biology exam 4
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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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