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Biology Study Guide Exam 38.1 Likebegetslike, moreorless.Asexualreproduction: thecreationof

geneticallyidenticaloffspringbyasingle

parent, withouttheparticipationofeggand

sperm. Inasexualreproduction, thereisonesimpleprincipleofinheritance: loneparent

andoffspringhaveidenticalgenes.Sexual

reproduction:

offspringsproducedbythismethodlookmore

aliketoitsparentsthantootherunrelated

individuals. Eachoffspringinheritsa

combinationofgenesfromitsparents, thus

sexualreproductioncanproducegreatvariationamongoffsprings.

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8.2 Cellsariseonlyfrompreexistingcells.In 1838 GermanphysicianRudolfVirchowstated: Whereacellexists, there

musthavebeenapreexistingcell. Perpetuationoflifeisbasedinthecell

cycle. Celldivisionplaysdifferentroles: Inunicellularorganismcanreproduce

anewentireorganism. Onlargerscalecanallowmulticellular

organismtoreproduceasexually. This

isthebaseforspermandegg

formation.

Alsoallowsexualreproducing

organismtodevelopfromasinglecell

intoanadultorganism. Servesasrenewalandrepairofcellsthatdie.8.3 Prokaryotesreproducebybinaryfission.

Prokaryoticcellsdividesbybinaryfission ( dividinginhalf). Its

chromosomesaremuchsmallerthaneukaryotics, duplicatingtheminan

orderlyfashionanddistributingthecopiesequallyintotwodaughtercells.Process:

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Eukaryoticchromosomesincludesomeproteinsthatgivestructureandcontrolactivityofitsgenes. Humanshave 46 chromosomes. Wellbeforethecellbegintodivide, eachchromosomesiscopied. Theresult

iscalledsisterchromatids, whichcontainedidenticalcopiesoftheDNA. Thetwochromatidsarejoinedtogetheratatightlynarrowwaistcalledthecentromere.Whenacelldivides, thesisterchromatidsof

aduplicatedchromosomesseparatefrom

eachother8.5 Thecellscyclemultiplescells.Celldivisionisthebasisofreproductionfor

everyorganism. Enablesamulticellularorganism

togrowtoadultsize. Alsoreplacesworn-outcellsor

damagedone. Keepthetotalnumberofcellina

matureindividualinconstant

balance.Humanshavearound 100 trillioncells. Somedividesonceaday, otherless

often. Otheralmostneverdividelikemusclecells.Cellcycle: anorderedsequenceofeventsthatextendsfromthetimeacell

isfisrtformedfromadividngparentcelluntilitsowndividonintotwocells. Consistsoftwostages: Agrowingstage (interphase) duringwhichthecelldoubleseverythingin

itscytoplasmandduplicatesthechromosomalDNA. Actualcelldivisioncalledmitosis.

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Mostofthecellcycleis

spentintheinterphase.

Duringthisstage:Thecellincreasesits

suppliesofproteins.Createsmore

cytoplasmatic

organelles.Growinsize.Chromosomes

duplicates.Interphaseisdivided

intothreesubphases: G1 phase (firstgap) Sphase G2phase (secondgap)Duringallsubphasescellgrows. ChromosomesareduplicatedonlyinS

phase. SstandsforsynthesisofDNA. AtthebeginningofthisstageallDNA

issingle, butattheendDNAchromosomesaredoubled.Themitoticphaseisthepartofthecellwhenitdivides. Thisphasesis

dividedintomitosisandcytokinesis. InMitosisthenucleusdividesandin

Cytokinesisthecytoplasmisdividedintwo. Eachnewlyproduceddaughter

cellmaythenproceedthroughG1andrepeatthecycle.Thisisuniqueto

eukaryoticcells.8.6 Celldivisionisacontinuumofdynamicchanges

Mitosisisdividedinto:1. Prophase2. Pro-metaphase3. Metaphase4. Anaphase

Eukaryotic Cell Cycle

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5. TelophaseChromosomesmovementdependsonthemitoticspindle, afootball-shaped

structureofmicrotubulesthatguidetheseparationofsetsofdaughtercells.

Theyemergedfromthecentrosomes. Theabsenceofthesestructuresdonotthwarttheprocess.

Interphaseisthephaseofthecellcyclein

whichthecellspendsthemajorityofits

timeandperformsthemajorityofits

purposesincludingpreparationforcell

division. Inpreparationforcelldivision, it

increasesitssizeandmakesacopyofits

DNA, whichismadeduringtheSphase.Interphaseisalsoconsideredtobethe

'living'phaseofthecell, inwhichthecell

obtainsnutrients, grows, readsitsDNA,

andconductsother"normal"cellfunctions.

