Biology in Focus - Chapter 10
Transcript of Biology in Focus - Chapter 10
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CAMPBELL BIOLOGY IN FOCUS
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Urry • Cain • Wasserman • Minorsky • Jackson • Reece
Lecture Presentations by Kathleen Fitzpatrick and Nicole Tunbridge
10Meiosis and Sexual Life Cycles
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Overview: Variations on a Theme
Living organisms are distinguished by their ability to reproduce their own kind
Heredity is the transmission of traits from one generation to the next
Variation is demonstrated by the differences in appearance that offspring show from parents and siblings
Genetics is the scientific study of heredity and variation
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Figure 10.1
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Concept 10.1: Offspring acquire genes from parents by inheriting chromosomes
In a literal sense, children do not inherit particular physical traits from their parents
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Inheritance of Genes
Genes are the units of heredity and are made up of segments of DNA
Genes are passed to the next generation via reproductive cells called gametes (sperm and eggs)
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Most DNA is packaged into chromosomes For example, humans have 46 chromosomes in their
somatic cells, the cells of the body except for gametes and their precursors
Each gene has a specific position, or locus, on a certain chromosome
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Comparison of Asexual and Sexual Reproduction
In asexual reproduction, a single individual passes genes to its offspring without the fusion of gametes
A clone is a group of genetically identical individuals from the same parent
In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents
Video: Hydra Budding
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Figure 10.2
(a) Hydra (b) Redwoods
Parent
0.5 mm
Bud
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Figure 10.2a
(a) Hydra
Parent
0.5 mm
Bud
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Figure 10.2b
(b) Redwoods
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Concept 10.2: 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
Human somatic cells have 23 pairs of chromosomes A karyotype is an ordered display of the pairs of
chromosomes from a cell The two chromosomes in each pair are called
homologous chromosomes, or homologs Chromosomes in a homologous pair are the same
length and shape and carry genes controlling the same inherited characters
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Figure 10.3
Application TechniquePair of homologousduplicated chromosomes
Centromere
Sisterchromatids
Metaphasechromosome
5 m
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Figure 10.3a
Application
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Figure 10.3b
TechniquePair of homologousduplicated chromosomes
Centromere
Sisterchromatids
Metaphasechromosome
5 m
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Figure 10.3c
5 m
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The sex chromosomes, which determine the sex of the individual, are called X and Y
Human females have a homologous pair of X chromosomes (XX)
Human males have one X and one Y chromosome The remaining 22 pairs of chromosomes are called
autosomes
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Each pair of homologous chromosomes includes one chromosome from each parent
The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father
A diploid cell (2n) has two sets of chromosomes For humans, the diploid number is 46 (2n 46)
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In a cell in which DNA synthesis has occurred, each chromosome is replicated
Each replicated chromosome consists of two identical sister chromatids
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Figure 10.4
Key
Centromere
Pair of homologouschromosomes(one from each set)
Sister chromatidsof one duplicatedchromosome
Two nonsisterchromatids ina homologous pair
2n 6Maternal set ofchromosomes (n 3)Paternal set ofchromosomes (n 3)
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A gamete (sperm or egg) contains a single set of chromosomes and is haploid (n)
For humans, the haploid number is 23 (n = 23) Each set of 23 consists of 22 autosomes and a
single sex chromosome In an unfertilized egg (ovum), the sex chromosome
is X In a sperm cell, the sex chromosome may be either
X or Y
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Fertilization is the union of gametes (the sperm and the egg)
The fertilized egg is called a zygote and has one set of chromosomes from each parent
The zygote produces somatic cells by mitosis and develops into an adult
Behavior of Chromosome Sets in the Human Life Cycle
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At sexual maturity, the ovaries and testes produce haploid gametes
Gametes are the only types of human cells produced by meiosis rather than mitosis
Meiosis results in one set of chromosomes in each gamete
Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number
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Figure 10.5
KeyHaploid (n)Diploid (2n)
Egg (n)
Haploid gametes (n 23)
Sperm (n)
