Meiosis and Sexual Life Cycles - R. · PDF file · 2015-08-25Comparison of Asexual...

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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

PowerPoint® Lecture Presentations for

BiologyEighth Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp

Chapter 13

Meiosis and SexualLife Cycles

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 inappearance that offspring show from parentsand siblings

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 13-1

Concept 13.1: Offspring acquire genes fromparents by inheriting chromosomes

• In a literal sense, children do not inheritparticular physical traits from their parents

• It is genes that are actually inherited


Inheritance of Genes

• Genes are the units of heredity, and are madeup of segments of DNA

• Genes are passed to the next generationthrough reproductive cells called gametes(sperm and eggs)

• Each gene has a specific location called alocus on a certain chromosome

• Most DNA is packaged into chromosomes

• One set of chromosomes is inherited fromeach parent


Comparison of Asexual and Sexual Reproduction

• In asexual reproduction, one parent producesgenetically identical offspring by mitosis

• A clone is a group of genetically identicalindividuals from the same parent

• In sexual reproduction, two parents give riseto offspring that have unique combinations ofgenes inherited from the two parents

Video: Hydra BuddingVideo: Hydra Budding


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Fig. 13-2

(a) Hydra (b) Redwoods

Parent

Bud

0.5 mmConcept 13.2: Fertilization and meiosis alternatein sexual life cycles

• A life cycle is the generation-to-generationsequence of stages in the reproductive historyof an organism


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 thesame length and carry genes controlling thesame inherited characters


Fig. 13-3APPLICATION

TECHNIQUE

Pair of homologousreplicated chromosomes

5 µm

Centromere

Sisterchromatids

Metaphasechromosome

• The sex chromosomes are called X and Y

• Human females have a homologous pair of Xchromosomes (XX)

• Human males have one X and one Ychromosome

• The 22 pairs of chromosomes that do notdetermine sex are called autosomes


• Each pair of homologous chromosomesincludes one chromosome from each parent

• The 46 chromosomes in a human somatic cellare two sets of 23: one from the mother andone from 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


Fig. 13-4

KeyMaternal set ofchromosomes (n = 3)Paternal set ofchromosomes (n = 3)

2n = 6

Centromere

Two sister chromatidsof one replicatedchromosome

Two nonsisterchromatids ina homologous pair

Pair of homologouschromosomes(one from each set)

• 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


• Fertilization is the union of gametes (thesperm and 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 HumanLife Cycle


• 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 lifecycles to maintain chromosome number


Fig. 13-5Key

Haploid (n)Diploid (2n)

Haploid gametes (n = 23)Egg (n)

Sperm (n)

MEIOSIS FERTILIZATION

Ovary Testis

Diploidzygote(2n = 46)

Mitosis anddevelopment

Multicellular diploidadults (2n = 46)

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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


• In animals, meiosis produces gametes, whichundergo no further cell division beforefertilization

• Gametes are the only haploid cells in animals

• Gametes fuse to form a diploid zygote thatdivides by mitosis to develop into amulticellular organism


Fig. 13-6

KeyHaploid (n)Diploid (2n)

n nGametes

nn n

Mitosis


MEIOSIS

2n 2nZygote2n

MitosisDiploidmulticellularorganism

(a) Animals

Spores

Diploidmulticellularorganism(sporophyte)

(b) Plants and some algae

2n

Mitosis

Gametes

Mitosisn

n n

Zygote

FERTILIZATION

nn

nMitosis

Zygote

(c) Most fungi and some protists


2n

Gametes

n

n

Mitosis

Haploid multi-cellular organism(gametophyte)

Haploid unicellular ormulticellular organism

Fig. 13-6aKey


Gametesn

n

n

2n 2nZygote


MitosisDiploidmulticellularorganism

(a) Animals

• Plants and some algae exhibit an alternationof generations

• This life cycle includes both a diploid andhaploid multicellular stage

• The diploid organism, called the sporophyte,makes haploid spores by meiosis


• 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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Fig. 13-6bKey


n n

n

n n

2n2n

Mitosis

Mitosis

Mitosis

Zygote

SporesGametes


Diploidmulticellularorganism(sporophyte)

Haploid multi-cellular organism(gametophyte)

(b) Plants and some algae

• 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


Fig. 13-6cKey


Mitosis Mitosis

Gametes

Zygote

Haploid unicellular ormulticellular organism


n

nn n

n

2n

(c) Most fungi and some protists

• 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


Concept 13.3: Meiosis reduces the number ofchromosome 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 inmitosis

• Each daughter cell has only half as manychromosomes as the parent cell


The Stages of Meiosis

• In the first cell division (meiosis I), homologouschromosomes separate

• Meiosis I results in two haploid daughter cellswith replicated chromosomes; it is called thereductional division

