13 lecture presentation
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LECTURE PRESENTATIONS
For CAMPBELL BIOLOGY, NINTH EDITION Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
© 2011 Pearson Education, Inc.
Lectures by
Erin Barley
Kathleen Fitzpatrick
Meiosis and Sexual
Life Cycles
Chapter 13
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Overview: Variations on a Theme
• Living organisms are distinguished by theirability to reproduce their own kind
• Genetics is the scientific study of heredity andvariation
• Heredity is the transmission of traits from onegeneration to the next
• Variation is demonstrated by the differences in
appearance that offspring show from parentsand siblings
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Concept 13.1: Offspring acquire genes from
parents by inheriting chromosomes
• In a literal sense, children do not inheritparticular physical traits from their parents
•
It is genes that are actually inherited
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Inheritance of Genes
• Genes are the units of heredity, and are madeup of segments of DNA
• Genes are passed to the next generation viareproductive cells called gametes (sperm and
eggs)• Each gene has a specific location called a
locus on a certain chromosome
• Most DNA is packaged into chromosomes
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Comparison of Asexual and Sexual
Reproduction
• In asexual reproduction, a single individualpasses 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 ofgenes inherited from the two parents
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Figure 13.2
(a) Hydra (b) Redwoods
Bud
Parent
0.5 mm
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Concept 13.2: Fertilization and meiosis
alternate in sexual life cycles
• A life cycle is the generation-to-generationsequence of stages in the reproductive history
of an organism
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Sets of Chromosomes in Human Cells
• Human somatic cells (any cell other than agamete) have 23 pairs of chromosomes
• A karyotype is an ordered display of the pairsof chromosomes from a cell
• The two chromosomes in each pair are calledhomologous chromosomes, or homologs
• Chromosomes in a homologous pair are the
same length and shape and carry genescontrolling the same inherited characters
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Figure 13.3
Pair of homologousduplicated chromosomes
Centromere
Sisterchromatids
Metaphase
chromosome
5 m
APPLICATION
TECHNIQUE
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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 Xchromosomes (XX)
• Human males have one X and one Ychromosome
• The remaining 22 pairs of chromosomes are
called autosomes
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• Each pair of homologous chromosomes includesone chromosome from each parent
• The 46 chromosomes in a human somatic cellare two sets of 23: one from the mother and onefrom the father
• A diploid cell (2n ) has two sets ofchromosomes
• 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 twoidentical sister chromatids
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Figure 13.4
Sister chromatids
of one duplicatedchromosome
Key
Maternal set of
chromosomes (n
3)Paternal set ofchromosomes (n 3)
Key
2n 6
Centromere
Two nonsisterchromatids ina homologous pair
Pair of homologouschromosomes(one from each set)
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• A gamete (sperm or egg) contains a single setof chromosomes, and is haploid (n )
• For humans, the haploid number is 23 (n = 23)
•
Each set of 23 consists of 22 autosomes and asingle sex chromosome
• In an unfertilized egg (ovum), the sexchromosome is X
• In a sperm cell, the sex chromosome may beeither X or Y
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• Fertilization is the union of gametes (the spermand the egg)
•
The fertilized egg is called a zygote and hasone set of chromosomes from each parent
• The zygote produces somatic cells by mitosisand develops into an adult
Behavior of Chromosome Sets in the
Human Life Cycle
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• At sexual maturity, the ovaries and testesproduce haploid gametes
• Gametes are the only types of human cellsproduced by meiosis, rather than mitosis
• Meiosis results in one set of chromosomes ineach gamete
• Fertilization and meiosis alternate in sexual life
cycles to maintain chromosome number
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Fi 13 5
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Figure 13.5
Key
Haploid (n )Diploid (2n )
Egg (n )
Haploid gametes (n 23)
Sperm (n )
Ovary Testis
Mitosis anddevelopment
Diploidzygote
(2n
46)
Multicellular diploid
adults (2n 46)
MEIOSIS FERTILIZATION
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The Variety of Sexual Life Cycles
• The alternation of meiosis and fertilization iscommon to all organisms that reproducesexually
• The three main types of sexual life cycles differin the timing of meiosis and fertilization
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• Gametes are the only haploid cells in animals• They are produces by meiosis and undergo no
