how cells reproduce chapter 8 part 2. 8.6 sexual reproduction and meiosis two modes of...
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How Cells Reproduce
Chapter 8
Part 2
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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
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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
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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
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Chromosome Pairs
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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
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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
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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
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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)
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Meiosis I and Meiosis II
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p. 145
each chromosome in the cell pairs with its homologous partner
then the partners separate
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p. 145
two chromosomes (unduplicated)
one chromosome (duplicated)
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Fig. 8-9a, p. 144
Reproductive organs of a human male
testis (where sperm
originate)
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Fig. 8-9b, p. 144
Reproductive organs of a human female
ovary (where eggs develop)
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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
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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
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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
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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
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Crossing Over
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Fig. 8-11a, p. 148
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Fig. 8-11b, p. 148
crossover
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Fig. 8-11c, p. 148
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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.
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Animation: Crossing over
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Meiosis II
The second nuclear division separates sister chromatids
Four haploid nuclei typically form, each with one complete set of unduplicated chromosomes
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Meiosis
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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
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Fig. 8-10b, p. 147
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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.
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Comparing Mitosis and Meiosis
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Animation: Comparing mitosis and meiosis
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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
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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
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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
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Comparing Life Cycles of Plants and Animals
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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
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Animation: Generalized life cycles
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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
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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
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Checkpoint Genes
BRCA genes are tumor suppressor genes whose products normally repair broken DNA
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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)
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Metastasis
Cancer cells may metastasize – break loose and colonize distant tissues
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Fig. 8-14, p. 150
4
3
1 benign tumor 2 malignant tumor
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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
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Skin Cancer: A Checkpoint Failure
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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
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Digging Into Data:HeLa Cells Are a Genetic Mess