. 6 Anaphase II. All sister chromatids separate. The now unduplicated chromo - somes head to the spindle poles. 7 Telophase II. A cluster of chromosomes reaches each spindle pole. A new nuclear envelope encloses each cluster, so four haploid (n) nuclei form. 8 Stepped Art

Animation: Meiosis

3D ANIMATION: Meiosis