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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
PowerPoint Lectures for Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Chapter 13Chapter 13
Meiosis and Sexual Life Cycles
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• Overview: Hereditary Similarity and Variation
• Living organisms
– Are distinguished by their ability to reproduce their own kind
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• Heredity
– Is the transmission of traits from one generation to the next
• Variation
– Shows that offspring differ somewhat in appearance from parents and siblings
Figure 13.1
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• Genetics
– Is the scientific study of heredity and hereditary variation
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• Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes
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Inheritance of Genes
• Genes
– Are the units of heredity
– Are segments of DNA
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• Each gene in an organism’s DNA
– Has a specific locus on a certain chromosome
• We inherit
– One set of chromosomes from our mother and one set from our father
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Comparison of Asexual and Sexual Reproduction
• In asexual reproduction
– One parent produces genetically identical offspring by mitosis
Figure 13.2
Parent
Bud
0.5 mm
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• In sexual reproduction
– Two parents give rise to offspring that have unique combinations of genes inherited from the two parents
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• Concept 13.2: Fertilization and meiosis alternate in sexual life cycles
• A life cycle
– Is the generation-to-generation sequence of stages in the reproductive history of an organism
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Sets of Chromosomes in Human Cells
• In humans
– Each somatic cell has 46 chromosomes, made up of two sets
– One set of chromosomes comes from each parent
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5 µmPair of homologouschromosomes
Centromere
Sisterchromatids
Figure 13.3
• A karyotype
– Is an ordered, visual representation of the chromosomes in a cell
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• Homologous chromosomes
– Are the two chromosomes composing a pair
– Have the same characteristics
– May also be called autosomes
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• Sex chromosomes
– Are distinct from each other in their characteristics
– Are represented as X and Y
– Determine the sex of the individual, XX being female, XY being male
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• A diploid cell
– Has two sets of each of its chromosomes
– In a human has 46 chromosomes (2n = 46)
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• In a cell in which DNA synthesis has occurred
– All the chromosomes are duplicated and thus each consists of two identical sister chromatids
Figure 13.4
Key
Maternal set ofchromosomes (n = 3)
Paternal set ofchromosomes (n = 3)
2n = 6
Two sister chromatidsof one replicatedchromosome
Two nonsisterchromatids ina homologous pair
Pair of homologouschromosomes(one from each set)
Centromere
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• Unlike somatic cells
– Gametes, sperm and egg cells are haploid cells, containing only one set of chromosomes
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Behavior of Chromosome Sets in the Human Life Cycle
• At sexual maturity
– The ovaries and testes produce haploid gametes by meiosis
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• During fertilization
– These gametes, sperm and ovum, fuse, forming a diploid zygote
• The zygote
– Develops into an adult organism
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Figure 13.5
Key
Haploid (n)
Diploid (2n)
Haploid gametes (n = 23)
Ovum (n)
SpermCell (n)
MEIOSIS FERTILIZATION
Ovary Testis Diploidzygote(2n = 46)
Mitosis anddevelopment
Multicellular diploidadults (2n = 46)
• The human life cycle
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The Variety of Sexual Life Cycles
• The three main types of sexual life cycles
– Differ in the timing of meiosis and fertilization
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• In animals
– Meiosis occurs during gamete formation
– Gametes are the only haploid cells
Gametes
Figure 13.6 A
Diploidmulticellular
organism
Key
MEIOSIS FERTILIZATION
n
n
n
2n2nZygote
Haploid
Diploid
Mitosis
(a) Animals
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MEIOSIS FERTILIZATION
nn
n
nn
2n2n
Haploid multicellularorganism (gametophyte)
Mitosis Mitosis
SporesGametes
Mitosis
Zygote
Diploidmulticellularorganism(sporophyte)
(b) Plants and some algaeFigure 13.6 B
• Plants and some algae
– Exhibit an alternation of generations
– The life cycle includes both diploid and haploid multicellular stages
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MEIOSIS FERTILIZATION
nn
n
n
n
2n
Haploid multicellularorganism
Mitosis Mitosis
Gametes
Zygote(c) Most fungi and some protistsFigure 13.6 C
• In most fungi and some protists
– Meiosis produces haploid cells that give rise to a haploid multicellular adult organism