• In the second cell division (meiosis II), sisterchromatids separate

• Meiosis II results in four haploid daughter cellswith unreplicated chromosomes; it is called theequational division


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Fig. 13-7-1Interphase

Homologous pair of chromosomesin diploid parent cell

Chromosomesreplicate

Homologous pair of replicated chromosomes

Sisterchromatids Diploid cell with

replicated chromosomes







Meiosis I

Homologouschromosomesseparate

1

Haploid cells withreplicated chromosomes







Meiosis I


1

Haploid cells withreplicated chromosomes

Meiosis II

2 Sister chromatidsseparate

Haploid cells with unreplicated chromosomes

• Meiosis I is preceded by interphase, in whichchromosomes are replicated to form sisterchromatids

• The sister chromatids are genetically identicaland joined at the centromere

• The single centrosome replicates, forming twocentrosomes

BioFlixBioFlix: Meiosis: Meiosis


Fig. 13-8

Prophase I Metaphase I Anaphase I Telophase I andCytokinesis Prophase II Metaphase II Anaphase II Telophase II and

Cytokinesis

Centrosome(with centriole pair)

Sisterchromatids Chiasmata

Spindle

Homologouschromosomes

Fragmentsof nuclearenvelope

Centromere(with kinetochore)

Metaphaseplate

Microtubuleattached tokinetochore

Sister chromatidsremain attached


Cleavagefurrow

Sister chromatidsseparate Haploid daughter cells

forming

• Division in meiosis I occurs in four phases:

– Prophase I

– Metaphase I

– Anaphase I

– Telophase I and cytokinesis


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Metaphase I

Fig. 13-8a

Prophase I Anaphase I Telophase I andCytokinesis



Spindle




Metaphaseplate




Cleavagefurrow

Prophase I

• Prophase I typically occupies more than 90%of the time required for meiosis

• Chromosomes begin to condense

• In synapsis, homologous chromosomesloosely pair up, aligned gene by gene


• 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


Metaphase I

• In metaphase I, tetrads line up at themetaphase plate, with one chromosome facingeach pole

• Microtubules from one pole are attached to thekinetochore of one chromosome of each tetrad

• Microtubules from the other pole are attachedto the kinetochore of the other chromosome


Fig. 13-8b

Prophase I Metaphase I



Spindle


Metaphaseplate




Anaphase I

• In anaphase I, pairs of homologouschromosomes separate

• One chromosome moves toward each pole,guided by the spindle apparatus

• Sister chromatids remain attached at thecentromere and move as one unit toward thepole


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Telophase I and Cytokinesis

• In the beginning of telophase I, each half of thecell has a haploid set of chromosomes; eachchromosome still consists of two sisterchromatids

• Cytokinesis usually occurs simultaneously,forming two haploid daughter cells


• 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


Fig. 13-8c

Anaphase I Telophase I andCytokinesis



Cleavagefurrow

• Division in meiosis II also occurs in fourphases:

– Prophase II

– Metaphase II

– Anaphase II

– Telophase II and cytokinesis

• Meiosis II is very similar to mitosis


Fig. 13-8d

Prophase II Metaphase II Anaphase II Telophase II andCytokinesis


forming

Prophase II

• In prophase II, a spindle apparatus forms

• In late prophase II, chromosomes (each stillcomposed of two chromatids) move toward themetaphase plate


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Metaphase II

• In metaphase II, the sister chromatids arearranged 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 tomicrotubules extending from opposite poles


Fig. 13-8e

Prophase II Metaphase II

Anaphase II

• In anaphase II, the sister chromatids separate

• The sister chromatids of each chromosomenow move as two newly individualchromosomes toward opposite poles


Telophase II and Cytokinesis

• In telophase II, the chromosomes arrive atopposite poles

• Nuclei form, and the chromosomes begindecondensing


• Cytokinesis separates the cytoplasm

• At the end of meiosis, there are four daughtercells, each with a haploid set of unreplicatedchromosomes

• Each daughter cell is genetically distinct fromthe others and from the parent cell


Fig. 13-8f

Anaphase II Telephase II andCytokinesis


forming

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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 chromosomessets from two (diploid) to one (haploid),producing cells that differ genetically from eachother and from the parent cell

• The mechanism for separating sisterchromatids is virtually identical in meiosis IIand mitosis


Fig. 13-9MITOSIS MEIOSIS

MEIOSIS I

Prophase I

Chiasma

Homologouschromosomepair

Chromosomereplication

Parent cell

2n = 6

Chromosomereplication

Replicated chromosome

Prophase

Metaphase Metaphase I

Anaphase ITelophase I

Haploidn = 3

Daughtercells of

meiosis I

AnaphaseTelophase

2n 2n

Daughter cellsof mitosis

n n n n

MEIOSIS II

Daughter cells of meiosis II

SUMMARY

Meiosis

Occurs during interphase before meiosis I begins

Two, each including prophase, metaphase, anaphase, andtelophase

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 many chromosomesas the parent cell; genetically different from the parentcell and from each other

