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)

Reproductive organs of a human male Figure 8.9: Animated! Meiosis of germ cells in reproductive organs gives rise to gametes. testis (where sperm originate) Fig. 8-9a, p. 144

Reproductive organs of a human female Figure 8.9: Animated! Meiosis of germ cells in reproductive organs gives rise to gametes. 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

Figure 8.11: Animated! Crossing over. Blue signifies a paternal chromosome, and pink, its maternal homologue. For clarity, we show only one pair of homologous chromosomes and one crossover, but more than one crossover may occur in each chromosome pair. Fig. 8-11a, p. 148

crossover Figure 8.11: Animated! Crossing over. Blue signifies a paternal chromosome, and pink, its maternal homologue. For clarity, we show only one pair of homologous chromosomes and one crossover, but more than one crossover may occur in each chromosome pair. Fig. 8-11b, p. 148

Figure 8.11: Animated! Crossing over. Blue signifies a paternal chromosome, and pink, its maternal homologue. For clarity, we show only one pair of homologous chromosomes and one crossover, but more than one crossover may occur in each chromosome pair. 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. crossover B) Close contact between the homologous chromosomes promotes crossing over between nonsister chromatids, so paternal and maternal chromatids exchange segments. Figure 8.11: Animated! Crossing over. Blue signifies a paternal chromosome, and pink, its maternal homologue. For clarity, we show only one pair of homologous chromosomes and one crossover, but more than one crossover may occur in each chromosome pair. 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

one pair of homologous chromosomes 1 Prophase I 2 Metaphase I 3 Anaphase I 4 Telophase I plasma membrane spindle microtubules one pair of homologous chromosomes Figure 8.10: Animated! Meiosis. Two pairs of chromosomes are illustrated in a diploid (2n) animal cell. Homologous chromosomes are indicated in blue and pink. Micrographs show meiosis in a lily plant cell (Lilium regale). 1 Prophase I. The (duplicated) chromosomes condense, and spindle microtubules attach to them as the nuclear envelope breaks up. 2 Metaphase I. The (duplicated) chromosomes are aligned midway between spindle poles. 3 Anaphase I. Homologous chromosomes separate. 4 Telophase I. Two clusters of chromosomes reach the spindle poles. A new nuclear envelope encloses each cluster, so two haploid (n) nuclei form. 5 Prophase II. The (still duplicated) chromosomes condense, and spindle microtubules attach to each sister chromatid as the nuclear envelope breaks up. 6 Metaphase II. The chromosomes are aligned midway between the spindle poles. 7 Anaphase II. Sister chromatids separate and become individual chromosomes (unduplicated). 8 Telophase II. A cluster of (unduplicated) chromosomes reaches each spindle pole. A new nuclear envelope encloses each cluster, so four haploid (n) nuclei form. centrosome nuclear envelope breaking up Fig. 8-10a, p. 146

Figure 8.10: Animated! Meiosis. Two pairs of chromosomes are illustrated in a diploid (2n) animal cell. Homologous chromosomes are indicated in blue and pink. Micrographs show meiosis in a lily plant cell (Lilium regale). 1 Prophase I. The (duplicated) chromosomes condense, and spindle microtubules attach to them as the nuclear envelope breaks up. 2 Metaphase I. The (duplicated) chromosomes are aligned midway between spindle poles. 3 Anaphase I. Homologous chromosomes separate. 4 Telophase I. Two clusters of chromosomes reach the spindle poles. A new nuclear envelope encloses each cluster, so two haploid (n) nuclei form. 5 Prophase II. The (still duplicated) chromosomes condense, and spindle microtubules attach to each sister chromatid as the nuclear envelope breaks up. 6 Metaphase II. The chromosomes are aligned midway between the spindle poles. 7 Anaphase II. Sister chromatids separate and become individual chromosomes (unduplicated). 8 Telophase II. A cluster of (unduplicated) chromosomes reaches each spindle pole. A new nuclear envelope encloses each cluster, so four haploid (n) nuclei form. Fig. 8-10b, p. 147

nuclear envelope breaking up centrosome plasma membrane spindle microtubules nuclear envelope breaking up centrosome one pair of homologous chromosomes There is no DNA replication between the two nuclear divisions. Figure 8.10: Animated! Meiosis. Two pairs of chromosomes are illustrated in a diploid (2n) animal cell. Homologous chromosomes are indicated in blue and pink. Micrographs show meiosis in a lily plant cell (Lilium regale). 1 Prophase I. The (duplicated) chromosomes condense, and spindle microtubules attach to them as the nuclear envelope breaks up. 2 Metaphase I. The (duplicated) chromosomes are aligned midway between spindle poles. 3 Anaphase I. Homologous chromosomes separate. 4 Telophase I. Two clusters of chromosomes reach the spindle poles. A new nuclear envelope encloses each cluster, so two haploid (n) nuclei form. 5 Prophase II. The (still duplicated) chromosomes condense, and spindle microtubules attach to each sister chromatid as the nuclear envelope breaks up. 6 Metaphase II. The chromosomes are aligned midway between the spindle poles. 7 Anaphase II. Sister chromatids separate and become individual chromosomes (unduplicated). 8 Telophase II. A cluster of (unduplicated) chromosomes reaches each spindle pole. A new nuclear envelope encloses each cluster, so four haploid (n) nuclei form. 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 Figure 8.14: Animated! Metastasis. 1 Benign cancer cells grow slowly and stay in their home tissue. 2 Malignant cancer cells can break away from their home tissue. 3 The metastasizing cells become attached to the wall of a blood vessel or lymph vessel. They release digestive enzymes that create an opening in the wall, then enter the vessel. 4 The cells creep or tumble along inside blood vessels, then leave the bloodstream the same way they got in. They start new tumors in new tissues. 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