further cell division before fertilization
•
Gametes fuse to form a diploid zygote thatdivides by mitosis to develop into a multicellularorganism
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Figure 13 6
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Figure 13.6
Key
Haploid (n )
Diploid (2n )
Gametes
MEIOSIS FERTILIZATION
Zygote
MitosisDiploidmulticellularorganism
(a) Animals
n
n
n
2n 2n 2n
2n 2n
n n
n
n n
n
n
n
n
n MEIOSIS
MEIOSIS
FERTILIZATION
FERTILIZATION
Mitosis Mitosis
Mitosis
Mitosis Mitosis
Gametes
Spores
Gametes
Zygote
Zygote
Haploid multi-cellular organism(gametophyte)
Diploidmulticellularorganism(sporophyte)
Haploid unicellular ormulticellular organism
(b) Plants and some algae (c) Most fungi and some protists
Figure 13 6a
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Figure 13.6a Key
Haploid (n )
Diploid (2n )
Gametes
MEIOSIS FERTILIZATION
Zygote
MitosisDiploidmulticellularorganism
(a) Animals
n
2n
n
n
2n
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• Plants and some algae exhibit an alternation ofgenerations
• This life cycle includes both a diploid andhaploid multicellular stage
• The diploid organism, called the sporophyte,makes haploid spores by meiosis
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• Each spore grows by mitosis into a haploidorganism called a gametophyte
• A gametophyte makes haploid gametes bymitosis
• Fertilization of gametes results in a diploidsporophyte
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Figure 13 6b
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Figure 13.6b
2n 2n
n
MEIOSIS FERTILIZATION
Mitosis Mitosis
Mitosis
GametesSpores
Zygote
Haploid multi-cellular organism(gametophyte)
Diploidmulticellularorganism(sporophyte)
(b) Plants and some algae
n n
n
n
Haploid (n )
Diploid (2n )
Key
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• In most fungi and some protists, the onlydiploid stage is the single-celled zygote; thereis no multicellular diploid stage
• The zygote produces haploid cells by meiosis
• Each haploid cell grows by mitosis into ahaploid multicellular organism
• The haploid adult produces gametes by
mitosis
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• Depending on the type of life cycle, eitherhaploid or diploid cells can divide by mitosis
• However, only diploid cells can undergomeiosis
• In all three life cycles, the halving and doublingof chromosomes contributes to geneticvariation in offspring
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Concept 13.3: Meiosis reduces the number of
chromosome sets from diploid to haploid
• Like mitosis, meiosis is preceded by thereplication of chromosomes
•
Meiosis takes place in two sets of celldivisions, called meiosis I and meiosis II
• The two cell divisions result in four daughtercells, 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
• After chromosomes duplicate, two divisionsfollow
– Meiosis I (reductional division): homologs pairup and separate, resulting in two haploid
daughter cells with replicated chromosomes – Meiosis II (equational division) sister
chromatids separate
• The result is four haploid daughter cells with
unreplicated chromosomes
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Figure 13.7-1
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g
Pair of homologouschromosomes indiploid parent cell
Duplicated pairof homologouschromosomes
Chromosomesduplicate
Sisterchromatids
Diploid cell withduplicatedchromosomes
Interphase
Figure 13.7-2
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Pair of homologouschromosomes indiploid parent cell
Duplicated pairof homologouschromosomes
Chromosomesduplicate
Sisterchromatids
Diploid cell withduplicatedchromosomes
Homologouschromosomes separate
Haploid cells withduplicated chromosomes
Meiosis I
1
Interphase
Figure 13.7-3 I h
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Pair of homologouschromosomes indiploid parent cell
Duplicated pairof homologouschromosomes
Chromosomesduplicate
Sisterchromatids
Diploid cell withduplicatedchromosomes
Homologouschromosomes separate
Haploid cells withduplicated chromosomes
Sister chromatidsseparate
Haploid cells with unduplicated chromosomes
Interphase
Meiosis I
Meiosis II
2
1
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• Meiosis I is preceded by interphase, when thechromosomes are duplicated to form sisterchromatids
• The sister chromatids are genetically identicaland joined at the centromere
• The single centrosome replicates, forming twocentrosomes
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BioFlix: Meiosis
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• Division in meiosis I occurs in four phases – Prophase I
– Metaphase I
– Anaphase I
– Telophase I and cytokinesis
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Figure 13.8
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MEIOSIS I: Separates homologous chromosomes
Prophase I Metaphase I Anaphase ITelophase I and
Cytokinesis
Centrosome(with centriole pair)
Sisterchromatids
Chiasmata
Spindle
Homologouschromosomes
Fragmentsof nuclearenvelope
Duplicated homologouschromosomes (red and blue)pair and exchange segments;2n 6 in this example.
