Meiosis and Sexual Reproduction

Chapter 9

Why Sex?

9.1 Genes and Alleles

Genes• Sequences of DNA that encode heritable traits

Alleles • Slightly different forms of the same gene• Each specifies a different version of gene product

Sexual and Asexual Reproduction

Asexual reproduction (1 parent)• Offspring inherit parent’s genes• Clones (identical copies of parent)

Sexual reproduction (2 parents)• Offspring differ from parents and each another• Different combinations of alleles • Different details of shared traits

Sexual Reproduction

Meiosis, gamete formation, and fertilization occur in sexual reproduction

Meiosis and fertilization shuffle parental alleles• Offspring inherit new combinations of alleles

Where Gametes Form

Flowering plant

Fig. 9.3a, p.140

anther (wheresexual sporesthat give rise tosperm form)

ovules inside anovary (where sexualspores that giveto eggs form)

Fig. 9.3b-c, p.140

testis(where sperm originate)

Human male Human female

ovary(where eggsdevelop)

Animation: Reproductive organs

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Key Concepts: SEXUAL VS. ASEXUAL REPRODUCTION

By asexual reproduction, one parent alone transmits its genetic information to offspring

By sexual reproduction, offspring typically inherit information from two parents that differ in their alleles

Alleles are different forms of the same gene; they specify different versions of a trait

9.2 What Meiosis Does

Meiosis • Nuclear division mechanism that precedes

gamete formation in eukaryotic cells• Halves parental chromosome number

Fertilization• Fusion of two gamete nuclei • Restores parental chromosome number• Forms zygote (first cell of new individual)

Meiosis and Fertilization

Fig. 9.12, p.150

gametes gametes

germ cell germ cell

each chromosomeduplicated during

interphase

MEIOSIS IIseparation of

sister chromatids

MEIOSIS Iseparation ofhomologues

diploid numberrestored atfertilization

zygote

2n

2n

n

Animation: Meiosis I and II

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Homologues

Sexual reproducers inherit pairs of chromosomes• 1 from maternal parent, 1 from paternal parent

The pairs are homologous (“the same”) • Except nonidentical sex chromosomes (X and Y)• Same length, shape, genes

All pairs interact at meiosis• One chromosome of each type sorts into gametes

Homologous Chromosomes

9.3 Tour of Meiosis

All chromosomes are duplicated during interphase, before meiosis

Two divisions, meiosis I and II, divide the parental chromosome number by two

Each forthcoming gamete is haploid (n)

Meiosis I

The first nuclear division

Each duplicated chromosome lines up with its homologous partner

The two homologous chromosomes move apart, toward opposite spindle poles

Prophase I

Chromosomes condense and align tightly with their homologues

Each homologous pair undergoes crossing over

Microtubules form the bipolar spindle

One pair of centrioles moves to the other side of the nucleus

Prophase I (cont.)

Nuclear envelope breaks up• Microtubules growing from each spindle pole

penetrate the nuclear region

Microtubules tether one or the other chromosome of each homologous pair

Prophase I

Metaphase I

Microtubules from both poles position all pairs of homologous chromosomes at the spindle equator

Metaphase I

Anaphase I

Microtubules separate each chromosome from its homologue, moving to opposite spindle poles

Other microtubules overlap midway between spindle poles, slide past each other to push poles farther apart

As anaphase I ends, one set of duplicated chromosomes nears each spindle pole

Anaphase I

Telophase I

Two nuclei form • Typically, the cytoplasm divides

All chromosomes are still duplicated• Each still consists of two sister chromatids

Telophase I

Meiosis II

The second nuclear division

Sister chromatids of each chromosome are pulled away from each other

Each is now an individual chromosome

Prophase II

Metaphase II

Anaphase II and Telophase II

In anaphase II, one chromosome of each type is moved toward opposite spindle poles• Occurs in both nuclei formed in meiosis I

By the end of telophase II, there are four haploid nuclei, each with unduplicated chromosomes

Anaphase II

Telophase II

plasma membrane

spindle equator (midway between the two poles)

one pair of homologous chromosomes

Prophase I Metaphase I Anaphase I Telophase I

Meiosis I

Fig. 9.5a, p.142

newly formingmicrotubules ofthe spindle

breakupof nuclearenvelope

centrosome witha pair of centrioles,moving to oppositesides of nucleus

Chromosomes were duplicated earlier, ininterphase.