Themajorityofeukaryoticcellsspendmost

oftheirtimeininterphase. Prophaseisastageof

mitosisinwhichthechromatincondensesinto

doublerod-shapedstructurescalled

chromosomesinwhichthechromatinbecomes

visible. Thisprocess, calledchromatin

condensation, ismediatedbythecondensing

complex. Sincethegeneticmaterialhasbeen

replicatedinthepriorinterphaseofthecell

cycle, therearetwoidenticalcopiesofeach

chromosomeinthecell. Thosecopiesare

calledsisterchromatidsandtheyareattached

toeachotherataDNAelementpresenton

everychromosomecalledthecentromere.

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Prometaphaseisthephaseofmitosis

followingprophaseandpreceding

metaphase, ineukaryoticsomaticcells.

Prometaphaseissometimessimplyincludedaspartoftheendofprophase

andearlymetaphase.Inprometaphase, thenuclear

membranebreaksapartintonumerous

"membranevesicles", andthe

chromosomesinsideformprotein

structurescalledkinetochores.

Kinetochoremicrotubulesemerging

fromthecentrosomesatthepoles

(ends) ofthespindlereachthe

chromosomesandattachtothe

kinetochores , throwingthe

chromosomesintoagitatedmotionMetaphaseisastageofmitosisinthe

eukaryoticcellcycleinwhichcondensedandhighlycoiledchromosomes, carrying

geneticinformation, aligninthemiddleof

thecellbeforebeingseparatedintoeachof

thetwodaughtercells. Metaphase

accountsforapproximately 4% ofthecell

cycle'sduration[citationneeded.Preceded

byeventsinprometaphaseandfollowedby

anaphase, microtubulesformedin

prophasehavealreadyfoundandattachedthemselvestokinetochoresinmetaphase.Inmetaphase, thecentromeresofthe

chromosomesconvenethemselvesonthe

metaphaseplate (orequatorialplate),an

imaginarylinethatisequidistantfromthe

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twocentrosomepoles. Anaphaseisthestageofmitosisormeiosiswhen

chromosomesaresplitandthesisterchromatidsmovetooppositepolesofthecell.Anaphaseaccountsforapproximately 1% ofthe

cellcycle'sduration. Itbeginswiththeregulated

triggeringofthemetaphase-to-anaphase

transition.Telophaseisthefinalstageinboth

meiosisandmitosisinaeukaryoticcell.Duringtelophase, theeffectsofprophase

andprometaphase (thenuclear

membraneandnucleolusdisintegrating)

arereversed. Twodaughternucleiformin

eachdaughtercell, andphosphatases

de-phosphorylatethenuclearlaminsat

theendsofthecell, formingnuclear

envelopesaroundeachnucleus.Two

theoriesastohowthishappensareVesiclefusionWhenfragmentsofthe

nuclearmembranefusetorebuildthe

nuclearmembrane

Reshapingoftheendoplasmicreticulumwherethepartsofthe

endoplasmicreticulumcontainingabsorbednuclearmembraneenvelopthe

nuclearspace, reformingaclosedmembrane.

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8.7 Cytokinesisdiffersforplantandanimalcells.Cytokinesisisthedivisionofthecellintwo. Beginsduringtelophase,

althoughitcanbegininlateanaphase. Inanimalcellsthisprocessoccursascleavage. Thefirstsignoftheseitstheappearanceofthecleavage

furrow: ashallowgrooveinthecellsurface.Atthesitethecytoplasmhasaringof

microfilamentsmadeofactin, associatedwith

moleculesoftheproteinmyosin.Cytokinesisisdifferentinplantcelldivision.

Duringtelophase, vesiclescontainingcellwallmaterialcollectatthemiddleoftheparentcell.

Thevesiclefuse

formingthecell

plate.Thecellplate

growsoutward.

Eventuallythe

cellplatefusewiththeplasma

membraneandthatresultsintheproduction

oftwodaughtercells.8.8 Anchorage, celldensity, andchemicalgrowthfactors

affectcelldivision

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Cellsneedtocontrolitsgrowthtoproperlymaintaintheequilibrium. Inlabtestwasfoundthat: Cellsfailtodivideifanessentialnutrientisleftout. Mammaliancellonlygrowswithanspecificgrowthfactor.Agrowthfactorisaproteinsecretedbycertainbodycellsthatstimulatesothercellstodivide.Differentcelltypesrespondspecificallytocertaingrowthfactors.Physicalfactorscanbeobservedindensity-dependentinhibition, a

