10.1Meiosis(AHL)

Essen1alidea:Meiosisleadstoindependentassortmentofchromosomesanduniquecomposi1onofallelesindaughtercells.

Thefamilyportraitshowslargeamountsofvaria1onwithinafamilydespitesharingalotofgenes.Thisshowsthepoten1alofcrossingoverandindependentassortmenttocreatenearinfinitevaria1oningametesandhenceinoffspringtoo.

ByChrisPaine

hIps://bioknowledgy.weebly.com/hIps://upload.wikimedia.org/wikipedia/commons/9/95/Coloured-family.jpg

Understandings,Applica1onsandSkillsStatement Guidance

10.1.U1 Chromosomesreplicateininterphasebeforemeiosis.10.1.U2 CrossingoveristheexchangeofDNAmaterialbetween

non-sisterhomologouschroma1ds.10.1.U3 Crossingoverproducesnewcombina1onsofalleleson

thechromosomesofthehaploidcells.10.1.U4 Chiasmataforma1onbetweennon-sisterchroma1ds

canresultinanexchangeofalleles.10.1.U5 HomologouschromosomesseparateinmeiosisI.10.1.U6 Sisterchroma1dsseparateinmeiosisII.10.1.U7 Independentassortmentofgenesisduetotherandom

orienta1onofpairsofhomologouschromosomesinmeiosisI.

10.1.S1 Drawingdiagramstoshowchiasmataformedbycrossingover.

Diagramsofchiasmatashouldshowsisterchroma1dss1llcloselyaligned,exceptatthepointwherecrossingoveroccurredandachiasmawasformed.

Review:3.3.U1Onediploidnucleusdividesbymeiosistoproducefourhaploidnuclei.

Editedfrom:hIps://commons.wikimedia.org/wiki/File:Diagram_of_meiosis.svg

Meiosisisareduc/ondivisionofthenucleustoformhaploidgametes

Onediploid(2N)bodycellscontainahomologouspairofeachchromosome(exceptforsexcells)

Fourhaploid(N)gametescontainoneofeachchromosome

Seconddivisionofthenucleus

Firstdivisionofthenucleus

Chromosomesarereplicatedtoformsisterchroma1ds

Review:3.3.U1Onediploidnucleusdividesbymeiosistoproducefourhaploidnuclei.

hIp://www.sumanasinc.com/webcontent/anima1ons/content/meiosis.html

hIp://highered.mheduca1on.com/sites/dl/free/0072437316/120074/bio19.swf

hIp://www.stolaf.edu/people/giannini/flashanimat/celldivision/meiosis.swf

hIp://www.biostudio.com/d_%20Meiosis.htm

Theanima1onsareagreatwaytovisualisetheprocess–watchandtakenotes.

Meosisisareduc/ondivisionofthenucleustoformhaploidgametes

Review:3.3.U3DNAisreplicatedbeforemeiosissothatallchromosomesconsistoftwosisterchroma1ds.

InterphaseIntheS-phaseoftheinterphasebeforemeiosisbegins,DNAreplica1ontakesplace.ChromosomesarereplicatedandthesecopiesareaIachedtoeachotheratthecentromere.TheaIachedchromosomeanditscopyareknownassisterchroma1ds.FollowingS-phase,furthergrowthandprepara1ontakeplaceformeiosis.

10.1.U1Chromosomesreplicateininterphasebeforemeiosis.Anhomologouspairofchromosomes…

Anhomologouspairofchromosomes……replicatesduringS-phaseofinterphase…

10.1.U1Chromosomesreplicateininterphasebeforemeiosis.

Anhomologouspairofchromosomes……replicatesduringS-phaseofinterphase…

…givingtwopairsofsisterchroma/ds,eachjoinedatthecentromere.

centromere

sisterchroma1ds

10.1.U1Chromosomesreplicateininterphasebeforemeiosis.

Review:3.3.S1Drawingdiagramstoshowthestagesofmeiosisresul1ngintheforma1onoffourhaploidcells.AND3.3.U4Theearlystagesofmeiosisinvolvepairingofhomologouschromosomesandcrossingoverfollowedbycondensa1on.

ProphaseI

hIp://www2.sunysuffolk.edu/gambier/micrographs/lateprophase3.htm

Crossing-overbetweennon-sisterchroma1dscantakeplace.Thisresultsinrecombina/onofallelesandisasourceofgene1cvaria1oningametes.

DNAsupercoilsandchromosomescondense

nuclearmembranedissolves

centriolesmigratetothepolesofthecell.

Thehomologouschromosomesassociatewitheachothertoformbivalents(synapsis).

Editedfrom:hIp://www.slideshare.net/gurus1p/meiosis-ahl

10.1.U2CrossingoveristheexchangeofDNAmaterialbetweennon-sisterhomologouschroma1ds.AND10.1.U4Chiasmataforma1onbetweennon-sisterchroma1dscanresultinanexchangeofalleles.

