Meiosis & Sexual Reproduction Chapter 10. Meiosis & Sexual Reproduction 2Outline Reduction in...

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


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.



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.



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


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)


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


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


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


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 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


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

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