Biology 201 Chapter 11

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Chapter 11

Lecture and Animation Outline

Sexual Reproductionand MeiosisChapter 11

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Sexual life cycle

• Made up of meiosis and fertilization• Diploid cells

– Somatic cells of adults have 2 sets of chromosomes

• Haploid cells– Gametes have only 1 set of chromosomes

• Offspring inherit genetic material from 2 parents

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Haploid sperm

Haploid egg

Diploid zygote

FertilizationPaternalhomologueMaternalhomologue

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• Life cycles of sexually reproducing organisms involve the alternation of haploid and diploid stages

• Some life cycles include longer diploid phases, some include longer haploid phases

• In most animals, diploid state dominates– Zygote first undergoes mitosis to produce diploid cells– Later in the life cycle, some of these diploid cells

undergo meiosis to produce haploid gametes

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n

2n

FERTILIZAOTINMEIOSISMEIOSIS

Sperm(haploid) n

Egg(haploid) n

Zygote(diploid) 2n

SomaticcellsGerm-linecells

Adult male(diploid) 2n Adult female

(diploid) 2n

MITOSIS

MITOSIS

Germ-linecells

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Features of Meiosis

• Meiosis includes two rounds of division– Meiosis I and meiosis II– Each has prophase, metaphase, anaphase, and

telophase stages• Synapsis

– During early prophase I– Homologous chromosomes become closely

associated– Includes formation of synaptonemal complexes

• Formation also called tetrad or bivalents

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• First meiotic division is termed the “reduction division”– Results in daughter cells that contain one homologue

from each chromosome pair• No DNA replication between meiotic divisions• Second meiotic division does not further reduce

the number of chromosomes– Separates the sister chromatids for each homologue

The Process of Meiosis• Meiosis I

– Prophase I– Metaphase I– Anaphase I– Telophase I

• Meiosis II– Prophase II– Metaphase II– Anaphase II– Telophase II

• Meiotic cells have an interphase period that is similar to mitosis with G1, S, and G2 phases

• After interphase, germ-line cells enter meiosis I

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Kinetochore

Sister chromatids

Homologues

a.

Centromere

Synaptonemalcomplex


Prophase I

• Chromosomes coil tighter and become visible, nuclear envelope disappears, spindle forms

• Each chromosome composed of 2 sister chromatids

• Synapsis– Homologues become closely associated– Crossing over occurs between nonsister chromatids– Remain attached at chiasmata

• Chiasmata move to the end of the chromosome arm before metaphase I

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Crossing over

• Genetic recombination between nonsister chromatids

• Allows the homologues to exchange chromosomal material

• Alleles of genes that were formerly on separate homologues can now be found on the same homologue

• Chiasmata – site of crossing over– Contact maintained until anaphase I

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Crossing overCohesin proteins

Prophase I Metaphase I Anaphase I

Site of crossoverCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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Chromosome (replicated)

Spindle

Chiasmata

In prophase I, the chromosomesbegin to condense, and thespindle of microtubules beginsto form. The DNA has beenreplicated, and eachchromosome consists of twosister chromatids attached atthe centromere. The cellillustrated here has fourchromosomes, or two pairs ofhomologues. Homologouschromosomes pair along theirentire length during synapsis.Crossing over occurs, formingchiasmata, which holdhomologous chromosomestogether.

Paired homologouschromosomes

Sisterchromatids


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Metaphase I

• Terminal chiasmata hold homologues together following crossing over

• Microtubules from opposite poles attach to each homologue– Not each sister chromatid as in mitosis

• Homologues are aligned at the metaphase plate side-by-side

• Orientation of each pair of homologues on the spindle is random

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Kinetochore microtubule

In metaphase I, the pairs ofhomologous chromosomesalign at the equator of the cell.Chiasmata keep homologouspairs together and microtubulesfrom opposite poles attach tosister kinetochores of eachhomologue, producing tension.A kinetochore microtubule fromone pole of the cell attaches toone homologue of achromosome, while akinetochore microtubule fromthe other cell pole attaches tothe other homologue of a pair.

Homologue pairon metaphase plate


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Anaphase I• Microtubules of the spindle shorten

– Chiasmata break• Homologues are separated from each other and move

to opposite poles– Sister chromatids remain attached to each other at their

centromeres• Each pole has a complete haploid set of

chromosomes consisting of one member of each homologous pair

• Independent assortment of maternal and paternal chromosomes

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Homologous chromosomes

Sister chromatids

In anaphase I, kinetochoremicrotubules shorten, andhomologous pairs are pulledapart. One duplicated homologuegoes to one pole of the cell, andthe other duplicated homologuegoes to the other pole. Sisterchromatids do not separate. Thisis in contrast to mitosis, in whichduplicated homologues line upindividually on the metaphaseplate, and sister chromatids arepulled apart in anaphase.