Centromere(with kinetochore)
Metaphaseplate
Microtubuleattached tokinetochore
Chromosomes line upby homologous pairs.
Sister chromatidsremain attached
Homologouschromosomesseparate
Each pair of homologouschromosomes separates.
Cleavagefurrow
Two haploid cellsform; each chromosomestill consists of twosister chromatids.
MEIOSIS I: Separates sister chromatids
Prophase II Metaphase II Anaphase IITelophase II and
Cytokinesis
Sister chromatidsseparate
Haploid daughtercells forming
During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing unduplicated chromosomes.
Figure 13.8a
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Prophase I Metaphase I Anaphase I Telophase I andCytokinesis
Centrosome
(with centriole pair)
Sisterchromatids
Chiasmata
Spindle
Homologouschromosomes
Fragmentsof nuclearenvelope
Duplicated homologouschromosomes (red and blue)pair and exchange segments;2n 6 in this example.
Centromere(with kinetochore)
Metaphaseplate
Microtubuleattached tokinetochore
Chromosomes line upby homologous pairs.
Sister chromatidsremain attached
Homologouschromosomesseparate
Each pair of homologouschromosomes separates.
Cleavagefurrow
Two haploidcells form; eachchromosomestill consistsof two sisterchromatids.
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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 chromatidsexchange DNA segments
• Each pair of chromosomes forms a tetrad, agroup of four chromatids
• Each tetrad usually has one or morechiasmata, X-shaped regions where crossingover 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 thekinetochore of one chromosome of each tetrad
• Microtubules from the other pole are attached tothe 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 thecentromere 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; eachchromosome still consists of two sisterchromatids
• Cytokinesis usually occurs simultaneously,forming two haploid daughter cells
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• In animal cells, a cleavage furrow forms; inplant cells, a cell plate forms
• No chromosome replication occurs betweenthe end of meiosis I and the beginning ofmeiosis II because the chromosomes arealready 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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Figure 13.8b
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Prophase II Metaphase II Anaphase IITelophase II and
Cytokinesis
Sister chromatidsseparate
Haploid daughtercells forming
During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing unduplicated chromosomes.
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Prophase II • In prophase II, a spindle apparatus forms
• In late prophase II, chromosomes (each stillcomposed of two chromatids) move towardthe 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 twosister chromatids of each chromosome are nolonger 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 chromosomenow move as two newly individualchromosomes 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 begindecondensing
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•
Cytokinesis separates the cytoplasm• At the end of meiosis, there are four daughter
cells, each with a haploid set of unreplicatedchromosomes
• Each daughter cell is genetically distinct fromthe others and from the parent cell
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A Comparison of Mitosis and Meiosis
•
Mitosis conserves the number of chromosomesets, producing cells that are geneticallyidentical to the parent cell
• Meiosis reduces the number of chromosomes
sets from two (diploid) to one (haploid),producing cells that differ genetically from eachother and from the parent cell
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Figure 13.9a
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Prophase
Duplicatedchromosome
MITOSIS
Chromosomeduplication
Parent cell
2n 6
Metaphase
AnaphaseTelophase
2n 2n
Daughter cellsof mitosis
MEIOSIS
MEIOSIS I
MEIOSIS II
Prophase I
Metaphase I
Anaphase I
Telophase I
Haploidn 3
Chiasma
Chromosomeduplication Homologous
chromosome pair
Daughtercells of
meiosis I
Daughter cells of meiosis II
n n n n
Figure 13.9b
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SUMMARY
Property Mitosis Meiosis
DNAreplication
Number ofdivisions
Synapsis ofhomologouschromosomes
Number ofdaughter cellsand geneticcomposition
Role in theanimal body
Occurs during interphase beforemitosis begins
One, including prophase, metaphase,anaphase, and telophase
Does not occur
Two, each diploid (2n ) and geneticallyidentical to the parent cell
Enables multicellular adult to arise fromzygote; produces cells for growth, repair,
and, in some species, asexual reproduction
Occurs during interphase before meiosis I begins
Two, each including prophase, metaphase, anaphase,and telophase