Ovary Testis
Diploidzygote(2n 46)
Mitosis anddevelopment
Multicellular diploidadults (2n 46)
MEIOSIS FERTILIZATION
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The Variety of Sexual Life Cycles
The alternation of meiosis and fertilization is common to all organisms that reproduce sexually
The three main types of sexual life cycles differ in the timing of meiosis and fertilization
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Gametes are the only haploid cells in animals They are produced by meiosis and undergo no
further cell division before fertilization Gametes fuse to form a diploid zygote that divides
by mitosis to develop into a multicellular organism
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Figure 10.6
KeyHaploid (n)Diploid (2n)
Gametes
MEIOSIS FERTILIZATION
MEIOSIS
Zygote
n n
n
2n 2n 2n 2n
FERTILIZATION
n nn
n n
SporesGametes
MitosisMitosis
Haploid multi-cellular organism(gametophyte)
ZygoteMitosisMitosisDiploid
multicellularorganism
(a) Animals (b) Plants and some algae
Diploidmulticellularorganism(sporophyte)
FERTILIZATION
nn
n
nn
Mitosis Mitosis
Haploid unicellular ormulticellular organism
Gametes
Zygote
2n
MEIOSIS
(c) Most fungi and some protists
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Figure 10.6a
KeyHaploid (n)Diploid (2n)Gametes
MEIOSIS FERTILIZATION
Zygote
n n
n
2n 2n
MitosisDiploidmulticellularorganism
(a) Animals
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Plants and some algae exhibit an alternation of generations
This life cycle includes both a diploid and haploid multicellular stage
The diploid organism, called the sporophyte, makes haploid spores by meiosis
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Each spore grows by mitosis into a haploid organism called a gametophyte
A gametophyte makes haploid gametes by mitosis Fertilization of gametes results in a diploid
sporophyte
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Figure 10.6b
MEIOSIS
2n2n
FERTILIZATION
n n nnn
SporesGametes
MitosisMitosis
Haploid multi-cellular organism(gametophyte)
ZygoteMitosis
(b) Plants and some algae
Diploidmulticellularorganism(sporophyte)
KeyHaploid (n)Diploid (2n)
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In most fungi and some protists, the only diploid stage is the single-celled zygote; there is no multicellular diploid stage
The zygote produces haploid cells by meiosis Each haploid cell grows by mitosis into a haploid
multicellular organism The haploid adult produces gametes by mitosis
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Figure 10.6c
KeyHaploid (n)Diploid (2n)
FERTILIZATION
nn
n
nn
MitosisMitosis
Haploid unicellular ormulticellular organism
Gametes
Zygote
2n
MEIOSIS
(c) Most fungi and some protists
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Depending on the type of life cycle, either haploid or diploid cells can divide by mitosis
However, only diploid cells can undergo meiosis In all three life cycles, the halving and doubling of
chromosomes contribute to genetic variation in offspring
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Concept 10.3: Meiosis reduces the number of chromosome sets from diploid to haploid
Like mitosis, meiosis is preceded by the replication of chromosomes
Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II
The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis
Each daughter cell has only half as many chromosomes as the parent cell
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The Stages of Meiosis
For a single pair of homologous chromosomes in a diploid cell, both members of the pair are duplicated
The resulting sister chromatids are closely associated all along their lengths
Homologs may have different versions of genes, each called an allele
Homologs are not associated in any obvious way except during meiosis
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Figure 10.7Interphase
Duplicated pairof homologouschromosomes
Pair of homologouschromosomes indiploid parent cell
Chromosomesduplicate
Diploid cell withduplicatedchromosomes
Sisterchromatids
Homologouschromosomesseparate
Sister chromatidsseparate
Haploid cells withduplicated chromosomes
Meiosis I
Meiosis II
Haploid cells with unduplicated chromosomes
1
2
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Figure 10.7a
Interphase
Duplicated pairof homologouschromosomes
Pair of homologouschromosomes indiploid parent cell
Chromosomesduplicate
Diploid cell withduplicatedchromosomes
Sisterchromatids
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Figure 10.7b
Homologouschromosomesseparate
Sister chromatidsseparate
Haploid cells withduplicated chromosomes
Meiosis I
Meiosis II
Haploid cells with unduplicated chromosomes
1
2
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Meiosis halves the total number of chromosomes very specifically
It reduces the number of sets from two to one, with each daughter cell receiving one set of chromosomes
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In the first meiotic division, homologous pairs of chromosomes pair and separate
In the second meiotic division, sister chromatids of each chromosome separate
Four new haploid cells are produced as a result
Animation: Meiosis
Video: Meiosis I in Sperm Formation
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Figure 10.8
MEIOSIS I: Separates homologous chromosomes
