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Meiosis & Sexual Reproduction
Chapter 10
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 2
OutlineOutlineReduction in Chromosome NumberReduction in Chromosome Number
Meiosis OverviewMeiosis Overview Homologous PairsHomologous Pairs
Genetic VariationGenetic Variation Crossing-OverCrossing-Over Independent AssortmentIndependent Assortment FertilizationFertilization
Phases of MeiosisPhases of Meiosis Meiosis IMeiosis I Meiosis IIMeiosis II
Meiosis Compared to MitosisMeiosis Compared to Mitosis
Human Life CycleHuman Life Cycle
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 3
Essential KnowledgeEssential Knowledge
3.A.2: In eukaryotes, heritable information is passed 3.A.2: In eukaryotes, heritable information is passed to the next generation via processes that include to the next generation via processes that include the cell cycle and mitosis or meiosis plus the cell cycle and mitosis or meiosis plus fertilization.fertilization.
c. Meiosis, a reduction division, followed by c. Meiosis, a reduction division, followed by fertilization ensures genetic diversity in sexually fertilization ensures genetic diversity in sexually reproducing organisms.reproducing organisms.
1. Meiosis ensures that each gamete receives one 1. Meiosis ensures that each gamete receives one complete haploid (1n) set of chromosomes.complete haploid (1n) set of chromosomes.
2. During meiosis, homologous chromosomes are 2. During meiosis, homologous chromosomes are paired, with one homologue originating from the paired, with one homologue originating from the maternal parent and the other from the paternal maternal parent and the other from the paternal parent. Orientation of the chromosome pairs is parent. Orientation of the chromosome pairs is random with respect to the cell poles.random with respect to the cell poles.
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 4
Essential KnowledgeEssential Knowledge
3.A.2: In eukaryotes, heritable information is passed to 3.A.2: In eukaryotes, heritable information is passed to the next generation via processes that include the cell the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization.cycle and mitosis or meiosis plus fertilization.
c. Meiosis, a reduction division, followed by fertilization c. Meiosis, a reduction division, followed by fertilization ensures genetic diversity in sexually reproducing ensures genetic diversity in sexually reproducing organisms.organisms.
3. Separation of the homologous chromosomes ensures 3. Separation of the homologous chromosomes ensures that each gamete receives a haploid (1n) set of that each gamete receives a haploid (1n) set of chromosomes composed of both maternal and chromosomes composed of both maternal and paternal chromosomes.paternal chromosomes.
4. During meiosis, homologous chromatids exchange 4. During meiosis, homologous chromatids exchange genetic material via a process called “crossing over,” genetic material via a process called “crossing over,” which increases genetic variation in the resultant which increases genetic variation in the resultant gametes. gametes.
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 5
Essential KnowledgeEssential Knowledge
3.A.2: In eukaryotes, heritable information is passed 3.A.2: In eukaryotes, heritable information is passed to the next generation via processes that include to the next generation via processes that include the cell cycle and mitosis or meiosis plus the cell cycle and mitosis or meiosis plus fertilization.fertilization.
c. Meiosis, a reduction division, followed by c. Meiosis, a reduction division, followed by fertilization ensures genetic diversity in sexually fertilization ensures genetic diversity in sexually reproducing organisms.reproducing organisms.
5. Fertilization involves the fusion of two gametes, 5. Fertilization involves the fusion of two gametes, increases genetic variation in populations by increases genetic variation in populations by providing for new combinations of genetic providing for new combinations of genetic information in the zygote, and restores the diploid information in the zygote, and restores the diploid number of chromosomes.number of chromosomes.