Prior to metaphase I, one set of microtubules had tethered one chromosome of each type to one spindle pole and another set tethered its homologue to the other spindle pole.

One of each duplicatedchromosome, maternal or paternal, moves to a spindle pole; its homologue moves to the opposite pole.

One of each typeof chromosome hasarrived at a spindlepole. In most species,the cytoplasm dividesat this time.

Telophase IAnaphase IMetaphase IProphase I

Meiosis I

Stepped Art

Fig. 9-5a, p.142

Fig. 9.5b, p.142

Prophase II Metaphase II Anaphase II Telophase II

Meiosis II

there is no DNA replication between the two divisions

Fig. 9.5b, p.142

In each cell, one of two centrioles moves to the opposite side of thecell, and a new bipolar spindle forms.

By now, microtubules fromboth spindle poles have finished a tug-of-war.

The sister chromatids of each chromosome move apart and are now individual, unduplicated

A new nuclear envelopeencloses each parcel ofchromosomes, so thereare now four nuclei.

Meiosis II

Prophase II Metaphase II Anaphase II Telophase II

Stepped Art

Fig. 9-5b, p.142

Haploid Daughter Cells

When cytoplasm divides, four haploid cells result

One or all may serve as gametes or, in plants, as spores that lead to gamete-producing bodies

Key Concepts:STAGES OF MEIOSIS

Diploid cells have a pair of each type of chromosome, one maternal and one paternal

Meiosis, a nuclear division mechanism, reduces the chromosome number

Meiosis occurs only in cells set aside for sexual reproduction

Key Concepts:STAGES OF MEIOSIS (cont.)

Meiosis sorts out a reproductive cell’s chromosomes into four haploid nuclei

Haploid nuclei are distributed to daughter cells by way of cytoplasmic division

Animation: Meiosis step-by-step

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9.4 Meiosis Introduces Variation in Traits

Two events in meiosis cause variation in traits in sexually reproducing species• Crossing over during prophase I of meiosis• Chromosome shuffling during metaphase I of

meiosis

Prophase I: Crossing Over

Nonsister chromatids of homologous chromosomes undergo crossing over• They exchange segments at the same place

along their length

Each ends up with new combinations of alleles not present in either parental chromosome

Crossing Over

Fig. 9.6, p.144

a A maternal chromosome (purple) and paternal chromosome (blue) were duplicated earlier, duringinterphase. They become visible in microscopes early in prophase I, when hey star to condense tothreadlike form.

b Each chromosome and its homologous partner zippertogether, so all four chromatidsare tightly aligned.

mom’salleleB

mom’salleleA

mom’salleleA

mom’salleleA

mom’salleleB

dad’sallele

a

dad’salleleb

dad’salleleb

c Here is a simple way to think about crossing over. (Chromosomes are still condensed and threadlike,and each is tightly aligned with its homologous partner.)

d Their intimate contactpromotes crossing over at different places along the length of nonsister chromatids.

e At the crossover site, paternal and maternal chromatids exchangecorresponding segments.

f Crossing over mixesup maternal and paternalalleles on homologouschromosomes.

Fig. 9.6a, p.144

Fig. 9.6b, p.144

Fig. 9.6c, p.144

Fig. 9.6d, p.144

Fig. 9.6e, p.144

Fig. 9.6f, p.144

Animation: Crossing over

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Metaphase I: Chromosome Shuffling

Homologous chromosomes align randomly during metaphase I

Microtubules can harness either a maternal or paternal chromosome of each homologous pair to either spindle pole

Either chromosome may end up in any new nucleus (gamete)

Chromosome Shuffling:Random Alignment

or

or

or

1 2 3

combinations possible

Fig. 9.7, p.145

1 2 3

combinations possible

or

or

or

Stepped Art

Fig. 9-7, p.145

Animation: Random alignment

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Key Concepts: CHROMOSOME RECOMBINATION AND SHUFFLING

During meiosis, each pair of maternal and paternal chromosomes swaps segments and exchanges alleles

Pairs get randomly shuffled, so forthcoming gametes end up with different mixes of maternal and paternal chromosomes

9.5 From Gametes to Offspring

Multicelled diploid and haploid bodies are typical in life cycles of plants and animals

Plants• Sporophyte: A multicelled plant body (diploid) that

makes haploid spores• Spores give rise to gametophytes (multicelled

plant bodies in which haploid gametes form)

