Meiosis & Sexual Reproduction Cell division / Asexual reproduction Mitosis – produce cells with same information identical daughter cells –

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2007-2008 Meiosis & Sexual Reproduction

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Cell division / Asexual reproduction Mitosis – produce cells with same information identical daughter cells – exact copies clones – same amount of DNA same number of chromosomes same genetic information Aaaargh! I ’ m seeing double!

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Asexual reproduction Single-celled eukaryotes – yeast (fungi) – Protists Paramecium Amoeba Simple multicellular eukaryotes – Hydra What are the disadvantages of asexual reproduction? What are the advantages? budding

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How about the rest of us? What if a complex multicellular organism (like us) wants to reproduce? – joining of egg + sperm Do we make egg & sperm by mitosis? 46 + 92 eggspermzygote What if we did, then…. Doesn’t work! No!

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Human female karyotype 46 chromosomes 23 pairs

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Human male karyotype 46 chromosomes 23 pairs

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Homologous chromosomes Paired chromosomes – both chromosomes of a pair carry “matching” genes control same inherited characters homologous = same information diploid 2n 2n = 4 single copied homologous chromosomes double copied homologous chromosomes

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How do we make sperm & eggs? Must reduce 46 chromosomes 23 – must reduce the number of chromosomes by half 23 46 egg sperm 46 meiosis 46 fertilization 23 gametes zygote

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Meiosis: production of gametes Alternating stages – chromosome number must be reduced diploid haploid 2n n – humans: 46 23 meiosis reduces chromosome number makes gametes – fertilization restores chromosome number haploid diploid n 2n haploid diploid

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Sexual reproduction lifecycle 1 copy haploid 1n 2 copies diploid 2n 1 copy haploid 1n meiosisfertilization In the next generation … We ’ re mixing things up here! A good thing? gametes

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Meiosis Reduction Division – special cell division for sexual reproduction – reduce 2n 1n – diploid haploid “two” “half” – makes gametes sperm, eggs Warning: meiosis evolved from mitosis, so stages & “machinery” are similar but the processes are radically different. Do not confuse the two!

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Steps of meiosis Meiosis 1 – interphase – prophase 1 – metaphase 1 – anaphase 1 – telophase 1 Meiosis 2 – prophase 2 – metaphase 2 – anaphase 2 – telophase 2 2nd division of meiosis separates sister chromatids (1n 1n) * just like mitosis * 1st division of meiosis separates homologous pairs (2n 1n) “reduction division”

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2n = 6 double stranded 2n = 6 single stranded Preparing for meiosis 1st step of meiosis – Duplication of DNA – Why bother? meiosis evolved after mitosis convenient to use “machinery” of mitosis DNA replicated in S phase of interphase of MEIOSIS (just like in mitosis) M1 prophase

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2n = 4 single stranded Meiosis 1 2n = 4 double stranded prophase 1 1st division of meiosis separates homologous pairs tetrad synapsis 1n = 2 double stranded telophase 1 2n = 4 double stranded metaphase 1 reduction

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Meiosis 2 1n = 2 double stranded metaphase 2 1n = 2 single stranded telophase 2 prophase 2 1n = 2 double stranded 2nd division of meiosis separates sister chromatids 4

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Interphase Mother cell Stages Of Meiosis: Meiosis I Meiosis II Prophase I: Tetrad formation/ crossing over Prophase I: Tetrad formation/ crossing over Metaphase I Telophase I Prophase I: Condensing Chromosomes Prophase I: Condensing Chromosomes Anaphase I 2n = 4 n = 2

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Telophase I Stages Of Meiosis: Meiosis II Metaphase II Anaphase II Telophase II The products of meiosis are 4 haploid cells each with a unique set of chromosomes. Prophase II n = 2

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Meiosis 1 & 2

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Trading pieces of DNA Crossing over – during Prophase 1, sister chromatids intertwine homologous pairs swap pieces of chromosome – DNA breaks & re-attaches tetrad synapsis prophase 1

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Crossing over 3 steps – cross over – breakage of DNA – re-fusing of DNA New combinations of traits What are the advantages of crossing over in sexual reproduction?

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Mitosis vs. Meiosis

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Mitosis – 1 division – daughter cells genetically identical to parent cell – produces 2 cells – 2n 2n – produces cells for growth & repair – no crossing over Meiosis – 2 divisions – daughter cells genetically different from parent – produces 4 cells – 2n 1n – produces gametes – crossing over

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mitosis zygote Putting it all together… 23 46 egg sperm 46 meiosis 46 23 fertilization development meiosis fertilization mitosis + development 46 gametes

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Sexual reproduction introduces genetic variation – 1.genetic recombination ( 2n, where n is the haploid number) independent assortment of chromosomes – random alignment of homologous chromosomes in Metaphase 1 – 2.crossing over mixing of alleles across homologous chromosomes – 3.random fertilization which sperm fertilizes which egg? Driving evolution – providing variation for natural selection The value of sexual reproduction metaphase1

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Independent Assortment Homologous chromosomes are positioned randomly so any copy can be passed to the gametes with any combination of other chromosomes There are 2 n combinations possible during meiosis with n the haploid number of chromosomes for the organism

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Variation from genetic recombination Independent assortment of chromosomes – meiosis introduces genetic variation – gametes of offspring do not have same combination of genes as gametes from parents random assortment in humans produces 2 23 (8,388,608) different combinations in gametes from Dadfrom Mom offspring new gametes made by offspring

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Variation from crossing over Crossing over creates completely new combinations of traits on each chromosome (recombinant chromosomes) – creates an infinite variety in gametes

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Variation from random fertilization Sperm + Egg = ? – any 2 parents will produce a zygote with over 70 trillion (2 23 x 2 23 ) possible diploid combinations

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Sexual reproduction creates variability Sexual reproduction allows us to maintain both genetic similarity & differences. Baldwin brothers Jonas Brothers Martin & Charlie Sheen, Emilio Estevez

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The Evolutionary Significance of Genetic Variation Within Populations Natural selection results in the accumulation of genetic variations favored by the environment Sexual reproduction contributes to the genetic variation in a population, which originates from mutations Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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Twins

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Monozygotic Twins (Identical) 1/3 of all twins are identical Twins that form from one zygote (one egg fertilized by one sperm). These twins have identical genes and must be the same sex. Having identical twins is random, not genetic

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Dizygotic Twins (fraternal) 2/3 of all twins are fraternal Twins that form from two zygotes (two eggs fertilized by two sperm) Can be the same sex or different sexes. The ability to have fraternal twins is thought to be genetic.

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What are the DISadvantages of sexual reproduction? Any Questions??

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