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 6Meiosis:Meiosis:
Halves the Chromosome NumberHalves the Chromosome Number
Special type of cell divisionSpecial type of cell division
Used only for sexual reproductionUsed only for sexual reproduction
Halves the chromosome number prior to Halves the chromosome number prior to fertilizationfertilization
Parents diploidParents diploid
Meiosis produces haploid gametesMeiosis produces haploid gametes
Gametes fuse in fertilization to form diploid Gametes fuse in fertilization to form diploid zygotezygote
Becomes the next diploid generationBecomes the next diploid generation
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 7Homologous Pairs ofHomologous Pairs of
ChromosomesChromosomes
In diploid body cells chromosomes occur in pairsIn diploid body cells chromosomes occur in pairs
Humans have 23 different types of chromosomesHumans have 23 different types of chromosomes
Diploid cells have two of each typeDiploid cells have two of each type
Chromosomes of the same type are said to be Chromosomes of the same type are said to be homologoushomologous They have the same lengthThey have the same length Their centromeres are positioned in the same placeTheir centromeres are positioned in the same place One came from the father (the paternal homolog) the One came from the father (the paternal homolog) the
other from the mother (the maternal homolog)other from the mother (the maternal homolog) When stained, they show similar banding patternsWhen stained, they show similar banding patterns Because they have genes controlling the same traits Because they have genes controlling the same traits
at the same positionsat the same positions
8Homologous Chromosomes
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 9Homologous Pairs ofHomologous Pairs of
ChromosomesChromosomes
Homologous chromosomes have genes controlling Homologous chromosomes have genes controlling the same trait at the same positionthe same trait at the same position Each gene occurs in duplicateEach gene occurs in duplicate A maternal copy from the motherA maternal copy from the mother A paternal copy from the fatherA paternal copy from the father
Many genes exist in several variant forms in a large Many genes exist in several variant forms in a large populationpopulation
Homologous copies of a gene may encode identical Homologous copies of a gene may encode identical or differing genetic informationor differing genetic information
The variants that exist for a gene are called allelesThe variants that exist for a gene are called allelesAn individual may have:An individual may have:
Identical alleles for a specific gene on both homologs Identical alleles for a specific gene on both homologs (homozygous for the trait), or(homozygous for the trait), or
A maternal allele that differs from the corresponding A maternal allele that differs from the corresponding paternal allele (heterozygous for the trait)paternal allele (heterozygous for the trait)
10Overview of Meiosis
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 11Phases of Meiosis I:Phases of Meiosis I:
Prophase I & Metaphase IProphase I & Metaphase I
Meiosis I (reductional division):Meiosis I (reductional division):Prophase IProphase I Each chromosome internally duplicated Each chromosome internally duplicated (consists of two identical sister chromatids)(consists of two identical sister chromatids)
Homologous chromosomes pair up – synapsisHomologous chromosomes pair up – synapsis
Physically align themselves against each other Physically align themselves against each other end to endend to end
End view would show four chromatids – TetradEnd view would show four chromatids – TetradMetaphase IMetaphase I Homologous pairs arranged onto the Homologous pairs arranged onto the metaphase platemetaphase plate
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 12Phases of Meiosis I:Phases of Meiosis I:
Anaphase I & Telophase IAnaphase I & Telophase I
Meiosis I (cont.):Meiosis I (cont.):Anaphase IAnaphase I Synapsis breaks upSynapsis breaks up Homologous chromosomes separate from one Homologous chromosomes separate from one anotheranother Homologues move towards opposite polesHomologues move towards opposite poles Each is still an internally duplicate chromosome Each is still an internally duplicate chromosome with two chromatidswith two chromatids
Telophase ITelophase I Daughter cells have one internally duplicate Daughter cells have one internally duplicate chromosome from each homologous pairchromosome from each homologous pair One (internally duplicate) chromosome of each One (internally duplicate) chromosome of each type (1n, haploid)type (1n, haploid)