Occurs during prophase I along with crossing overbetween nonsister chromatids; resulting chiasmatahold pairs together due to sister chromatid cohesion
Four, each haploid (n ), containing half as manychromosomes as the parent cell; genetically differentfrom the parent cell and from each other
Produces gametes; reduces number of chromosomesby half and introduces genetic variability among the
gametes
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•
Three events are unique to meiosis, and allthree occur in meiosis l
– Synapsis and crossing over in prophase I:Homologous chromosomes physically connect
and exchange genetic information – At the metaphase plate, there are paired
homologous chromosomes (tetrads), instead ofindividual replicated chromosomes
– At anaphase I, it is homologous chromosomes,instead of sister chromatids, that separate
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•
Sister chromatid cohesion allows sisterchromatids of a single chromosome to staytogether through meiosis I
• 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 ofhomologs) and at the centromeres in anaphase II(separation of sister chromatids)
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Concept 13.4: 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 genescalled alleles
• Reshuffling of alleles during sexual reproductionproduces genetic variation
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Origins of Genetic Variation Among
Offspring
• The behavior of chromosomes during meiosisand fertilization is responsible for most of thevariation that arises in each generation
• Three mechanisms contribute to geneticvariation
– Independent assortment of chromosomes
– Crossing over – Random fertilization
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Independent Assortment of Chromosomes •
Homologous pairs of chromosomes orientrandomly at metaphase I of meiosis
• In independent assortment, each pair ofchromosomes sorts maternal and paternal
homologs into daughter cells independently ofthe other pairs
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•
The number of combinations possible whenchromosomes assort independently intogametes is 2n , where n is the haploid number
• For humans (n = 23), there are more than 8
million (223) possible combinations ofchromosomes
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Figure 13.10-3
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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 recombinantchromosomes, which combine DNA inheritedfrom each parent
• Crossing over begins very early in prophase I,
as homologous chromosomes pair up gene bygene
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•
In crossing over, homologous portions of twononsister chromatids trade places
• Crossing over contributes to genetic variationby combining DNA from two parents into a
single chromosome
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Figure 13.11-1 Prophase I of meiosis
Nonsister chromatidsheld together
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of meiosis held togetherduring synapsis
Pair of homologs
Figure 13.11-2 Prophase I of meiosis
Nonsister chromatidsheld together
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of meiosis held togetherduring synapsis
Pair of homologs
Chiasma
Centromere
TEM
Figure 13.11-3 Prophase I of meiosis
Nonsister chromatidsheld together
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of meiosis held togetherduring synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Figure 13.11-4 Prophase I of meiosis
Nonsister chromatidsheld together
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of meiosis held togetherduring synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Figure 13.11-5 Prophase I of meiosis
Nonsister chromatidsheld together
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of meiosis held togetherduring synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Daughtercells
Recombinant chromosomes
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Random Fertilization •
Adds to genetic variation because any spermcan fuse with any ovum (unfertilized egg)
• The fusion of two gametes (each with 8.4million possible chromosome combinations
from independent assortment) produces azygote with any of about 70 trillion diploidcombinations
•
Crossing over adds even more variation• Each zygote has a unique genetic identity
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Animation: Genetic VariationRight-click slide / select “Play”
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The Evolutionary Significance of Genetic
Variation Within Populations
• Natural selection results in the accumulation ofgenetic variations favored by the environment
• Sexual reproduction contributes to the geneticvariation in a population, which originates frommutations
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