Sexual Reproductionand Meiosis

Chapter 11

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Ramifications of meiosis

• MOST animals and plants reproduce sexually

• plays a key role in generating the tremendous genetic diversity of evolution

• Genetic contributions from 2 cells instead of 1

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History of meiosis

• 1876- Walther Fleming discovers chromosomes

• 1879- Cytologist Edouard van Beneden discovers different # of chromosomes in different cell in roundworm

• 1883-proposes idea that gametes contain ½ # of chromosomes

• These join to form a zygote

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Overview of Meiosis

Meiosis is a form of cell division that leads to the production of gametes.

gametes: egg cells and sperm cells-contain half the number of chromosomes of

an adult body cellAdult body cells (somatic cells) are diploid,

containing 2 sets of chromosomes.Gametes are haploid, containing only 1 set

of chromosomes.

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Overview of Meiosis

Sexual reproduction includes the fusion of gametes (fertilization or syngamy) to produce a diploid zygote.

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.

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GERM-LINE CELLS

• Cells that will eventually be gametes are set aside early in embryonic development

• Gamete producing germ-line cells are

• Diploid

• Undergo meiosis to produce haploid gametes

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Features of Meiosis

Meiosis includes two rounds of division – meiosis I and meiosis II.

During meiosis I, homologous chromosomes (homologues) become closely associated with each other. This is synapsis.

Proteins between the homologues hold them in a synaptonemal complex. Sometimes called a tetrad.

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Features of Meiosis

Crossing over: genetic recombination between non-sister chromatids

-physical exchange of regions of the chromatids

chiasmata: sites of crossing over

The homologues are separated from each other in anaphase I.

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Features of Meiosis

Meiosis involves two successive cell divisions with no replication of genetic material in the 2nd, just a separation of the sister chromatids for each homologue.

This results in a reduction of the chromosome number from diploid to haploid. The first division is called “reduction division”.

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The Process of Meiosis

Prophase I:

-chromosomes coil tighter

-nuclear envelope dissolves

-homologues become closely associated in synapsis

-crossing over occurs between non-sister chromatids (different from mitosis)

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The Process of Meiosis

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

-the orientation of each pair of homologues on the spindle is random

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The Process of Meiosis

Anaphase I:

-microtubules of the spindle shorten

-homologues are separated from each other

-sister chromatids remain attached to each other at their centromeres

Genes on different chromosomes assort differently (independent assortment)

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The Process of Meiosis

Telophase I:

-nuclear envelopes form around each set of chromosomes

-each new nucleus is now haploid

-sister chromatids are no longer identical because of crossing over

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The Process of Meiosis

Meiosis II resembles a mitotic division:-prophase II: nuclear envelopes dissolve

and 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;

cytokinesis follows

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Final result

• 4 different haploid cells

• in animals become gametes

• Plants, fungi, protists increased number of gametes

• Varying # of chromosome sets found in some insects and plants – this results in polyploidy- (1 or more entire sets of chromosomes added to a diploid gamete)

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ERRORS IN MEIOSIS

• Nondisjunction-

• Aneuploid gametes-

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Meiosis vs. Mitosis

Meiosis 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.

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Meiosis vs. Mitosis

Meiosis produces haploid cells that are not identical to each other.

Genetic differences in these cells arise from:

-crossing over

-random alignment of homologues in metaphase I (independent assortment)

Mitosis produces 2 cells identical to each other.

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