ThehomologouspairassociatesduringprophaseI,throughsynapsis…

…makingabivalent(ortetrad).

10.1.U2CrossingoveristheexchangeofDNAmaterialbetweennon-sisterhomologouschroma1ds.AND10.1.U4Chiasmataforma1onbetweennon-sisterchroma1dscanresultinanexchangeofalleles.

Crossing-overmighttakeplacebetweennon-sisterchroma/dsinprophaseI…

…leadingtorecombina/onofalleles.

Theloca1onofcrossing-overbetweennon-sisterchroma/dsiscalledthechiasma(pl.chiasmata).

10.1.U2CrossingoveristheexchangeofDNAmaterialbetweennon-sisterhomologouschroma1ds.AND10.1.U4Chiasmataforma1onbetweennon-sisterchroma1dscanresultinanexchangeofalleles.

hIp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Chiasmata013.jpg

Theforma/onofchiasmaiscommonandisthoughttobeessen/alformeiosis

Howmanychiasmacanyouiden1fyinthemicrographs?

b.

Inhumanstheaveragenumberofchiasmataperbivalent(tetrad)isjustovertwo.

hIp://nzetc.victoria.ac.nz/etexts/Bio13Tuat01/Bio13Tuat01_063a(h280).jpg

a.

Chiasmaoccurbetweenthesisterchroma1dsandthereforeshouldbeappearthecentralspace.

10.1.U2CrossingoveristheexchangeofDNAmaterialbetweennon-sisterhomologouschroma1ds.AND10.1.U4Chiasmataforma1onbetweennon-sisterchroma1dscanresultinanexchangeofalleles.

hIp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Chiasmata013.jpg

Theforma/onofchiasmaiscommonandisthoughttobeessen/alformeiosis

Howmanychiasmacanyouiden1fyinthemicrographs?

b.

Inhumanstheaveragenumberofchiasmataperbivalent(tetrad)isjustovertwo.

hIp://nzetc.victoria.ac.nz/etexts/Bio13Tuat01/Bio13Tuat01_063a(h280).jpg

a.

1

2 1

2 3 4

5centromere

Chiasmaoccurbetweenthesisterchroma1dsandthereforeshouldbeappearthethecentralspace.

10.1.U3Crossingoverproducesnewcombina1onsofallelesonthechromosomesofhaploidcells.Crossing-over Increasesgene;cvaria;onthrough

recombina;onoflinkedalleles.

SynapsisHomologouschromosomesassociate

ChiasmaForma/onNeighboringnon-sisterchroma1ds

arecutatthesamepoint.AHollidayjunc1onformsastheDNA

ofthecutsec1onsaIachtotheopenendoftheoppositenon-sister

chroma1d.

Recombina/onAsaresult,allelesare

swappedbetweennon-sisterchroma1ds.

10.1.U3Crossingoverproducesnewcombina1onsofallelesonthechromosomesofthehaploidcells.

Crossing-Over Increasesgene;cvaria;onthroughrecombina;onoflinkedalleles.

Crossingoverleadstomorevaria/oningametes.Thisisthestandardnota1onforwri1nggenotypesofallelesonlinkedgenes(moreofthislaterwhenwestudy10.2InheritanceAHL)

Editedfrom:hIp://www.slideshare.net/gurus1p/meiosis-ahl

10.1.S1Drawingdiagramstoshowchiasmataformedbycrossingover.

Keypointswhendrawingchiasmata:•  Usecolourofshadingtoindicatetheeachofthesisterchroma1ds•  Rememberthehomologouschromosomesareinsynapsis–beforeand

ahercrossingoverthechromosomesshouldbeshownclosetogether.

centromere

chiasmaRecombinedchroma1ds

Un-recombinedchroma1ds

Sisterchroma1ds

Homologouspair

Crossing-Over Drawingcleardiagramshelpswhencommunica;ngwithothers

Review:3.3.S1Drawingdiagramstoshowthestagesofmeiosisresul1ngintheforma1onoffourhaploidcells.AND3.3.U5Orienta1onofpairsofhomologouschromosomespriortosepara1onisrandom.

courtesyof:hIp://www.flickr.com/carolinabio

MetaphaseI

Randomorienta/onoccurs-eachbivalentalignsindependentlyandhencethedaughternucleigetadifferentmixofchromosomes.Thisisasignificantsourceofgene1cvaria1on:thereare2npossibleorienta1onsinmetaphaseIandII.Thatis223inhumans–or8,388,068differentcombina1onsingametes!

Thebivalentslineupattheequator.

