Sexual Reproductionand Meiosis

Chapter 11

1

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

• Allows offspring to inherit genetic material from 2 parents

2

4

• 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 more diploid cells

– multicellularity

– Later in the life cycle, some of these diploid cells undergo meiosis to produce haploid gametes

– (sperm & eggs)

Meiosis • DNA replicates (chromosome # doubled)• Meiosis I

– Stages:• Prophase I – synapsis occurs• Metaphase I – homologous chromosome pairs line up (4 chromosomes)• Anaphase I – homologous chromosomes seperate• Telophase I

– Results in 2 cells, but still diploid number• Meiosis II

– Stages• Prophase II• Metaphase II • Anaphase II – sister chromosomes seperate• Telophase II

– Results in 4 cells, haploid number, one of each homologous pair in every cell

6

7

Features of Meiosis

• Meiosis includes 2 rounds of division– Meiosis I and meiosis II– Each has prophase, metaphase, anaphase, and

telophase stages

• Synapsis (crossing-over)– During early prophase I– Homologous chromosomes become closely

associated (eventually swapping DNA)– Includes formation of synaptonemal complexes

• Formation also called tetrad or bivalents

8

• 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– Meiosis II is very much like mitosis (but half the DNA)

Prophase I• Chromosomes coil tighter and

become visible, nuclear envelope disappears, spindle forms

• Each chromosome composed of 2 sister chromatids

• Synapsis (crossing-over) – Homologues become closely

associated– Crossing over occurs between

nonsister chromatids– Nonsister chromatids remain

attached at chiasmata• Chiasmata move to the end

of the chromosome arm before metaphase I

10

12

Crossing over• Occurs between nonsister chromatids • Allows the maternal and paternal

homologues to exchange chromosomal material (recombination)

• 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

13

Metaphase I• Terminal chiasmata hold homologues

together following crossing over

• Microtubules from opposite poles attach to each homologue (not each sister chromatid)

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

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

• (This sets up independent assortment)

14

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Many random combinations of the homologous pairing of 3 chromosomes

15

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 occurs

16

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

17

Meiosis II• Resembles a mitotic division

– But with a haploid rather than diploid cell

• 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

18

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 some species the nuclear Envelope does not re-form in telophase I, removing the need for Nuclear envelope breakdown.

Prophase II

19

In metaphase II, chromosomesconsisting of sister chromatidsjoined at the centromere alignalong the metaphase plate ineach cell.

Now, kinetochore microtubules from opposite poles attach to kinetochores of sister chromatids, as in mitosis.

Metaphase II

20

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

Anaphase II

21

The nuclear membranes re-form around four different clusters ofchromosomes.

After cytokinesis, four haploid cells result.

No two cells are alikedue to the random alignment ofhomologous pairs atmetaphase I and crossing overduring prophase I.

Telophase II

Final result of Meiosis

• Four cells containing haploid sets of chromosomes• In animals, these cells develop directly into gametes

– In Spermtogenesis: 4 sperm– In Oogenesis: 1 egg, 3 polar bodies

• In plants, fungi, and many protists, haploid cells then divide mitotically– Produce greater number of gametes

22

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

23

24

Meiosis vs. MitosisMeiosis is characterized by 4 features:

1. Synapsis and crossing over

2. Sister chromatids remain joined at their centromeres throughout meiosis I

3. Kinetochores of sister chromatids attach to the same pole in meiosis I

4. DNA replication is suppressed between meiosis I and meiosis II

25

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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.

26

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

27

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

MEIOSIS II

Prophase 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)