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6-3 How Gametes Form: Meiosis

Although some organisms are able to use mitosisand cell division to reproduce, most multicellular organisms use specialized cells for reproduction.

OBJECTIVES. Define gamete, and state its function in sexual

reproduction. Explain how meiosis maintains chromo-some

number throughout generations. Summarize the events of meiosis. Define crossing-over, and explain its impact on

evolution.

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Gametes A gamete is a reproductive

cell an egg or a sperm.

In sexual reproduction, twogametes fuse to form the firstcell of a new individual, asshown.

If gametes were diploidinstead of haploid, the newindividual would have twice asmany chromosomes as its

parents.

fertilization

2N zygote

gamete

gametes

Gametes are found only inorganisms that reproducesexually, as do most animals.

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Meiosis A type of nuclear division

called meiosis formsgametes .

meiosis I

meiosis II

The second meioticdivision (meiosis II)

separates identical copiesof chromosomes.

Like mitosis, meiosis isusually followed by cell

division.

Meiosis is a two-stagenuclear division in which

the chromosome number is halved.

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A Closer Look:

Meiosismale female

In meiosis, eukaryotes produce reproductivecells that contain half asmany chromosomes asthe parent cell

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One member of eachchromosome pair is from oneparent; the other is from theother parent.

A Closer Look:Meiosis

The diploid (2n) cell nucleishown here each have four chromosomes.

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First the amount of DNAdoubles.

A Closer Look:Meiosis

Then the similar

chromosomes pair with oneanother.

Occasionally, paired

chromosomes will exchangesegments in a process calledcrossing-over.

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Cell division now occurs, producing four haploid N)cells.

A Closer Look:Meiosis

N N N N

Meiosis II

The second meiotic division (Meiosis II) separates thetwo copies of each chromosome (the chromatids).

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Click image to view movie.

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During sperm formation, the four cells containing these nucleidevelop heads and tails.

A Closer Look:Meiosissperm

eggDuring egg formation, only one of the cells containing these nucleibecomes a mature egg; the other three cells usually die.

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Most of your life cycle is spent as a diploidorganism. Only certain cells gametes arehaploid.

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Fertilization When two haploid gametes unite in fertilization,

the resulting cell is diploid .

It received one of each type of chromosome

from the egg of the female parent and one of each type of chromosome from the sperm of the male parent.

These chromosomes form pairs homologous pairs .

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Crossing Over During the first division of meiosis, the two members of each

chromosome pair move together and line up side by side. Proteins hold the two pairs of chromosomes so closely together

that they appear indistinguishable While paired the chromosomesmay exchange segments of DNA.

This exchange of corresponding segments of DNA is called

crossing-over. Crossing-over results in two chromosomes that are no longer

identical. Each chromosome now contains genetic material from each

parent. Exchanging segments of DNA between the members of a pair of

chromosomes results in new combinations of genes. These new combinations of genes act as one source of variation

within a species, necessary for natural selection to occur.

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SECTION REVIEW

What is a gamete?

What is its role in organisms that repro-ducesexually?

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SECTION REVIEW

Why does meiosis ensure that offspring havethe same chromosome number as parents?

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SECTION REVIEW

What is the result of meiosis I?

of meiosis II?

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SECTION REVIEW

What is crossing-over?

What is its influence on evolution?

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References & Acknowledgements

Johnson, George B.: Biology , Visualizing Life,HOLT, RINEHART & WINSTON; Austin,1998. Miller, Kenneth R. & Levine, Joseph: Biology ,

Prentice Hall; Upper Saddle River, New Jersey,2004.

Campbell, Neil A., Williamson, Brad & Heyden,Robin J. BIOLOGY , Exploring Life, Prentice Hall;Needham, Massachusetts, 2004.

Biggs, Alton, Hagins; Whitney Crispen; Kapica,Chris; Lundgren, Linda; Rillero, Peter; Tallman,

Kathleen G.; Zike, Dinah; National GeographicSociety; Biology , The Dynamics of Life, McGrawHill, Glencoe, New York, New York, 2004.

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Photo Credits

Aaron Haupt

Corbis Digital Stock

Elaine Shay

Mark Thayer

PhotoDisc

Alton Biggs