Paternal homologue Maternal homologue Diploid zygote€¦ · chromosomes begin to condense, and the spindle of microtubules begins to form. The DN A has been replicated, and each

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

Haploid sperm

Haploid egg

Diploid zygote

Fertilization Paternal homologue

Maternal homologue

1


n 2n

FERTILIZAOTIN MEIOSIS MEIOSIS

Sperm (haploid) n

Egg (haploid) n

Zygote (diploid) 2n

Somatic cells

Germ-line cells

Adult male (diploid) 2n Adult female

(diploid) 2n

MITOSIS

MITOSIS

Germ-line cells

2


a.

b. c.

Sister chromatids

Homologues

Kinetochore Diploid cell

Chromosome duplication

Meiosis I

Meiosis II

Haploid cells

Centromere

Synaptonemal complex

Synaptonemal complex

Homologous chromosomes

138 nm b: Reprinted, with permission, from the Annual Review of Genetics, Volume 6 © 1972 by Annual Reviews, www.annualreviews.org

3


Site of crossover

4


Meiosis I Mitosis

Metaphase I

Anaphase I

Metaphase

Anaphase

Chiasmata hold homologues together. The kinetochores of sister chromatids fuse and function as one. Microtubules can attach to only one side of each centromere.

Microtubules pull the homologous chromosomes apart, but sister chromatids are held together.

Homologues do not pair; kinetochores of sister chromatids remain separate; microtubules attach to both kinetochores on opposite sides of the centromere.

Microtubules pull sister chromatids apart.

5


6


MEIOSIS I Prophase I Metaphase I Anaphase I Telophase I

Chromosome (replicated) Spindle

Chiasmata

Kinetochore microtubule


Sister chromatids Chromosome Nonidentical sister chromatids

In prophase I of meiosis I, the chromosomes begin to condense, and the spindle of microtubules begins to form. The DN A has been replicated, and each chromosome consists of two sister chromatids attached at the centromere. In the cell illustrated here, there are four chromosomes, or two pairs of homologues. Homologous chromosomes pair up and become closely associated during synapsis. Crossing over occurs, forming chiasmata, which hold homologous chromosomes together.

In metaphase I, the pairs of homologous chromosomes align along the metaphase plate. Chiasmata help keep the pairs together and produce tension when microtubules from opposite poles attach to sister kinetochores of each homologue. A kinetochore microtubule from one pole of the cell attaches to one homologue of a chromosome, while a kinetochore microtubule from the other cell pole attaches to the other homologue of a pair.

In anaphase I, kinetochore microtubules shorten, and homologous pairs are pulled apart. One duplicated homologue goes to one pole of the cell, while the other duplicated homologue goes to the other pole. Sister chromatids do not separate.This is in contrast to mitosis, where duplicated homologues line up individually on the metaphase plate, kinetochore microtubules from opposite poles of the cell attach to opposite sides of one homologue's centromere, and sister chromatids are pulled apart in anaphase.

In telophase I, the separated homologues form a cluster at each pole of the cell, and the nuclear envelope re-forms around each daughter cell nucleus. Cytokinesis may occur . The resulting two cells have half the number of chromosomes as the original cell: In this example, each nucleus contains two chromosomes (versus four in the original cell). Each chromosome is still in the duplicated state and consists of two sister chromatids, but sister chromatids are not identical because crossing over has occurred.

Paired homologous chromosomes

Homologue pair on metaphase plate


Sister chromatids

© Clare A. Hasenkampf/Biological Photo Service

40 µm 40 µm 40 µm 40 µm

7


MEIOSIS II Prophase II Metaphase II Anaphase II Telophase II

Sister chromatids

Sister chromatids Spindle

Nuclear membrane breaking down Chromosome

40 µm 40 µm 40 µm 40 µm

Following a typically brief interphase, with no S phase, meiosis II begins. During prophase II, a new spindle apparatus forms in each cell, and the nuclear envelope breaks down. In some species the nuclear envelope does not re-form in telophase I removing the need for nuclear envelope breakdown.

In metaphase II, a completed spindle apparatus is in place in each cell. Chromosomes consisting of sister chromatids joined at the centromere align along the metaphase plate in each cell. No w , kinetochore microtubules from opposite poles attach to kinetochores of sister chromatids.

When microtubules shorten in anaphase II, the centromeres split, and sister chromatids are pulled to opposite poles of the cells.

In telophase II, the nuclear membranes re-form around four di f ferent clusters of chromosomes. After cytokinesis, four haploid cells result. No two cells are alike due to the random alignment of homologous pairs at metaphase I and crossing over during prophase I.

Nuclear membrane re-forming

Kinetochore microtubule

© Clare A. Hasenkampf/Biological Photo Service 8

Parent cell (2n)

MEIOSIS I

Prophase I Metaphase I Anaphase I Telophase I

Prophase Metaphase Anaphase T elophase

Homologous chromosomes do not pair.

Individual homologues align on metaphase plate.

Paternal homologue


Chromosome replication

Chromosome replication

Homologous chromosomes pair; synapsis and crossing over occur.

Paired homologous chromosomes align on metaphase plate.

Maternal homologue

MITOSIS


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

Two daughter cells (each 2n)

Homologous chromosomes separate; sister chromatids remain together.

9


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.

Four daughter cells (each n)

10

Paternal homologue Maternal homologue Diploid zygote€¦ · chromosomes begin to condense, and the spindle of microtubules begins to form. The DN A has been replicated, and each

Documents