From Gametes to Offspring

Animals• Germ cells in the reproductive organs give rise to

sperm or eggs• Fusion of a sperm and egg at fertilization results

in a zygote

Comparing Plant And Animal Life Cycles

Fig. 9.8a, p.146

meiosisDIPLOID

fertilization

zygote

gametes spores

HAPLOID

Fig. 9.8a, p.146

meiosis

meiosis

meiosis

a Plant life cycle

(2n)

(2n)

(n)

multicelledgametophyte

multicelledsporophyte

(n) (n)

Fig. 9.8b, p.146

Fig. 9.8b, p.146

meiosis DIPLOIDfertilization

zygote

HAPLOID

meiosis

(2n)

(2n)

(n)gametes

multicelledbody

b Animal life cycle

Animation: Generalized life cycles

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Introducing Variation in Offspring

Three events cause new combinations of alleles in offspring: • Crossing over during prophase I (meiosis)• Random alignment of maternal and paternal

chromosomes at metaphase I (meiosis)• Chance meeting of gametes at fertilization

All three contribute to variation in traits

Sperm Formation in Animals

a Growth b Meiosis I andcytoplasmic division

c Meiosis II and cytoplasmic division

spermatids (haploid)

secondary spermatocytes

(haploid)

primary spermatocyte

(diploid)

Fig. 9.9, p.147

diploid malegerm cell

sperm (mature,haploid male

gametes)

Animation: Sperm formation

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Egg Formation in Animals

a Growth b Meiosis I and cytoplasmic division

c Meiosis II and cytoplasmic division

ovum (haploid)

primary oocyte (diploid)

secondary oocyte

(haploid)

first polar body

(haploid)

three polar bodies

(haploid)

Fig. 9.10a, p.147

oogonium(diploidfemale

germ cell)

Animation: Egg formation

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Key Concepts: SEXUAL REPRODUCTION IN LIFE CYCLES

In animals, gametes form by different mechanisms in males and females

In most plants, spore formation and other events intervene between meiosis and gamete formation

9.6 Comparing Mitosis and Meiosis

Both mitosis and meiosis require bipolar spindle to move and sort duplicated chromosomes

Some mechanisms of meiosis resemble those of mitosis, and may have evolved from them• Example: DNA repair enzymes function in both

Differences in Mitosis and Meiosis

Mitosis maintains parental chromosome number• Duplicates genetic information• Occurs in body cells

Meiosis halves chromosome number• Introduces new combinations of alleles in

offspring• Occurs only in cells for sexual reproduction

Comparing Mitosis and Meiosis

Comparing Mitosis and Meiosis

Comparing Mitosis and Meiosis

Telophase IProphase I Anaphase IMetaphase I

Fig. 9.11a, p.148

In a diploid (2n) germ cell,duplicated chromosomesnow condense. The bipolarspindle forms and tethers thechromosomes. Crossoversoccur between homologues.

Each maternal chromosomeand its paternal homologueare randomly aligned midwaybetween the two spindlepoles. Either one may getattached to either pole.

Homologouspartnersseparateand moveto oppositepoles.

There are two clustersof chromosomes. Newnuclear envelopes mayform and the cytoplasmmay divide beforemeiosis II begins.

Fig. 9.11b, p.149

In a diploid (2n) body cell, the duplicated chromosomes nowcondense. Bipolar spindle formsand tethers the chromosomes.

All chromosomesaligned at thespindle equator.

Sister chromatids of each chromosomemoved to oppositespindle poles.

Two diploid (2n) nuclei form. After cytoplasmicdivision, there are two diploid body cells.

Prophase Metaphase Anaphase Telophase

Mitosis

Fig. 9.11c, p.149

no interphaseand no DNAreplicationbetween thetwo nucleardivisions

Prophase II Metaphase II Anaphase II Telophase IIAll chromosomes stillduplicated. New spindleforms in each nucleus,tethers chromosomesto spindle poles.

All chromosomesaligned at thespindle equator.

Sister chromatids ofeach chromosomemoved to oppositespindle poles.

Four haploid (n) nucleiform. After cytoplasmicdivision, haploid cellsfunction as gametesor spores.

Key Concepts: MITOSIS AND MEIOSIS COMPARED

Recent molecular evidence suggests that meiosis originated through mechanisms that already existed for mitosis and, before that, for repairing damaged DNA

Animation: Comparing mitosis and meiosis

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Video: Why Sex?

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