2Cell division / Asexual reproduction Mitosisproduce cells with same informationidentical daughter cellsexact copiesclonessame amount of DNAsame number of chromosomessame genetic informationAaaargh! I’m seeing double!
3Asexual reproduction Single-celled eukaryotes yeast (fungi)ProtistsParameciumAmoebaSimple multicellular eukaryotesHydrabuddingbuddingWhat are the disadvantages of asexual reproduction?What are the advantages?
4+ 46 46 92 How about the rest of us? What if a complex multicellular organism (like us) wants to reproduce?joining of egg + spermDo we make egg & sperm by mitosis?No!What if we did, then….46+4692eggspermzygoteDoesn’t work!
7Homologous chromosomes Paired chromosomesboth chromosomes of a pair carry “matching” genescontrol same inherited charactershomologous = same informationsingle stranded homologous chromosomesdiploid 2n2n = 4double stranded homologous chromosomes
8How do we make sperm & eggs? Must reduce 46 chromosomes 23must reduce the number of chromosomes by half234623zygote46egg23meiosis4623fertilizationspermgametes
9Meiosis: production of gametes Alternating stageschromosome number must be reduceddiploid haploid2n nhumans: 46 23meiosis reduces chromosome numbermakes gametesfertilization restores chromosome numberhaploid diploidn 2nhaploiddiploid
10Sexual reproduction lifecycle 2 copiesdiploid2n1 copyhaploid1n1 copyhaploid1nfertilizationmeiosisIn the next generation… We’re mixingthings up here!A good thing?gametesgametes
11Meiosis Reduction Division special cell division for sexual reproductionreduce 2n 1ndiploid haploid“two” “half”makes gametessperm, eggsWarning: meiosis evolved from mitosis, so stages & “machinery” are similar but the processes are radically different. Do not confuse the two!
13Double division of meiosis DNA replicationRepeat after me!I can’t hear you!Meiosis 11st division of meiosis separates homologous pairsMeiosis 22nd division of meiosis separates sister chromatids
14Preparing for meiosis 1st step of meiosis Duplication of DNA meiosis evolved after mitosisconvenient to use “machinery” of mitosisDNA replicated in S phase of interphase of MEIOSIS (just like in mitosis)2n = 6singlestranded2n = 6doublestrandedM1 prophase
19Trading pieces of DNA Crossing over during Prophase 1, sister chromatids intertwinehomologous pairs swap pieces of chromosomeDNA breaks & re-attachesprophase 1synapsistetrad
20What are the advantages of crossing over in sexual reproduction? 3 stepscross overbreakage of DNAre-fusing of DNANew combinations of traitsSexual reproduction is advantageous to species that benefit from genetic variability. However, since evolution occurs because of changes in an individual's DNA, crossing over and chromosome segregation is likely to result in progeny that are less well-adapted than their parents. On the other hand, asexual reproduction ensures the production of progeny as fit as the parent since they are identical to the parent. Remember the adage, “if it's not broken, don't fix it.” There are several hypotheses regarding the evolution of sexual reproduction. One is associated with repairing double-stranded DNA breaks induced by radiation or chemicals. The contagion hypothesis suggests that sex arose from infection by mobile genetic elements. The Red Queen hypothesis theorizes that sex is needed to store certain recessive alleles in case they are needed in the future. Along similar lines, eukaryotic cells build up large numbers of harmful mutations. Sex, as explained by Miller's rachet hypothesis, may simply be a way to reduce these mutations. The “whole truth” is likely a combination of these factors. Regardless of how and why, the great diversity of vertebrates and higher plants and their ability to adapt to the highly varied habitats is indeed a result of their sexual reproduction.
22Mitosis vs. Meiosis Mitosis Meiosis 1 division daughter cells genetically identical to parent cellproduces 2 cells2n 2nproduces cells for growth & repairno crossing overMeiosis2 divisionsdaughter cells genetically different from parentproduces 4 cells2n 1nproduces gametescrossing over
23Putting it all together… meiosis fertilization mitosis + developmentgametes46234623464646464623meiosis4646egg464623zygotefertilizationmitosisspermdevelopment
24The value of sexual reproduction Sexual reproduction introduces genetic variationgenetic recombinationindependent assortment of chromosomesrandom alignment of homologous chromosomes in Metaphase 1crossing overmixing of alleles across homologous chromosomesrandom fertilizationwhich sperm fertilizes which egg?Driving evolutionproviding variation for natural selectionmetaphase1
25Variation from genetic recombination Independent assortment of chromosomesmeiosis introduces genetic variationgametes of offspring do not have same combination of genes as gametes from parentsrandom assortment in humans produces 223 (8,388,608) different combinations in gametesoffspringfrom Momfrom Dadnew gametesmade by offspring
26Variation from crossing over Crossing over creates completely new combinations of traits on each chromosomecreates an infinite variety in gametes
27Variation from random fertilization Sperm + Egg = ?any 2 parents will produce a zygote with over 70 trillion (223 x 223) possible diploid combinations
28Sexual reproduction creates variability Sexual reproduction allows us to maintain both genetic similarity & differences.Jonas BrothersBaldwin brothersMartin & Charlie Sheen, Emilio Estevez
29Sperm production Spermatogenesis continuous & prolific process EpididymisTestisgerm cell(diploid)Coiledseminiferoustubulesprimaryspermatocyte(diploid)MEIOSIS Isecondaryspermatocytes(haploid)MEIOSIS IIVas deferensspermatids(haploid)spermatozoaSpermatogenesiscontinuous & prolific processeach ejaculation = million spermCross-section ofseminiferous tubule
30Egg production Oogenesis eggs in ovaries halted before Anaphase 1 Meiosis 1 completed during maturationMeiosis 2 completed after fertilization1 egg + 2 polar bodiesunequal divisionsMeiosis 1 completedduring egg maturationovulationWhat is the advantage of this development system?Meiosis 2 completedtriggered by fertilization
31Putting all your egg in one basket! OogenesisMEIOSIS IMEIOSIS IIfirst polar bodysecondpolar bodyovum(haploid)secondaryoocyteprimary(diploid)germinal cellprimary folliclesmature follicle withsecondary oocyteruptured follicle (ovulation)corpus luteumdevelopingfolliclefertilizationfallopian tubeafter fertilization
32Differences across kingdoms Not all organisms use haploid & diploid stages in same waywhich one is dominant (2n or n) differsbut still alternate between haploid & diploidmust for sexual reproduction