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Meiosis and genetic variationMeiosis and genetic variation

IB Biology HL E. McIntyre IB Biology HL E. McIntyre

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GenomeGenome

GenomeGenome: Complete complement of an : Complete complement of an organism’s DNA.organism’s DNA.– Includes Includes genesgenes (control traits) and non- (control traits) and non-

coding DNA organized in coding DNA organized in chromosomeschromosomes..

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GenesGenes

Eukaryotic DNA is organized Eukaryotic DNA is organized in chromosomes.in chromosomes.– Genes have specific places on Genes have specific places on

chromosomes. chromosomes.

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HeredityHeredity

HHeredityeredity – – way of way of transferring genetic transferring genetic information to information to offspringoffspring

Chromosome theory of Chromosome theory of heredityheredity: : chromosomes carry chromosomes carry genesgenes..

GeneGene – – “unit of “unit of heredity”.heredity”.

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ReproductionReproduction AsexualAsexual

– Many single-celled organisms reproduce Many single-celled organisms reproduce by splitting, budding, parthenogenesis.by splitting, budding, parthenogenesis.

– Some multicellular organisms can Some multicellular organisms can reproduce asexually, produce reproduce asexually, produce clonesclones ((offspring genetically identical to parentoffspring genetically identical to parent).).

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Sexual reproductionSexual reproduction

Fusion of two gametes to produce a single Fusion of two gametes to produce a single zygote.zygote.

Introduces greater genetic variation, allows Introduces greater genetic variation, allows genetic recombination. genetic recombination.

With exception of self-fertilizing organisms With exception of self-fertilizing organisms (e.g. some plants), zygote has gametes (e.g. some plants), zygote has gametes from two different parents.from two different parents.

77Attack………

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ChromosomesChromosomes

Karyotype: Karyotype: – ordered display of an individual’s chromosomes.ordered display of an individual’s chromosomes.– Collection of chromosomes from mitotic cells.Collection of chromosomes from mitotic cells.– Staining can reveal visible band patterns, gross Staining can reveal visible band patterns, gross

anomalies.anomalies.

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KaryotypingKaryotyping

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http://gslc.genetics.utah.edu/units/disorders/karyotype/karyotype.cfmhttp://gslc.genetics.utah.edu/units/disorders/karyotype/karyotype.cfm

Karyotype ActivityKaryotype Activity

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HomologuesHomologues Chromosomes exist in homologous Chromosomes exist in homologous

pairs in diploid cells.pairs in diploid cells.

Exception: Sex chromosomes (X, Y).

Other chromosomes are known as autosomes, they have homologues.

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In humans …In humans … 23 chromosomes donated by each parent 23 chromosomes donated by each parent

(total = 46 or 23 pairs).(total = 46 or 23 pairs). Gametes (sperm/ova):Gametes (sperm/ova):

– Contain 22 autosomes and 1 sex chromosome.Contain 22 autosomes and 1 sex chromosome.– Are haploid (haploid numberAre haploid (haploid number “n” = 23 in humans “n” = 23 in humans).).

Fertilization/syngamy results in zygote with 2 Fertilization/syngamy results in zygote with 2 haploid sets of chromosomes - now haploid sets of chromosomes - now diploiddiploid..– Diploid cell; Diploid cell; 2n = 462n = 46. (. (n=23 in humansn=23 in humans))

Most cells in the body produced by Most cells in the body produced by mitosismitosis.. Only gametes are produced by Only gametes are produced by meiosis.meiosis.

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Chromosome numbersChromosome numbersAll are even numbers – diploid (2n) sets of homologous chromosomes!

Ploidy = number of copies of each chromosome. Diploidy

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Meiosis – key differences from mitosisMeiosis – key differences from mitosis Meiosis Meiosis reduces the number of chromosomes by halfreduces the number of chromosomes by half.. Daughter cellsDaughter cells differ from parentdiffer from parent, and each other., and each other. Meiosis involves Meiosis involves two divisionstwo divisions, Mitosis only one., Mitosis only one. Meiosis I involves:Meiosis I involves:

– SynapsisSynapsis – homologous chromosomes pair up. – homologous chromosomes pair up. ChiasmataChiasmata form ( form (crossing overcrossing over of non-sister of non-sister chromatids).chromatids).

– In Metaphase I, In Metaphase I, homologous pairs line uphomologous pairs line up at at metaphase plate.metaphase plate.

– In Anaphase I, In Anaphase I, sister chromatids do NOT separatesister chromatids do NOT separate..– Overall, Overall, separation of homologous pairs of separation of homologous pairs of

chromosomeschromosomes, rather than sister chromatids of , rather than sister chromatids of individual chromosome.individual chromosome.

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AnimationAnimation

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Meiosis 1Meiosis 1

First division of meiosisFirst division of meiosis Prophase 1: Prophase 1: Each chromosome dupicates Each chromosome dupicates

and remains closely associated. These are and remains closely associated. These are called sister chromatids. called sister chromatids. Crossing-overCrossing-over can can occur during the latter part of this stage. occur during the latter part of this stage.

Metaphase 1Metaphase 1: Homologous chromosomes : Homologous chromosomes align at the equatorial plate. align at the equatorial plate.

Anaphase 1Anaphase 1: Homologous pairs separate : Homologous pairs separate with sister chromatids remaining together. with sister chromatids remaining together.

