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

p. 145

each chromosome in the cell pairs with its homologous partner

then the partners separate

p. 145

two chromosomes (unduplicated)

one chromosome (duplicated)

Fig. 8-9a, p. 144

Reproductive organs of a human male

testis (where sperm

originate)

Fig. 8-9b, p. 144

Reproductive organs of a human female

ovary (where eggs develop)

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

Fig. 8-11b, p. 148

crossover

Fig. 8-11c, p. 148

Fig. 8-11c, p. 148

Stepped Art

C) Crossing over mixes uppaternal and maternal alleles onhomologous chromosomes.

crossover

B) Close contact between the homologous chromosomes promotes crossing over between nonsister chromatids, so paternal and maternal chromatids exchange segments.

A) Here, we focus on only two genes.One gene has alleles A and a; the otherhas alleles B and b.

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

Fig. 8-10a, p. 146

1 Prophase I 2 Metaphase I 3 Anaphase I 4 Telophase I

one pair of homologous chromosomes

plasma membrane

spindle microtubules

nuclear envelope breaking up

centrosome

Fig. 8-10b, p. 147

Fig. 8-10b, p. 147

Stepped Art

one pair of homologous chromosomes

plasma membrane

spindle microtubules

nuclear envelope breaking up

centrosome

There is no DNA replication between the two nuclear divisions.

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

Fig. 8-14, p. 150

4

3

1 benign tumor 2 malignant tumor

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