Editedfrom:hIp://www.slideshare.net/gurus1p/meiosis-ahl

10.1.U7Independentassortmentofgenesisduetotherandomorienta1onofpairsofhomologouschromosomesinmeiosisI.MetaphaseI

Thedirec1oninwhichonebivalentalignsdoesnotaffectthealignmentofotherbivalents.

Alleleshavea50percentchanceofmovingtoapar1cularpole.

Thereforedifferentallelecombina1onsshouldalwaysbeequallypossible(ifthegenelociareondifferentchromosomes–thisdoesnotholdforlinkedgenes)

Editedfrom:hIp://www.slideshare.net/gurus1p/meiosis-ahl

10.1.U5HomologouschromosomesseparateinmeiosisI.InanaphaseI,thehomologouspairisseparatedbutthesisterchroma/dsremainaRached.

Thisisthereduc/ondivision.

Review:3.3.S1Drawingdiagramstoshowthestagesofmeiosisresul1ngintheforma1onoffourhaploidcells.

courtesyof:hIp://www.flickr.com/carolinabio

MetaphaseII

Editedfrom:hIp://www.slideshare.net/gurus1p/meiosis-ahl

SpindlefibresformandaIachatthecentromeres.

Pairsofsisterchroma1dsalignattheequator.

10.1.U6Sisterchroma1dsseparateinmeiosisII.MetaphaseIIPairsofsisterchroma1dsalignattheequator.SpindlefibresformandaIachatthecentromeres.

Randomorienta1onagaincontributestovaria1oninthegametes,thoughnottosuchanextentasinmetaphaseI.Thisisbecausethereisonlyadifferencebetweenchroma1dswherecrossing-overhastakenplace.

Review:3.3.S1Drawingdiagramstoshowthestagesofmeiosisresul1ngintheforma1onoffourhaploidcells.

courtesyof:hIp://www.flickr.com/carolinabio

AnaphaseII

Editedfrom:hIp://www.slideshare.net/gurus1p/meiosis-ahl

Spindlefibrescontractandthecentromeresaresplit.

Thesisterchroma1dsareseparated.Thechroma/ds(nowcalledchromosomesarepulledtoopposingpoles.

10.1.U6Sisterchroma1dsseparateinmeiosisII.

TelophaseII

Newhaploidnucleiareformed.Cytokinesisbegins,splijngthecells.Theendresultofmeiosisisfourhaploidgametecells.Fer1liza1onofthesehaploidgameteswillproduceadiploidzygote.

n.b.duetocrossingovereachofthefournewnucleiislikelytobegene;callydifferent.

Checkyourlanguage.Thisimageshows…

A.  Fourseparatechromosomes.

B.  Abivalent.

C.  Onepairofsisterchroma1ds.

D. Non-disjunc1on.

Checkyourlanguage.Thisimageshows…

A.  Fourseparatechromosomes.

B.  Abivalent.

C.  Onepairofsisterchroma1ds.

D. Non-disjunc1on.

Checkyourlanguage.Thisimageshows…

A.  Twoseparatechromosomes.

B.  Abivalent.

C.  Onepairofsisterchroma1ds.

D. Crossing-over.

Checkyourlanguage.Thisimageshows…

A.  Twoseparatechromosomes.

B.  Abivalent.

C.  Onepairofsisterchroma1ds.

D. Crossing-over.

Checkyourlanguage.Thisimageshows…

A.  Twoseparatechromosomes.

B.  Abivalent.

C.  Onepairofsisterchroma1ds.

D. Homologouschromosomes.

Checkyourlanguage.Thisimageshows…

A.  Twoseparatechromosomes.

B.  Abivalent.

C.  Onepairofsisterchroma1ds.

D. Homologouschromosomes.

30

Checkyourlanguage.Thisimageshows…

A.  8separatechromosomes.

B.  Twobivalents.

C.  Twopairsofsisterchroma1ds.

D. Twohomologouschromosomes.

Checkyourlanguage.Thisimageshows…

A.  8separatechromosomes.

B.  Twobivalents.

C.  Twopairsofsisterchroma1ds.

D. Twohomologouschromosomes.

Review:3.3.U7Crossingoverandrandomorienta1onpromotesgene1cvaria1on.

ProphaseI MetaphaseI

Crossing-overbetweennon-sisterchroma1dsresultsinrecombina/onofalleles

Randomorienta/onofthehomologouschromosomesmeansthereare2npossibleorienta1onsinmetaphaseIandII.Thatis223inhumans–or8,388,068differentcombina1onsingametes!

MetaphaseII

Becausebothcrossing-overandrandomorienta/onoccurduringmeiosistheresultisiseffec1velyinfinitegene/cvaria/oninthehaploidgamete.

n.b.foraneworganismtoarisesexuallymeiosisoccursinbothparentsfurtherincreasingthegene;cvaria;on

Review:3.3.U7Crossingoverandrandomorienta1onpromotesgene1cvaria1on.