Telophase 1Telophase 1: Two daughter cells are formed : Two daughter cells are formed with each daughter containing only one with each daughter containing only one chromosome of the homologous pair.chromosome of the homologous pair.

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Meiosis IIMeiosis IISecond division of meiosis: Second division of meiosis: Gamete formationGamete formation Prophase 2Prophase 2: DNA does not replicate. : DNA does not replicate. Metaphase 2Metaphase 2: Chromosomes align at : Chromosomes align at

the equatorial plate. the equatorial plate. Anaphase 2Anaphase 2: Centromeres divide and : Centromeres divide and

sister chromatids migrate separately to sister chromatids migrate separately to each pole. each pole.

Telophase 2Telophase 2: Cell division is complete. : Cell division is complete. Four haploid daughter cells are Four haploid daughter cells are obtained.obtained.

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Mitosis vs. MeiosisMitosis vs. Meiosis

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Meiosis creates genetic variationMeiosis creates genetic variation During normal cell growth, During normal cell growth, mitosismitosis produces produces

daughter cells identical to parent celldaughter cells identical to parent cell (2n (2n to 2n) to 2n)

MeiosisMeiosis results in results in genetic variationgenetic variation by by shufflingshuffling of maternal and paternal of maternal and paternal chromosomes and chromosomes and crossing overcrossing over..

No daughter cells formed during No daughter cells formed during meiosis are genetically identical to either meiosis are genetically identical to either mother or father mother or father

During sexual reproductionDuring sexual reproduction,, fusion of fusion of thethe unique haploid gametes produces truly unique haploid gametes produces truly unique offspringunique offspring..

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Ever wonder why siblings don’t Ever wonder why siblings don’t look alike even though they come look alike even though they come

from the same DNA?from the same DNA?

Wait no more…..Wait no more…..

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Independent assortmentIndependent assortment

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Independent assortmentIndependent assortment

Number of combinations: 2Number of combinations: 2nn

e.g. 2 chromosomes in haploid2n = 4; n = 22n = 22 = 4 possible combinations

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In humansIn humans

e.g. 23 chromosomes in haploid2n = 46; n = 232n = 223 = ~ 8 million possible combinations!

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Crossing overCrossing overChiasmata – sites of crossing over, occur in synapsis. Exchange of genetic material between non-sister chromatids.

Crossing over produces recombinant chromosomes.

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Crossing Over AnimationCrossing Over Animation

http://www.biostudio.com/d_%20Meselson%20Radding%20Model%20Crossing%20Over.htmhttp://www.biostudio.com/d_%20Meselson%20Radding%20Model%20Crossing%20Over.htm

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Harlequin chromosomesHarlequin chromosomes

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Random fertilizationRandom fertilizationAt least 8 million combinations from Mom, and At least 8 million combinations from Mom, and

another 8 million from Dad …another 8 million from Dad …

>64 trillion combinations for a diploid zygote!!! >64 trillion combinations for a diploid zygote!!! Ykes……that’s a lot of sex…ual reproductionYkes……that’s a lot of sex…ual reproduction

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Meiosis & sexual life cyclesMeiosis & sexual life cycles Life cycleLife cycle = sequence = sequence

of stages in organisms of stages in organisms reproductive history; reproductive history; conception to conception to reproductionreproduction..

SomaticSomatic cells = any cells = any cell other than cell other than gametesgametes, most of the , most of the cells in the body.cells in the body.

GametesGametes produced by produced by meiosis.meiosis.

Generalized animal life cycle

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Sex is costly!Sex is costly! Large amounts of energy required to find a Large amounts of energy required to find a

mate and do the mating: specialized mate and do the mating: specialized structures and behavior required structures and behavior required

Intimate contact provides route for infection Intimate contact provides route for infection by parasites (AIDS, syphillis, etc.) by parasites (AIDS, syphillis, etc.)

Genetic costs: in sex, we pass on only half of Genetic costs: in sex, we pass on only half of genes to offspring. genes to offspring.

Males are an expensive luxuryMales are an expensive luxury - in most - in most species they contribute little to rearing species they contribute little to rearing offspringoffspring..

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But …But … More genetic diversity: more potential for survival More genetic diversity: more potential for survival

of species when environmental conditions of species when environmental conditions changechange.. – Shuffling of genes in meiosis Shuffling of genes in meiosis – Crossing-over in meiosis Crossing-over in meiosis – Fertilization: combines genes from 2 separate Fertilization: combines genes from 2 separate

individuals individuals DNA back-up and repairDNA back-up and repair..

– Asexual organisms don't have back-up copies of genes, Asexual organisms don't have back-up copies of genes, sexual organisms have two sets of chromosomes and sexual organisms have two sets of chromosomes and one can act as a back-up if the other is damaged. one can act as a back-up if the other is damaged.

– Sexual mechanisms, especially recombination, are Sexual mechanisms, especially recombination, are used to repair damaged DNA - the undamaged used to repair damaged DNA - the undamaged chromosome acts as a template and eventually both chromosome acts as a template and eventually both chromosomes end up with the correct gene. chromosomes end up with the correct gene.

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Important LinkImportant Link

Use the link & watch the animations carefullyUse the link & watch the animations carefully

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter12/animations.htmlhttp://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter12/animations.html

Remember, YOU contribute only half the genetic material to your offspring. So when you are choosing a mate for reproduction remember Indiana Jones & the Temple of Doom.

-” Choose, but choose wisely.”