Produces gametes; reduces number of chromosomes by halfand introduces genetic variability amoung the gametes

Mitosis

Occurs during interphase beforemitosis begins

One, including prophase, metaphase,anahase, 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

Property

DNAreplication

Number ofdivisions

Synapsis ofhomologouschromosomes

Number ofdaughter cellsand geneticcomposition

Role in theanimal body

• Three events are unique to meiosis, and allthree occur in meiosis l:

– Synapsis and crossing over in prophase I:Homologous chromosomes physically connectand exchange genetic information

– At the metaphase plate, there are pairedhomologous chromosomes (tetrads), insteadof individual replicated chromosomes

– At anaphase I, it is homologouschromosomes, instead of sister chromatids,that separate


• Sister chromatid cohesion allows sisterchromatids of a single chromosome to staytogether through meiosis I

• Protein complexes called cohesins areresponsible for this cohesion

• In mitosis, cohesins are cleaved at the end ofmetaphase

• In meiosis, cohesins are cleaved along thechromosome arms in anaphase I (separation ofhomologs) and at the centromeres in anaphaseII (separation of sister chromatids)


Fig. 13-10EXPERIMENT

RESULTS

Shugoshin+ (normal)+Spore case Fluorescent label

Metaphase I

Shugoshin–

Anaphase I

Metaphase II

Anaphase II

Maturespores

OR

Spore Two of three possible arrange-ments of labeled chromosomes

Shugoshin+ Shugoshin–

Spor

e ca

ses

(%)

100806040200

? ?

??

? ?

??

Concept 13.4: Genetic variation produced insexual life cycles contributes to evolution

• Mutations (changes in an organism’s DNA) arethe original source of genetic diversity

• Mutations create different versions of genescalled alleles

• Reshuffling of alleles during sexualreproduction produces 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


Independent Assortment of Chromosomes

• Homologous pairs of chromosomes orientrandomly at metaphase I of meiosis

• In independent assortment, each pair ofchromosomes sorts maternal and paternalhomologues into daughter cells independentlyof the other pairs


• The number of combinations possible whenchromosomes assort independently intogametes is 2n, where n is the haploid number

• For humans (n = 23), there are more than8 million (223) possible combinations ofchromosomes


Fig. 13-11-1

Possibility 1 Possibility 2

Two equally probablearrangements ofchromosomes at

metaphase I

Fig. 13-11-2



metaphase I

Metaphase II

Fig. 13-11-3



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 genesinherited from each parent

• Crossing over begins very early in prophase I,as homologous chromosomes pair up gene bygene


• In crossing over, homologous portions of twononsister chromatids trade places

• Crossing over contributes to genetic variationby combining DNA from two parents into asingle chromosome


Fig. 13-12-1Prophase Iof meiosis

Pair ofhomologs

Nonsisterchromatidsheld togetherduring synapsis


Pair ofhomologs


Chiasma

CentromereTEM


Pair ofhomologs


Chiasma

Centromere

Anaphase ITEM


Pair ofhomologs


Chiasma

Centromere

Anaphase I

Anaphase II

TEM

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Pair ofhomologs


Chiasma

Centromere

Anaphase I

Anaphase II

Daughtercells

Recombinant chromosomes

TEM

Random Fertilization

• Random fertilization adds to genetic variationbecause any sperm can fuse with any ovum(unfertilized egg)

• The fusion of two gametes (each with 8.4million possible chromosome combinationsfrom independent assortment) produces azygote with any of about 70 trillion diploidcombinations


• Crossing over adds even more variation

• Each zygote has a unique genetic identity

Animation:Animation: Genetic Variation Genetic Variation


The Evolutionary Significance of Genetic VariationWithin 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


Fig. 13-UN1

Prophase I: Each homologous pair undergoessynapsis and crossing over between nonsisterchromatids.

Metaphase I: Chromosomes line up as homolo-gous pairs on the metaphase plate.

Anaphase I: Homologs separate from each other;sister chromatids remain joined at the centromere.

You should now be able to:

1. Distinguish between the following terms:somatic cell and gamete; autosome and sexchromosomes; haploid and diploid

2. Describe the events that characterize eachphase of meiosis

3. Describe three events that occur duringmeiosis I but not mitosis

4. Name and explain the three events thatcontribute to genetic variation in sexuallyreproducing organisms


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Meiosis and Sexual Life Cycles - R. · PDF file · 2015-08-25Comparison of Asexual...

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