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 13Phases of Meiosis I:Phases of Meiosis I:
Cytokinesis I & InterkinesisCytokinesis I & Interkinesis
Meiosis I (cont.):Meiosis I (cont.):Cytokinesis ICytokinesis I Two daughter cellsTwo daughter cells Both with one internally duplicate chromosome Both with one internally duplicate chromosome of each typeof each type HaploidHaploidMeiosis I is reductional (halves chromosome Meiosis I is reductional (halves chromosome number)number)
InterkinesisInterkinesisSimilar to mitotic interphaseSimilar to mitotic interphaseUsually shorterUsually shorterNo replication of DNANo replication of DNA
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 14Genetic Variation:Genetic Variation:
Crossing OverCrossing Over
Meiosis brings about genetic variation in two key Meiosis brings about genetic variation in two key ways:ways: Crossing-over between homologous chromosomes, Crossing-over between homologous chromosomes,
andand Independent assortment of homologous chromosomesIndependent assortment of homologous chromosomes
1. Crossing Over:1. Crossing Over: Exchange of genetic material between nonsister Exchange of genetic material between nonsister
chromatids during meiosis Ichromatids during meiosis I At synapsis, a nucleoprotein lattice (called the At synapsis, a nucleoprotein lattice (called the
synaptonemal complex) appears between homologuessynaptonemal complex) appears between homologues Holds homologues togetherHolds homologues together Aligns DNA of nonsister chromatids Aligns DNA of nonsister chromatids Allows crossing-over to occurAllows crossing-over to occur
Then homologues separate and are distributed to Then homologues separate and are distributed to different daughter cellsdifferent daughter cells
15Crossing Over
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 16Genetic Variation:Genetic Variation:
Independent AssortmentIndependent Assortment
2. Independent assortment:2. Independent assortment:When homologues align at the When homologues align at the metaphase plate:metaphase plate:
They separate in a random mannerThey separate in a random manner
The maternal or paternal homologue may The maternal or paternal homologue may be oriented toward either pole of mother be oriented toward either pole of mother cellcell
Causes random mixing of blocks of Causes random mixing of blocks of alleles into gametesalleles into gametes
17Independent Assortment
Recombination
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 18Genetic Variation:Genetic Variation:
FertilizationFertilization
When gametes fuse at fertilization:When gametes fuse at fertilization:
Chromosomes donated by the parents are Chromosomes donated by the parents are combinedcombined
In humans, (223)In humans, (223)2 2 = 70,368,744,000,000 = 70,368,744,000,000 chromosomally different zygotes are possiblechromosomally different zygotes are possible
If crossing-over occurs only onceIf crossing-over occurs only once
(423)(423)22, or 4,951,760,200,000,000,000,000,000,000 , or 4,951,760,200,000,000,000,000,000,000 genetically different zygotes are possiblegenetically different zygotes are possible
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 19Genetic Variation:Genetic Variation:
SignificanceSignificance
Asexual reproduction produces genetically Asexual reproduction produces genetically identical clonesidentical clones
Sexual reproduction cause novel genetic Sexual reproduction cause novel genetic recombinationsrecombinations
Asexual reproduction is advantageous when Asexual reproduction is advantageous when environment is stableenvironment is stable
However, if environment changes, genetic However, if environment changes, genetic variability introduced by sexual reproduction variability introduced by sexual reproduction may be advantageousmay be advantageous
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 20Phases of Meiosis II:Phases of Meiosis II:
Similar to MitosisSimilar to Mitosis
Metaphase IIMetaphase II OverviewOverview UnremarkableUnremarkable Virtually indistinguishable from mitosis of two haploid Virtually indistinguishable from mitosis of two haploid
cellscells Prophase II – Chromosomes condenseProphase II – Chromosomes condense Metaphase II – chromosomes align at metaphase Metaphase II – chromosomes align at metaphase
plateplate Anaphase IIAnaphase II Centromere dissolvesCentromere dissolves Sister chromatids separate and become daughter Sister chromatids separate and become daughter