Prophase I Metaphase I Anaphase I Telophase I andCytokinesis Prophase II Metaphase II Anaphase II Telophase II and
Cytokinesis
MEIOSIS II: Separates sister chromatids
Sisterchromatids
Centromere(with kinetochore) Sister chromatids
remain attachedCentrosome(with centriolepair)
Metaphaseplate
ChiasmataSpindle
Cleavagefurrow
Homologouschromosomesseparate
Microtubuleattached tokinetochore
Fragmentsof nuclearenvelope
Homologouschromosomes
Sister chromatidsseparate
Haploiddaughter
cells forming
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Figure 10.8a
MEIOSIS I: Separates homologous chromosomes
Prophase I Metaphase I Anaphase I Telophase I andCytokinesis
Sisterchromatids
Centromere(with kinetochore) Sister chromatids
remain attachedCentrosome(with centriolepair)
Metaphaseplate
ChiasmataSpindle
Cleavagefurrow
Homologouschromosomesseparate
Microtubuleattached tokinetochore
Fragmentsof nuclearenvelope
Homologouschromosomes
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Figure 10.8b
MEIOSIS II: Separates sister chromatids
Prophase II Metaphase II Anaphase II Telophase II andCytokinesis
Sister chromatidsseparate
Haploiddaughter
cells forming
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Prophase I Prophase I typically occupies more than 90% of the
time required for meiosis Chromosomes begin to condense In synapsis, homologous chromosomes loosely pair
up, aligned gene by gene
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In crossing over, nonsister chromatids exchange DNA segments
Each homologous pair has one or more X-shaped regions called chiasmata
Chiasmata exist at points where crossing over has occurred.
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Metaphase I In metaphase I, tetrads line up at the metaphase
plate, with one chromosome facing each pole Microtubules from one pole are attached to the
kinetochore of one chromosome of each tetrad Microtubules from the other pole are attached to the
kinetochore of the other chromosome
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Anaphase I In anaphase I, pairs of homologous chromosomes
separate One chromosome moves toward each pole, guided
by the spindle apparatus Sister chromatids remain attached at the centromere
and move as one unit toward the pole
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Telophase I and Cytokinesis In the beginning of telophase I, each half of the cell
has a haploid set of chromosomes; each chromosome still consists of two sister chromatids
Cytokinesis usually occurs simultaneously, forming two haploid daughter cells
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In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms
No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated
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Division in meiosis II also occurs in four phases Prophase II Metaphase II Anaphase II Telophase II and cytokinesis
Meiosis II is very similar to mitosis
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Prophase II In prophase II, a spindle apparatus forms In late prophase II, chromosomes (each still
composed of two chromatids) move toward the metaphase plate
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Metaphase II In metaphase II, the sister chromatids are arranged
at the metaphase plate Because of crossing over in meiosis I, the two
sister chromatids of each chromosome are no longer genetically identical
The kinetochores of sister chromatids attach to microtubules extending from opposite poles
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Anaphase II In anaphase II, the sister chromatids separate The sister chromatids of each chromosome now
move as two newly individual chromosomes toward opposite poles
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Telophase II and Cytokinesis In telophase II, the chromosomes arrive at
opposite poles Nuclei form, and the chromosomes begin
decondensing
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At the end of meiosis, there are four daughter cells, each with a haploid set of unduplicated chromosomes
Each daughter cell is genetically distinct from the others and from the parent cell
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A Comparison of Mitosis and Meiosis
Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell
Meiosis reduces the number of chromosome sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell
Meiosis includes two divisions after replication, each with specific stages
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Three events are unique to meiosis, and all three occur in meiosis l Synapsis and crossing over in prophase I:
Homologous chromosomes physically connect and exchange genetic information
Homologous pairs at the metaphase plate: Homologous pairs of chromosomes are positioned there in metaphase I
Separation of homologs during anaphase I
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Figure 10.9MITOSIS MEIOSIS
Prophase
Duplicatedchromosome
Metaphase
AnaphaseTelophase
2n 2n
Daughter cellsof mitosis
Sister chromatidsseparate.
Individualchromosomesline up.
Chromosomeduplication
Parent cell
2n = 6Chromosomeduplication
Pairs ofchromosomesline up.
Chiasma MEIOSIS I
Prophase I
Metaphase I
Homologouschromosomepair
Anaphase ITelophase I
MEIOSIS II
Homologsseparate.
Sisterchromatidsseparate.