– The haploid adult carries out mitosis, producing cells that will become gametes
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• Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid
• Meiosis
– Takes place in two sets of divisions, meiosis I and meiosis II
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The Stages of Meiosis
• An overview of meiosis
Figure 13.7
Interphase
Homologous pairof chromosomesin diploid parent cell
Chromosomesreplicate
Homologous pair of replicated chromosomes
Sisterchromatids Diploid cell with
replicatedchromosomes
1
2
Homologous chromosomes separate
Haploid cells withreplicated chromosomes
Sister chromatids separate
Haploid cells with unreplicated chromosomes
Meiosis I
Meiosis II
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• Meiosis I
– Reduces the number of chromosomes from diploid to haploid
• Meiosis II
– Produces four haploid daughter cells
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Centrosomes(with centriole pairs)
Sisterchromatids
Chiasmata
Spindle
Tetrad
Nuclearenvelope
Chromatin
Centromere(with kinetochore)
Microtubuleattached tokinetochore
Tertads line up
Metaphaseplate
Homologouschromosomesseparate
Sister chromatidsremain attached
Pairs of homologouschromosomes split up
Chromosomes duplicateHomologous chromosomes
(red and blue) pair and exchangesegments; 2n = 6 in this example
INTERPHASE MEIOSIS I: Separates homologous chromosomes
PROPHASE I METAPHASE I ANAPHASE I
• Interphase and meiosis I
Figure 13.8
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TELOPHASE I ANDCYTOKINESIS
PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II ANDCYTOKINESIS
MEIOSIS II: Separates sister chromatids
Cleavagefurrow Sister chromatids
separate
Haploid daughter cellsforming
During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes
Two haploid cellsform; chromosomesare still doubleFigure 13.8
• Telophase I, cytokinesis, and meiosis II
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A Comparison of Mitosis and Meiosis
• Meiosis and mitosis can be distinguished from mitosis
– By three events in Meiosis l
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• Synapsis and crossing over
– Homologous chromosomes physically connect and exchange genetic information
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• Tetrads on the metaphase plate
– At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates
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• Separation of homologues
– At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell
– In anaphase II of meiosis, the sister chromatids separate
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Figure 13.9
MITOSIS MEIOSIS
Prophase
Duplicated chromosome(two sister chromatids)
Chromosomereplication
Chromosomereplication
Parent cell(before chromosome replication)
Chiasma (site ofcrossing over)
MEIOSIS I
Prophase I
Tetrad formed bysynapsis of homologouschromosomes
Metaphase
Chromosomespositioned at themetaphase plate
Tetradspositioned at themetaphase plate
Metaphase I
Anaphase ITelophase I
Haploidn = 3
MEIOSIS II
Daughtercells of
meiosis I
Homologuesseparateduringanaphase I;sisterchromatidsremain together
Daughter cells of meiosis II
n n n n
Sister chromatids separate during anaphase II
AnaphaseTelophase
Sister chromatidsseparate duringanaphase
2n 2nDaughter cells
of mitosis
2n = 6
• A comparison of mitosis and meiosis
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• Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution
• Reshuffling of genetic material in meiosis
– Produces genetic variation
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Origins of Genetic Variation Among Offspring
• In species that produce sexually
– The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generation
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Independent Assortment of Chromosomes
• Homologous pairs of chromosomes
– Orient randomly at metaphase I of meiosis
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• In independent assortment
– Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs
Figure 13.10
Key
Maternal set ofchromosomesPaternal set ofchromosomes
Possibility 1
Two equally probable arrangements ofchromosomes at
metaphase I
Possibility 2
Metaphase II
Daughtercells
Combination 1 Combination 2 Combination 3 Combination 4
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Crossing Over
• Crossing over
– Produces recombinant chromosomes that carry genes derived from two different parents
Figure 13.11
Prophase Iof meiosis
Nonsisterchromatids
Tetrad
Chiasma,site ofcrossingover
Metaphase I
Metaphase II
Daughtercells
Recombinantchromosomes
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Random Fertilization
• The fusion of gametes
– Will produce a zygote with any of about 64 trillion diploid combinations
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Evolutionary Significance of Genetic Variation Within Populations
• Genetic variation
– Is the raw material for evolution by natural selection
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• Mutations
– Are the original source of genetic variation
• Sexual reproduction
– Produces new combinations of variant genes, adding more genetic diversity