1. How Cells Reproduce
Chapter 8
Part 2

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

3. 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

4. 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

5. Chromosome Pairs

6. 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

7. 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

8. 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

9. 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)

10. Meiosis I and Meiosis II

11. each chromosome in the cell pairs with its homologous partner
then the partners separate
p. 145

12. two chromosomes (unduplicated) one chromosome (duplicated)

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. Crossing Over

20. 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

21. 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

22. 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

23. 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

24. Animation: Crossing over

25. Meiosis II The second nuclear division separates sister chromatids
Four haploid nuclei typically form, each with one complete set of unduplicated chromosomes

26. Meiosis

27. one pair of homologous chromosomes1
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

28. 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

29. 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

30. Comparing Mitosis and Meiosis

31. Animation: Comparing mitosis and meiosis

32. 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

33. 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

34. 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

35. Comparing Life Cycles of Plants and Animals

36. 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

37. Animation: Generalized life cycles

38. 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

39. 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

40. Checkpoint Genes
BRCA genes are tumor suppressor genes whose products normally repair broken DNA

41. 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)

42. Metastasis
Cancer cells may metastasize – break loose and colonize distant tissues

43. 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

44. 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

45. Skin Cancer: A Checkpoint Failure

46. 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

47. Digging Into Data: HeLa Cells Are a Genetic Mess