Daughter cells of meiosis IIn n n n
Daughtercells of
meiosis I
SUMMARYMitosis MeiosisProperty
DNA replication
Number of divisions
Synapsis of homologouschromosomes
Number of daughter cellsand genetic composition
Role in the animal body
Occurs during interphase before mitosis begins
One, including prophase, prometaphase,metaphase, anaphase, and telophase
Does not occur
Two, each diploid (2n) and geneticallyidentical to the parent cell
Enables multicellular adult to arise from zygote;produces cells for growth, repair, and, in somespecies, asexual reproduction
Occurs during interphase before meiosis I begins
Two, each including prophase, metaphase, anaphase, and telophase
Occurs during prophase I along with crossing over between nonsister chromatids;resulting chiasmata hold pairs together due to sister chromatid cohesion
Four, each haploid (n), containing half as many chromosomes as the parent cell;genetically different from the parent cell and from each other
Produces gametes; reduces number of chromosome sets by half and introducesgenetic variability among the gametes
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Figure 10.9a
MITOSIS MEIOSIS
Prophase
Duplicatedchromosome
Metaphase
AnaphaseTelophase
2n 2n
Daughter cellsof mitosis
Sister chromatidsseparate.
Individualchromosomesline up.
Chromosomeduplication
Parent cell
2n = 6Chromosomeduplication
Pairs ofchromosomesline up.
Chiasma MEIOSIS I
Prophase I
Metaphase I
Homologouschromosomepair
Anaphase ITelophase I
MEIOSIS II
Homologsseparate.
Sisterchromatidsseparate.
Daughter cells of meiosis IIn n n n
Daughtercells of
meiosis I
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Figure 10.9aa
MITOSIS MEIOSIS
Prophase
Duplicatedchromosome
Metaphase
Individualchromosomesline up.
Chromosomeduplication
Parent cell
2n = 6Chromosomeduplication
Pairs ofchromosomesline up.
ChiasmaMEIOSIS I
Prophase I
Metaphase I
Homologouschromosomepair
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Figure 10.9ab
MITOSIS MEIOSISAnaphaseTelophase
2n 2n
Daughter cellsof mitosis
Sister chromatidsseparate.
Anaphase ITelophase I
MEIOSIS II
Homologsseparate.
Sisterchromatidsseparate.
Daughter cells of meiosis IIn n n n
Daughtercells of
meiosis I
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Figure 10.9b
DNAreplication
Number ofdivisions
Synapsis ofhomologouschromosomes
Number ofdaughter cellsand geneticcomposition
Role in theanimal body
Occurs during interphasebefore mitosis begins
One, including prophase,prometaphase, metaphase,anaphase, and telophase
Does not occur
Two, each diploid (2n) andgenetically identical to theparent cell
Enables multicellular adult toarise from zygote; producescells for growth, repair, and,in some species, asexualreproduction
Occurs during interphase before meiosis Ibegins
Two, each including prophase, metaphase,anaphase, and telophase
Occurs during prophase I along with crossingover between nonsister chromatids; resultingchiasmata hold pairs together due to sisterchromatid cohesion
Four, each haploid (n), containing half asmany chromosomes as the parent cell;genetically different from the parent cell andfrom each other
Produces gametes; reduces number ofchromosome sets by half and introducesgenetic variability among the gametes
SUMMARYMeiosisMitosisProperty
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Figure 10.9ba
DNAreplication
Number ofdivisions
Synapsis ofhomologouschromosomes
Number ofdaughter cellsand geneticcomposition
Role in theanimal body
Occurs during interphasebefore mitosis begins
One, including prophase,prometaphase, metaphase,anaphase, and telophase
Does not occur
Two, each diploid (2n) andgenetically identical to theparent cell
Enables multicellular adult to arisefrom zygote; produces cells forgrowth, repair, and, in somespecies, asexual reproduction
MitosisProperty
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Figure 10.9bb
Occurs during interphase before meiosis Ibegins
Two, each including prophase, metaphase,anaphase, and telophase
Occurs during prophase I along with crossingover between nonsister chromatids; resultingchiasmata hold pairs together due to sisterchromatid cohesion
Four, each haploid (n), containing half asmany chromosomes as the parent cell;genetically different from the parent cell andfrom each other
Produces gametes; reduces number ofchromosome sets by half and introducesgenetic variability among the gametes
Meiosis
DNAreplication
Number ofdivisions
Synapsis ofhomologouschromosomes
Number ofdaughter cellsand geneticcomposition
Role in theanimal body
Property
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Sister chromatid cohesion keeps sister chromatids of a single chromosome attached during meiosis and mitosis
Protein complexes called cohesins are responsible for this cohesion
In mitosis, cohesins are cleaved at the end of metaphase
In meiosis, cohesins are cleaved along the chromosome arms in anaphase I (separation of homologs) and at the centromeres in anaphase II (separation of sister chromatids)
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Meiosis I is called the reductional division because it halves the number of chromosome sets per cell from diploid (2n) to haploid (n)