chromosomeschromosomes Telophase II and cytokinesis IITelophase II and cytokinesis II Four haploid cellsFour haploid cells All genetically uniqueAll genetically unique
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 21
Meiosis versus MitosisMeiosis versus Mitosis
MeiosisMeiosis Requires Requires twotwo nuclear nuclear
divisionsdivisions Chromosomes Chromosomes synapsesynapse and and
cross overcross over Centromeres Centromeres survivesurvive
Anaphase IAnaphase I HalvesHalves chromosome chromosome
numbernumber Produces Produces fourfour daughter daughter
nucleinuclei Produces daughter cells Produces daughter cells
genetically genetically differentdifferent from from parent and each otherparent and each other
Used only for Used only for sexualsexual reproductionreproduction
MitosisMitosis Requires Requires oneone nuclear nuclear
divisiondivision Chromosomes Chromosomes do notdo not
synapsesynapse nor nor cross overcross over Centromeres Centromeres dissolvedissolve in in
mitotic anaphasemitotic anaphase PreservesPreserves chromosome chromosome
numbernumber Produces Produces twotwo daughter daughter
nucleinuclei Produces daughter cells Produces daughter cells
genetically genetically identicalidentical to to parent and to each otherparent and to each other
Used for Used for asexualasexual reproduction and reproduction and growthgrowth
22Meiosis Compared to Mitosis
23Meiosis I Compared to Mitosis
24Meiosis II Compared to Mitosis
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 25Life Cycle Basics:Life Cycle Basics:
PlantsPlants
Haploid multicellular “individuals” alternate with Haploid multicellular “individuals” alternate with diploid multicellular “individuals” diploid multicellular “individuals”
The haploid individual:The haploid individual: Known as the gametophyteKnown as the gametophyte May be larger or smaller than the diploid individualMay be larger or smaller than the diploid individual
The diploid individual:The diploid individual: Known as the sporophyteKnown as the sporophyte May be larger or smaller than the haploid individualMay be larger or smaller than the haploid individual
Mosses are haploid most of their life cycleMosses are haploid most of their life cycleFerns & higher plants have mostly diploid life cyclesFerns & higher plants have mostly diploid life cyclesIn fungi and most algae, only the zygote is diploidIn fungi and most algae, only the zygote is diploidIn plants, algae, & fungi, gametes produced by In plants, algae, & fungi, gametes produced by
haploid individualshaploid individuals
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 26Life Cycle Basics:Life Cycle Basics:
AnimalsAnimals
In familiar animals:In familiar animals:““Individuals” are diploid; produce haploid Individuals” are diploid; produce haploid gametesgametes
Only haploid part of life cycle is the gametesOnly haploid part of life cycle is the gametesThe products of meiosis are always gametesThe products of meiosis are always gametesMeiosis occurs only during gametogenesisMeiosis occurs only during gametogenesis Production of sperm Production of sperm
SpermatogenesisSpermatogenesis All four cells become spermAll four cells become sperm
Production of eggs Production of eggs OogenesisOogenesis Only one of four nuclei get cytoplasmOnly one of four nuclei get cytoplasm
Becomes the egg or ovumBecomes the egg or ovumOthers wither away as polar bodiesOthers wither away as polar bodies
Meiosis & Sexual Meiosis & Sexual ReproductionReproduction 27
The Human Life CycleThe Human Life Cycle
Sperm and egg are produced by meiosisSperm and egg are produced by meiosisA sperm and egg fuse at fertilizationA sperm and egg fuse at fertilizationResults in a zygoteResults in a zygote
The one-celled stage of an individual of the next The one-celled stage of an individual of the next generationgeneration
Undergoes mitosisUndergoes mitosis
Results in multicellular embryo that gradually takes Results in multicellular embryo that gradually takes on features determined when zygote was formedon features determined when zygote was formed
All growth occurs as mitotic divisionAll growth occurs as mitotic divisionAs a result of mitosis, each somatic cell in bodyAs a result of mitosis, each somatic cell in body
Has same number of chromosomes as zygoteHas same number of chromosomes as zygote Has genetic makeup determined when zygote was Has genetic makeup determined when zygote was
formedformed
28Gametogenesis in Mammals
Meiosis & Sexual Reproduction
Ending Slide Chapter 10