ProphaseI MetaphaseI

Crossing-overbetweennon-sisterchroma1dsresultsinrecombina/onofalleles

Randomorienta/onofthehomologouschromosomesmeansthereare2npossibleorienta1onsinmetaphaseIandII.Thatis223inhumans–or8,388,068differentcombina1onsingametes!

MetaphaseII

Becausebothcrossing-overandrandomorienta/onoccurduringmeiosistheresultisiseffec1velyinfinitegene/cvaria/oninthehaploidgamete.

Whichpartoftheproces

s,MeiosisIorMeiosisII,

producesthemostgene/c

varia/on?Giveeviden

ce

tosupportyouranswe

r.

10.1.U7Independentassortmentofgenesisduetotherandomorienta1onofpairsofhomologouschromosomesinmeiosisI.

Mendel’sLawofIndependentAssortment“Thepresenceofanalleleofoneofthegenesinagametehasnoinfluenceoverwhichalleleofanothergeneispresent.”

AandBaredifferentgenesondifferentchromosomes.

Aisdominantovera.Bisdominantoverb.

Thisonlyholdstrueforunlinkedgenes(genesondifferentchromosomes).

RandomOrienta/onvsIndependentAssortment“Thepresenceofanalleleofoneofthegenesinagametehasnoinfluenceoverwhichalleleofanothergeneispresent.”

RandomOrienta/onreferstothebehaviorofhomologouspairsofchromosomes(metaphaseI)orpairsofsisterchroma1ds(metaphaseII)inmeiosis.Independentassortmentreferstothebehaviourofallelesofunlinkedgenesasaresultofgameteproduc/on(meiosis).

Duetorandomorienta/onofthechromosomesinmetaphaseI,theallelesoftheseunlinkedgeneshavebecomeindependentlyassortedintothegametes.

Anima/onfromSumanas:hIp://www.sumanasinc.com/webcontent/anima1ons/content/independentassortment.html

10.1.U7Independentassortmentofgenesisduetotherandomorienta1onofpairsofhomologouschromosomesinmeiosisI.

MendelandMeiosis “Thepresenceofanalleleofoneofthegenesinagametehasnoinfluenceoverwhichalleleofanothergeneispresent.”

Anima/onfromSumanas:hIp://www.sumanasinc.com/webcontent/anima1ons/content/independentassortment.html

Mendeldeducedthatcharacteris1csweredeterminedbytheinterac1onbetweenpairsofalleleslongbeforethedetailsofmeiosiswereknown.WhereMendelstatesthatpairsofallelesofageneseparateindependentlyduringgameteproduc;on,wecannowaIributethistorandomorienta/onofchromosomesduringmetaphaseI. Mendelmadethisdeduc1onwhenworkingwith

peaplants.Heinves1gatedtwoseparatetraits(colorandshape)andperformedmanytestcrosses,recordingthera1osofphenotypesproducedinsubsequentgenera1ons.Itwasratherfortunatethatthesetwotraitshappenedtobeonseparatechromosomes(unlinkedgenes)!Rememberbackthenhedidnotknowaboutthecontentsofthenucleus.ChromosomesandDNAwereyettobediscovered.Wewillusehisworkasanexampleofdihybridcrossesinthenextsec1on.

10.1.U7Independentassortmentofgenesisduetotherandomorienta1onofpairsofhomologouschromosomesinmeiosisI.

NatureofScience:Makingcarefulobserva1ons—carefulobserva1onandrecordkeepingturnedupanomalousdatathatMendel’slawofindependentassortmentcouldnotaccountfor.ThomasHuntMorgandevelopedtheno1onoflinkedgenestoaccountfortheanomalies.(1.8)

The‘anomalous’datawasrepeatedandfoundtobepredictable.TheexperimentsleadMorganandhiscolleaguestoreviseMendelianheredity(1915)toincludecertainkeytenets:•  Discretepairsoffactorsarelocatedonchromosomes(latertobecalledgenes)

•  Certaincharacteris1csaresex-linked•  Othercharacteris1csarealsosome1mesassociated

ThomasHuntMorgandevelopedtheideaofsex-linkedgenes

hIp://www.nature.com/scitable/content/ne0000/ne0000/ne0000/ne0000/122977784/1_2.jpgColumbiaUniversityFlyRoom

hIps://www.dnalc.org/view/15005-Thomas-Hunt-Morgan.html

hIps://gene1csandevolu1onch10.files.wordpress.com/2015/01/kbtnk9dz-13672096431.jpg?w=672&h=327

Morgan’sexperiments(1909-1914)withfruitfliesproducedresultsthatcouldnotbeexplainedbyMendel’sworkonheredityasitstood.

Bibliography/Acknowledgments

BobSmullen