Meiosis Imkita

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Autosomes

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· 22 different pairs of chromosomes

· there are two of each that are identical to each other

· one member of each pair is of maternal and paternal origin

· humans have 44 somatic cells (44 + XX in females and 44 + XY in males)

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Gonosomes

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· a pair of sex chromosomes

· in females, the gonosomes are referred to as XX chromosomes· in males, the gonodomes are referred to as XY chromosomes

·

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Somatic cells

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· diploid (2n)

· 46 chromosomes

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Gamete

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· haploid (n)

· 23 chromosomes

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Meiosis

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The division of one cell into four where all cells have half the number of chromosomes as the parent cell and all four cells are genetically different from each other

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When does meiosis occur?

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During gametogenesis, where gametes are formed

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Why meiosis is important

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The halving effect of of meiosis overcomes the doubling effect of fertilization and thus maintaining a constant number of chromosomes each generation

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Interphase

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· DNA replocates so that the genetic material is doubled in the chromatin network in preparation for Meiosis I

· single-stranded chromosomes become double-stranded

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Prophase I

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· nuclear membrane begins to disappear

· chromatin network unwinds to form distinct chromosomes

· each chromosome is made of identical chromatids

· chromosomes come together in homologous pairs

· crossing-over takes place between chromatids of these homologous pairs

· the centrosome splits into two centrioles which move to opposite poles

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Metaphase I

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· spindle threads form between the centrioles

· chromosomes arrange along the equator in homologous pairs

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Anaphase I

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· spindle threads contract

· two chromosomes (each with two chromatids) of each homologous pair are pulled to opposite poles

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Telophase I

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· two groups of chromosomes appear, forming a nucleus at each pole

· the cytoplasm divides to form two new cells

· the two new cells have half the number of chromosomes in each and are also genetically different (because of crossing-over)

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Prophase II

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· in both cells, each chromosome is visible as two chromatids

· centrosomes splits into centrioles which move to opposite poles

· the nuclear membrane begins to disappear

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Metaphase II

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· chromosomes become arranged along the equator singly attached to spindle threads

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Anaphase II

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· the chromosomes split

· the two chromatids making up each chromosome are pulled to opposite poles when the threads contract

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Telophase II

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· two groups of chromosomes occur - one at each end of the cell forming two new nuclei

· the cytoplasm of each two cells divides to form four new cells (two formed from each cell)

· the nucleus in the four cells has half the number of chromosomes (23 chromosomes)

· the four cells are genetically different because of crossing-over

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The process of crossing-over

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· happens in Prophase I

· homologous chromosomes lie close together and become involved in crossing-over (they are referred to as bivalent when this happens

· one chromatid of each chromosome overlaps with a chromatid of its homologous pair

· the point where they cross over is called the chiasma

· when they seperate, each chromosome has one chromatid with genetic material from its homologous partner

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The result of crossing-over

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· gametes produced will be different to each other and therefore results in variation in offspring

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The difference between Meiosis I and Meiosis II

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Why are cells produced by meiosis different?

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· crossing-over during Prophase I results in exchange of genetic material

· chromosomes are arranged randomly along the equator in Metaphase I

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Why is meiosis important?

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· leads to the formation of haploid gametes/spores

· the halving effect of meiosis neutralizes the doubling effect of fertilization

· crossing-over introduces genetic variation for better survival

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The difference between mitosis and meiosis

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Meiosis in the life cycle of humans and most plants

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· in most plants and animals (Kingdom Plantae and Animalia), meiosis takes place when gametes form

· these gametes have a haploid number of chromosomes compared to an adult

· the fusion of gametes (during fertilization) result in a diploid amount of chromosomes being restored in the zygote which then goes through mitosis to produce a multicellular adult

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Meiosis in the life cycle of algae and fungi

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· in some algae (Kingdom Protista), e.g. Spirogyra, and some fungi, e.g. Rhizopus, the adults are haploid and give rise to haploid gametes which result in diploid gametes after fertilization

· meiosis occurs in the zygote/zygospore to halve the chromosome number so that new offspring are haploid

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Meiosis in the life cycle of moss and fern

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· mosses and ferns have an alternation of generations (haploid gametophyte and diploid sporophyte)

· haploid gametophytes produce haploid gametes during meisosis

· diploid sporophytes produce diploid spores during meiosis

· the halving of meiosis allows for this alternation of generations