Slide 1 of 35 Copyright Pearson Prentice Hall 11-4 Meiosis

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Copyright Pearson Prentice Hall

11-4 Meiosis11-4 Meiosis

11-4 Meiosis

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Each organism must inherit a single copy of every gene from each of its “parents.”

WHY?.....

Gametes are formed by a process that separates the two sets of genes so that each gamete ends up with just one set.

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Chromosome Number

Chromosome Number

All organisms have different numbers of chromosomes.

A body cell in an adult fruit fly has 8 chromosomes: 4 from the fruit fly's male parent, and 4 from its female parent.

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Chromosome Number

These two sets of chromosomes are homologous.

Each of the 4 chromosomes that came from the male parent has a corresponding chromosome from the female parent.

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Chromosome Number

A cell that contains both sets of homologous chromosomes is said to be diploid.

The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N.

For Drosophila, the diploid number is 8, which can be written as 2N=8.

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Chromosome Number

The gametes of sexually reproducing organisms contain only a single set of chromosomes, and therefore only a single set of genes.

These cells are haploid. Haploid cells are represented by the symbol N.

For Drosophila, the haploid number is 4, which can be written as N=4.

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Phases of Meiosis

What happens during the process of meiosis?

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Phases of Meiosis

Phases of Meiosis

Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.

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Meiosis involves two divisions, meiosis I and meiosis II.

By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells.

Phases of Meiosis

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Phases of Meiosis

Meiosis I

Prophase I Metaphase I Anaphase I Telophase I and

Cytokinesis

Interphase I

Meiosis I

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Phases of Meiosis

Cells undergo a round of DNA replication, forming duplicate chromosomes.

Cell grows.

Make organelles

Make proteins

Interphase I

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Phases of Meiosis

Each chromosome pairs with its corresponding homologous chromosome to form a tetrad: crossing over can occur.

Centrioles move to sides

Spindle grows

There are 4 chromatids in a tetrad.

MEIOSIS I Prophase II

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Phases of Meiosis

When homologous chromosomes form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing-over.

Crossing-over produces new combinations of alleles.

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Phases of Meiosis

Spindle fibers attach to the chromosomes.

Some spindles push against each other

Homologous pairs line up in the middle

MEIOSIS I Metaphase I

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Phases of Meiosis

MEIOSIS I Anaphase IThe fibers pull the

homologous chromosomes toward opposite ends of the cell: some spindles shorten.

Cell elongates: some spindles get bigger

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Phases of Meiosis

MEIOSIS I Telophase I and CytokinesisNuclear membranes form.

The cell separates into two cells.

The two cells produced by meiosis I have chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I.

Spindle disappears.

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Phases of Meiosis

Meiosis II

The two cells produced by meiosis I now enter a second meiotic division.

Unlike meiosis I, neither cell goes through chromosome replication.

Each of the cell’s chromosomes has 2 chromatids.

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Phases of Meiosis

Meiosis II

Telophase II and

CytokinesisProphase II

Metaphase II Anaphase IITelophase I and Cytokinesis I

Meiosis II

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Phases of Meiosis

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.

Centrioles move

Spindle starts to form

NO MORE CROSSING OVER!

MEIOSIS IIProphase II

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Phases of Meiosis

The chromosomes line up in the center of cell.

Spindles attach

MEIOSIS II Metaphase II

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Phases of Meiosis

The sister chromatids separate and move toward opposite ends of the cell; spindle shortens

Cell elongates; spindles grow.

MEIOSIS II Anaphase II

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Phases of Meiosis

Meiosis II results in four haploid (N) daughter cells.

Nuclear membrane reappears

Spindle disappears

Chromosomes spread back out.

MEIOSIS II Telophase II and Cytokinesis

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Gamete Formation

Gamete Formation

In male animals, meiosis results in four equal-sized gametes called sperm.

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Gamete Formation

In many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in reproduction.

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Comparing Mitosis and Meiosis

How is meiosis different from mitosis?

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Mitosis results in the production of two genetically identical diploid cells. Meiosis produces four genetically different haploid cells.

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Mitosis

• Cells produced by mitosis have the same number of chromosomes and alleles as the original cell.

• Mitosis allows an organism to grow and replace cells.

• Some organisms reproduce asexually by mitosis.

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Meiosis

• Cells produced by meiosis have half the number of chromosomes as the parent cell.

• These cells are genetically different from the diploid cell and from each other.

• Meiosis is how sexually reproducing organisms produce gametes.

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If the body cells of humans contain 46 chromosomes, a single sperm cell should have

a. 46 chromosomes.

b. 23 chromosomes.

c. 92 chromosomes.

d. between 23 and 46 chromosomes.

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During meiosis, the number of chromosomes per cell is cut in half through the separation of

a. daughter cells.

b. homologous chromosomes.

c. gametes.

d. chromatids.

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The formation of a tetrad occurs during

a. anaphase I.

b. metaphase II.

c. prophase I.

d. prophase II.

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In many female animals, meiosis results in the production of

a. only 1 egg.

b. 1 egg and 3 polar bodies.

c. 4 eggs.

d. 1 egg and 2 polar bodies.

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Compared to egg cells formed during meiosis, daughter cells formed during mitosis are

a. genetically different, while eggs are genetically identical.

b. genetically different, just as egg cells are.

c. genetically identical, just as egg cells are.

d. genetically identical, while egg cells are genetically different.

END OF SECTION

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Slide 1 of 35 Copyright Pearson Prentice Hall 11-4 Meiosis