Download Meiosis and Mitosis

Meiosis and Mitosis
Dr. Bob Dobalina
Page 1 of 3
Meiosis and Mitosis
Lecture Objectives:
1. Differentiate mitosis from meiosis.
2. Describe how chromosomes replicate during mitosis and meiosis.
3. Describe how meiosis facilitates the three major features of Mendelian genetics:
segregation, independent assortment and genetic recombination.
4. Explain the medical consequences that result from errors in cell division 5. Identify the
origins of errors in cell division.
Meiosis and Mitosis
Dr. Bob Dobalina
I.
Page 2 of 3
An Overview of Mitosis & Meiosis
A. Cell Division
1. Mitosis
2. Meiosis
B. Cell Division Definitions
1. Mitosisoccursduringcelldivisionofsomaticcells.Adiploidparentcell
generatestwoidenticaldiploiddaughtercells.Normally,recombination
(discussedlater)doesnottakeplace.
2. Meiosisistheprocessbywhichsexcells(gametes)areformed.Adiploid
progenitorcellgeneratesfourhaploidgametes.
a. Therearetwocelldivisionsbutonlyonedoublingofthe
chromosomes.
i. Theprocessultimatelyresultsinhalvingtheamountof
geneticmaterialinthegametes,from46(diploid)to23
(haploid)inthematureeggorsperm.Onechromosomein
eachpairofhomologouschromosomesintheparentcell
willbeallocatedatrandomintoeachofthedaughtercells.
Thus,thenormaleggandspermeachcontain23
chromosomesintermixedofdifferingparentalorigin.
Whentheyuniteatfertilization,therewillagainbe46
chromosomesinthefertilizedegg.Recombinationis
frequentduringmeiosis.
II.
DefinitionsandCellMachinery
A. Chromosomes
1. Chromosomesarethestructuresthatholdallthegeneticmaterial.They
arerodshapedcondensationsofnucleicmaterial(DNA)andcomein
differentsizes.Eachhasuniquecharacteristicswhichallowsusto
differentiateonefromtheother.
2. Chromosomes,likegenes,comeinpairs-ideally,exactlyhalfthe
chromosomalmaterial–onememberofeachpair–camefromeach
parent.T
B. Centromere
1. Theconstrictionofthechromosome,whichinthecenterofalltheaction
duringcelldivision,iscalledthecentromere.
2. AfterDNAreplication,eachchromosomehastwoidenticalsister
chromatids.
Meiosis and Mitosis
Dr. Bob Dobalina
Page 3 of 3
C. Kinetochore
1. Thekinetochoreisaproteinstructurethatformsonthecentromereto
whichthemicrotubules,literallytheworkhorsesincelldivision,attach.
Therearethreetypesofmicrotubulesthatfunctionaspartofthemitotic
spindleapparatus.
2. Thekinetochoremicrotubulesattachtothekinetochoreatthe
centromere.Theyareresponsibleforpullingthesisterchromatidsof
eachchromosomeapartattheendofmetaphase.
a. Thepolar
Image 1:
microtubules
overlapinthe
midlineandare
responsiblefor
pushingthepolesof
thespindle
apparatusapart.
Theastral
microtubulesradiate
inalldirectionsfrom
thecentrosomesandaidinspindleorientationimmediately
beforemitosis.
b. Thecentrosomesconsistoftwocentriolesandaretheorganelles
thataretheprimarymicrotubularorganizingcenterthatsplitsand
formsthespindleapparatus.
D. CellCycleReview
1. Mitosisisactuallytheshortestofthefourstagesofthecellcycle.
Immediatelyaftermitosis,ifacellisdestinedtodivideagain,itenters
Gap1orG1wherethereisnoDNAsynthesis.Ifacellisnotdestinedto
divideagain,itentersG0.SomecellsspendaverylongtimeinG1–days
orevenyears;otherspassthroughG1inamatterofhours.
2. G1isfollowedbythe
Sphase,thestageofDNAsynthesiswheretheDNAmoleculereplicates
andisfollowedbyafurthergapintimewaitingformitosistobegin.By
theendofSphase,theDNAcontentofthecellhasdoubled,andduring
thenextphase,Gap2orG2,eachchromosomeconsistsoftwoidentical
DNAmolecules,thesisterchromatids.
a. DuringG2,essentialproteinsandcofactorsareproduced
necessaryformitosistooccur.G2isendedbymitosis.