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Telophase I

• Nuclear envelope re-forms around each daughter nucleus

• Sister chromatids are no longer identical because of crossing over (prophase I)

• Cytokinesis may or may not occur after telophase I

• Meiosis II occurs after an interval of variable length

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Chromosome

Nonidentical sister chromatids

In telophase I, the separatedhomologues form a cluster ateach pole of the cell, and thenuclear envelope re-formsaround each daughter cellnucleus. Cytokinesis may occur.The resulting two cells havehalf the number ofchromosomes of the originalcell: In this example, eachnucleus contains twochromosomes (versus four inthe original cell). Eachchromosome consists of twosister chromatids, but sisterchromatids are not identicalbecause crossing over hasoccurred.

Homologouschromosomes


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Meiosis II• Resembles a mitotic division• Prophase II: nuclear envelopes dissolve and

new spindle apparatus forms• Metaphase II: chromosomes align on

metaphase plate• Anaphase II: sister chromatids are separated

from each other• Telophase II: nuclear envelope re-forms

around 4 sets of daughter chromosomes; cytokinesis follows

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Spindle

Nuclear membrane breaking down

Following a brief interphase,with no S phase, meiosis IIbegins. During prophase II,a new spindle apparatus formsin each cell, and the nuclearenvelope breaks down. In somespecies the nuclear envelopedoes not re-form in telophase I,obviating the need for nuclearenvelope breakdown.


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Sister chromatids

Chromosome

In metaphase II, chromosomesconsisting of sister chromatidsjoined at the centromere alignalong the metaphase plate ineach cell. Now, kinetochoremicrotubules from oppositepoles attach to kinetochores ofsister chromatid, as in mitosis.


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Sister chromatids

When microtubules shorten inanaphase II, sister chromatidsare pulled to opposite polesof the cells, as in mitosis.

Kinetochoremicrotubule


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In telophase II, the nuclearmembranes re-form aroundfour different clusters ofchromosomes. Aftercytokinesis, four haploid cellsresult. No two cells are alikedue to the random alignment ofhomologous pairs atmetaphase I and crossing overduring prophase I.

Nuclearmembranere-forming


Final result

• Four cells containing haploid sets of chromosomes

• In animals, develop directly into gametes• In plants, fungi, and many protists, divide

mitotically– Produce greater number of gametes– Adults with varying numbers of gametes

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Errors in Meiosis

• Nondisjunction – failure of chromosomes to move to opposite poles during either meiotic division

• Aneuploid gametes – gametes with missing or extra chromosomes

• Most common cause of spontaneous abortion in humans

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Meiosis vs. Mitosis

Meiosis is characterized by 4 features:1. Synapsis and crossing over2. Sister chromatids remain joined at their centromeres throughout meiosis I3. Kinetochores of sister chromatids attach to the same pole in meiosis I4. DNA replication is suppressed between meiosis I and meiosis II

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Meiosis I Mitosis

Metaphase I

Anaphase I

Metaphase

Anaphase

Crossovers andsister chromatidcohesion lockhomologuestogether.Microtubulesconnect to thekinetochores ofsister chromatids sothat homologues arepulled towardopposite poles.

Microtubules pullthe homologouschromosomesapart, but sisterchromatids areheld together atthe centromere.

Homologues donot pair;kinetochores ofsister chromatidsremain separate;microtubulesattach to bothkinetochores onopposite sides ofthe centromere.

Microtubulespull sisterchromatidsapart.

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Parent cell (2n)

MEIOSIS I

Prophase I Metaphase I Anaphase I Telophase I

Prophase Metaphase Anaphase Telophase

Homologous chromosomesdo not pair. Individual homologues align

on metaphase plate.

Paternalhomologue

Homologouschromosomes

Homologous chromosomes pair;synapsis and crossing over occur.

Paired homologous chromosomesalign on metaphase plate.

Maternalhomologue

MITOSIS

Sister chromatids separate, cytokinesis occurs, and two cellsresult, each containing the original number of homologues.

Twodaughtercells(each 2n)

Homologous chromosomes separate;sister chromatids remain together.

Chromosomereplication

Chromosomereplication

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MEIOSIS IIProphase II Metaphase II Anaphase II Telophase II

Chromosomes align, sister chromatids separate, and four haploid cells result,each containing half the original number of homologues.

Fourdaughtercells(each n)

Education

Biology 201 Chapter 11