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How Cells Reproduce Chapter 8 Part 2 8.6 Sexual Reproduction and Meiosis Two modes of reproduction: asexual and sexual Asexual reproduction • Reproductive mode by which offspring arise from one parent and inherit that parent’s genes only • Offspring of asexual reproduction are clones Clone • A genetically identical copy of an organism Sexual Reproduction Offspring of sexual reproduction vary in shared traits Sexual reproduction • Reproductive mode by which offspring arise from two parents and inherit genes from both Inheriting Chromosome Pairs Offspring of most sexual reproducers inherit pairs of chromosomes, one of each pair from the mother and the other from the father Except for a pair of nonidentical sex chromosomes, the members of a chromosome pair have the same length, shape, and set of genes – these are homologous chromosomes Chromosome Pairs Introducing Alleles Paired genes on homologous chromosomes often vary slightly in DNA sequence as alleles Alleles • Forms of a gene that encode slightly different versions of the gene’s product Alleles are the basis of traits Variation in Traits Sexual reproduction mixes up alleles from two parents, resulting in new combinations of alleles (and traits) in offspring Variations in allele combinations are introduced during meiosis Meiosis Halves the Chromosome Number Meiosis occurs in immature reproductive cells (germ cells) of sexually reproducing eukaryotes, forming male and female haploid gametes Gamete • Mature, haploid reproductive cell Haploid (n) • Having one of each type of chromosome characteristic of the species Meiosis Halves the Chromosome Number Meiosis sorts the chromosomes into new nuclei twice (meiosis I and meiosis II) Duplicated chromosomes of a diploid nucleus (2n) are distributed into four haploid nuclei (n) Meiosis I and Meiosis II each chromosome in the cell pairs with its homologous partner then the partners separate p. 145 two chromosomes (unduplicated) one chromosome (duplicated) p. 145 Reproductive organs of a human male testis (where sperm originate) Fig. 8-9a, p. 144 Reproductive organs of a human female ovary (where eggs develop) Fig. 8-9b, p. 144 Restoring Diploid Number Diploid number is restored at fertilization, when two haploid (n) gametes fuse to form a zygote Fertilization • Fusion of a sperm nucleus and an egg nucleus, resulting in a single-celled zygote Zygote • Diploid (2n) cell formed by fusion of gametes • First cell of a new individual, with two sets of chromosomes, one from each parent 8.7 Meiosis In meiosis, two nuclear divisions halve the parental chromosome number • Meiosis I • Meiosis II Meiosis shuffles parental combinations of alleles, introducing variation in offspring • Crossing over in prophase I • Random assortment in metaphase I Meiosis I In the first nuclear division, duplicated homologous chromosomes line up and cross over, then move apart, toward opposite spindle poles Two new nuclear envelopes form around the two clusters of still-duplicated chromosomes Crossing Over Crossing over is recombination between nonsister chromatids of homologous chromosomes which produces new combinations of parental alleles Crossing over • Homologous chromosomes exchange corresponding segments during prophase I of meiosis Crossing Over Fig. 8-11a, p. 148 crossover Fig. 8-11b, p. 148 Fig. 8-11c, p. 148 A) Here, we focus on only two genes. One gene has alleles A and a; the other has alleles B and b. B) Close contact between the homologous chromosomes promotes crossing over between nonsister chromatids, so paternal and maternal chromatids exchange segments. crossover C) Crossing over mixes up paternal and maternal alleles on homologous chromosomes. Stepped Art Fig. 8-11c, p. 148 Animation: Crossing over Meiosis II The second nuclear division separates sister chromatids Four haploid nuclei typically form, each with one complete set of unduplicated chromosomes Meiosis 1 Prophase I 2 Metaphase I spindle plasma membrane microtubules 3 Anaphase I 4 Telophase I one pair of homologous chromosomes centrosome nuclear envelope breaking up Fig. 8-10a, p. 146 Fig. 8-10b, p. 147 plasma membrane spindle microtubules one pair of homologous chromosomes centrosome nuclear envelope breaking up There is no DNA replication between the two nuclear divisions. Stepped Art Fig. 8-10b, p. 147 Comparing Mitosis and Meiosis Animation: Comparing mitosis and meiosis 8.8 From Gametes to Offspring Meiosis and cytoplasmic division precede the development of haploid gametes in animals and spores in plants The union of two haploid gametes at fertilization results in a diploid zygote Gamete Formation in Plants In plants, two kinds of multicelled bodies form • Familiar plants are diploid sporophytes that make haploid spores Sporophyte • Diploid, spore-producing body of a plant Gametophyte • A haploid, multicelled body in which gametes form during the life cycle of plants Gamete Formation in Animals Germ cells in the reproductive organs of animals give rise to sperm or eggs Sperm • Mature male gamete Egg • Mature female gamete, or ovum Comparing Life Cycles of Plants and Animals Fertilization The fusion of two haploid gamete nuclei during fertilization restores the parental chromosome number in the zygote, the first cell of the new individual Animation: Generalized life cycles 8.9 When Control is Lost The cell cycle has built-in checkpoints that allow problems to be corrected before the cycle advances Checkpoint gene products are gene expression controls that advance, delay, or block the cell cycle in response to internal and external conditions Checkpoints and Tumors Checkpoint genes whose products inhibit meiosis are called tumor suppressors Disruption of checkpoint gene products, such as by mutations or viruses, causes tumors that may end up as cancer Failure of cell cycle checkpoints results in the uncontrolled cell divisions that characterize cancer Checkpoint Genes BRCA genes are tumor suppressor genes whose products normally repair broken DNA Cancer Moles and other tumors are neoplasms; a benign neoplasm is noncancerous A malignant neoplasm (cancer) occurs when abnormally dividing cells disrupt body tissues, physically and metabolically Malignant neoplasms can break free and invade other tissues (metastasize) Metastasis Cancer cells may metastasize – break loose and colonize distant tissues 4 3 1 benign tumor 2 malignant tumor Fig. 8-14, p. 150 Three Characteristics of Cancer Cells 1. Grow and divide abnormally 2. Often have an abnormal plasma membrane, cytoskeleton, or metabolism 3. Often have weakened capacity for adhesion because recognition proteins are altered or lost Skin Cancer: A Checkpoint Failure 8.10 Impacts/Issues Revisited The HeLa cell line was established more than 50 years ago without Henrietta Lacks knowledge or consent Today, consent forms are required to take tissue samples, and it is illegal to sell one’s own organs or tissues Digging Into Data: HeLa Cells Are a Genetic Mess