Meiosis II is called the equational division because the haploid cells divide to produce haploid daughter cells
The mechanism of sister chromatid separation in meiosis II is identical to that in mitosis
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Concept 10.4: Genetic variation produced in sexual life cycles contributes to evolution
Mutations (changes in an organism’s DNA) are the original source of genetic diversity
Mutations create different versions of genes called alleles
Reshuffling of alleles during sexual reproduction produces genetic variation
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Origins of Genetic Variation Among Offspring
The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation
Three mechanisms contribute to genetic variation Independent assortment of chromosomes Crossing over Random fertilization
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Independent Assortment of Chromosomes
Homologous pairs of chromosomes orient randomly at metaphase I of meiosis
In independent assortment, each pair of chromosomes sorts maternal and paternal homologs into daughter cells independently of the other pairs
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The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number
For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes
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Figure 10.10-1
Possibility 1 Possibility 2
Two equally probablearrangements ofchromosomes at
metaphase I
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Figure 10.10-2
Possibility 1 Possibility 2
Two equally probablearrangements ofchromosomes at
metaphase I
Metaphase II
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Figure 10.10-3
Possibility 1 Possibility 2
Two equally probablearrangements ofchromosomes at
metaphase I
Metaphase II
Daughtercells
Combination 1 Combination 2 Combination 3 Combination 4
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Crossing Over
Crossing over produces recombinant chromosomes, which combine DNA inherited from each parent
Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene
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In crossing over, homologous portions of two nonsister chromatids trade places
Crossing over contributes to genetic variation by combining DNA, producing chromosomes with new combinations of maternal and paternal alleles
Animation: Genetic Variation
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Figure 10.11-1Prophase Iof meiosis
Pair ofhomologs
Nonsister chromatidsheld togetherduring synapsis
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Figure 10.11-2Prophase Iof meiosis
Pair ofhomologs
Chiasma
CentromereTEM
Synapsis andcrossing over
Nonsister chromatidsheld togetherduring synapsis
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Figure 10.11-3Prophase Iof meiosis
Pair ofhomologs
Chiasma
CentromereTEM
Anaphase I
Synapsis andcrossing over
Breakdown ofproteins holding sisterchromatid arms together
Nonsister chromatidsheld togetherduring synapsis
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Figure 10.11-4Prophase Iof meiosis
Pair ofhomologs
Chiasma
CentromereTEM
Anaphase I
Synapsis andcrossing over
Breakdown ofproteins holding sisterchromatid arms together
Anaphase II
Nonsister chromatidsheld togetherduring synapsis
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Figure 10.11-5Prophase Iof meiosis
Pair ofhomologs
Chiasma
CentromereTEM
Anaphase I
Synapsis andcrossing over
Breakdown ofproteins holding sisterchromatid arms together
Anaphase II
Daughtercells
Recombinant chromosomes
Nonsister chromatidsheld togetherduring synapsis
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Figure 10.11a
Chiasma
Centromere
TEM
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Random Fertilization
Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg)
The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations
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Crossing over adds even more variation Each zygote has a unique genetic identity
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The Evolutionary Significance of Genetic Variation Within Populations
Natural selection results in the accumulation of genetic variations favored by the environment
Sexual reproduction contributes to the genetic variation in a population, which originates from mutations
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Asexual reproduction is less expensive than sexual reproduction
Nonetheless, sexual reproduction is nearly universal among animals
Overall, genetic variation is evolutionarily advantageous
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Figure 10.12
200 m
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Figure 10.UN01
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Figure 10.UN02
Prophase I: Each homologous pair undergoessynapsis and crossing over between nonsisterchromatids with the subsequent appearanceof chiasmata.
Metaphase I: Chromosomes line up ashomologous pairs on the metaphase plate.
Anaphase I: Homologs separate from eachother; sister chromatids remain joined atthe centromere.
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Figure 10.UN03
FH