phenomenoninwhichcrowdedcellsstopdividing. Studiesshowsthat

physicalcontactofcell-surfaceproteinsbetweenadjacentcellsis

responsibleforinhibitingcelldivision.Alsoanchoragedependenceisseen. Theymustbeincontactwithasolidsurfacetodivide.8.9 GrowthfactorssignalthecellcyclecontrolsystemMostcellareanchoredandsurroundedbynutrients. Yettheynotundergo

divisionunlesssignaledbyothercells.Thecellcyclecontrolssystemisacyclicallyoperatingsetofmoleculesin

thecellthatbothtriggersandcoordinateskeyeventsinthecellcycle. Cell

cycleisnotlikefallingdominoes. Theyresetbycontrolsinsidethecellitself.Acheckpointinthecellcycleiscritical. Animalscellgenerallyhavebuilt-in

stopsignalsthathaltthecellcycleat checkpointuntiloverriddenbybygo-aheadsignals.

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Majorcheckpointincellcycle: DuringtheG1andtheG2 AndtheMphaseIntracellularsignalsdetectedbythecontrolsystemtellwhetherkeycellular

processesuptoeachpointhavebeencompletedandthuswetherornotthecell

cycleshouldproceedpastthatpoint.Alsorecievesmessagesfromoutsidethecell.FormostcelltheG1pointisthemostcritical. WhenthecelldoesnotpasttheG1

thecellexitthecellcycleandenterstheG0phase. Thisstageisnon-dividingwhichiswherenerveandmusclecellsare.

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8.10 Growingoutofcontrol, cancercellsproducemalignant

tumorsCancerisadiseaseofthecellcycle. Thecellsdivideswithnocontrolandif

uncheckeditcanleadtothekillingoftheorganism.Theprocessbeginswhenanormalcellundergoestransformation. The

bodysimmunesystemrecognizesthethreatandgetsridofit.Ifthecellevadesdestructionitturnsintoatumor: abnormallygrowingmass

inthebody. Ifthelumpremainsinthesameplaceitiscalledabenign

tumor. Ifitisnot, itiscalledthenamalignanttumor.The

spreading

of

cancer

cell

is

called

metastasis

.Cancer

is

named

after

the

placeisformed. Carcinomas: originatedintheexternalorinternalcoveringofthebody.

( skinorliningoftheintestine) Sarcoma: ariseintissuesthatsupportthebody.(boneandmuscle) LeukemiaandLymphomas: arebloodrelated.(blood, bonemarrow,

spleen, andlymphnodes)

Cancercells: Donotexhibitdensity-dependentinhibition Continuetodivideevenathighdensities Mayhavedefectivecellcyclecontrolsystem

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Synthesizefactorsthatallowthemtogrowoutofcontrol Inculturetheyareimmortal, candividedaslongastheyhavesupplyof

nutrients

8.12 Chromosomesarematchedinhomologouspairs.Typicalcellinthehumanbodyiscalledasomaticcellandhave 46

chromosomes. Overalltheyare 23 pairsofsisterchromatids. Two

chromosomescomposingapairarecalledhomologouschromosomes.

Theyrecalledthatwaybecausebothcarrythegenesofthesameinherited

characteristics.

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Thegenesthatdeterminesacharacteristicpatternislocatedinaparticular

place , orlocus, ( pluralloci) ononechromosome.ThetwochromosomesXandYareanexception. FemalehaveXXand

malesXY. OnlysmallpartsofXandYarehomologous. Theyrecalledthesexchromosomes. Theother 22 pairsarecalledautosomes.Inapairone

comefromthemotherandtheotherfromthefather.8.13 Gameteshaveasinglesetofchromosomes.Anycellwiththetotalnumberofchromosomesiscalledadiploidcell, and

thetotalofchromosomesiscalledthediploidnumber. ( abbreviated 2n) For

humansthediploidnumberis 46, 2n=46. Allcellsinhumanbodyarediploid

excepttheeggandsperm. Theyreknownasthegametes. Eachonehasa

singlesetof 22 autosomeschromosomes, plusasinglechromosomesXor

Y. Acellwithasinglechromosomesiscalledahaploidcell.Fertilization: thefusionofthespermwiththeegg.Theresultiscalledthezygote, whichsdiploid. Gametesaremadebymeiosis, whichoccursonlyinreproductiveorgans.

Meiosisonlyproduceadaughtercellwithhalfofchromosomes.8.14 Meiosis

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8.15 MitosisandMeiosishaveimportantsimilaritiesand

differencesAlleventsuniqueinmeiosisoccurduringmeiosisI. InprophaseI,

duplicatedhomologouspairtoformtetrads, fourchromatids, whicheach

pairofsisterchromatidsjoinedattheircentromeres.

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