Meiotic Cell Division

• Sexual reproduction is the most common way for eukaryotic organisms to produce offspring

• Parents make gametes with half the amount of genetic material (haploid)

• These gametes fuse with each other during fertilization to generate a new organism

Sexual Reproduction

• Simple eukaryotes are isogamous– They produce gametes that are morphologically similar

• Most eukaryotic species are heterogamous– These produce gametes that are morphologically

different• Sperm cells

– Relatively small and mobile

• Oocytes or ova– Usually large and nonmobile

– Store large amounts of nutrients

• Microspores (Pollen)• Macrospores (Ovules)

Gametes

How Does One Make a Haploid Gamete?• Answer – meiosis

• Haploid cells are produced from diploid cells during gametogenesis

• The chromosomes must be distributed to reduce the chromosome number to half its original value

• but simultaneously sorted to assure that each chromosome (& its genes) is represented in each gamete

Mitosis vs Meiosis

• Produces two diploid daughter cells

• Produces daughter cells that ARE genetically identical

• Produce four haploid daughter cells

• Produces daughter cells that are NOT genetically identical

• Meiosis begins after a cell has progressed through G1, S, & G2

• Meiosis involves two successive divisions– Meiosis I and II

– Each of these is subdivided into• Prophase• Metaphase• Anaphase• Telophase

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Meiosis

Meiosis

• Prophase I is further subdivided into periods known as – Leptotena– Zygotena– Pachytena– Diplotena– Diakinesis

A recognition process

A total of 4 chromatids

Figure 3.11

Periods of Prophase I

Synaptonemal Complex

A physical exchange of chromosome pieces

A tetrad 2 bivalents

Periods of Prophase I

Spindle apparatus complete;pairs of chromatids attached to kinetochore microtubules

Stages of Meiosis I

• Bivalents are organized along the metaphase plate

• Homologous pairs of sister chromatids aligned side by side – A pair of sister chromatids is

linked to one of the poles– And the homologous pair is

linked to the opposite pole– The arrangement is random

with regards to the (blue and red) homologues

Figure 3.13

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Pairs of sister chromatids separate from each otherThe centromere remains between sister chromatids

Stages of Meiosis I

Meiosis • Telophase I & cytokinesis of meiosis I is followed meiosis II

• Meiosis I has reduced the number of chromosomes in the daughter cells to the ½ the diploid number

• However, each homolog is still composed of 2 recombinant sister chromatids – The genetic content is still 2n

• Meiosis II reduces the genetic content to n

Stages of Meiosis II

1 of each type of chromosome (n) in each daughter cell (gamete)

Separation of Alleles During Meiosis

Prophase I

Metaphase I

Anaphase ITelophase I

Meiosis II

Haploid cells

Heterozygous (Yy) cell froma plant with yellow seeds

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Meiosis II

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Heterozygous diploidcell (YyRr) toundergo meiosis

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Separation of Alleles During Meiosis