Cell division.pdf

cell division

1. Cell Division
Dr. PratibhaBisen
Dept. Plant Breeding & Genetics
College of Agriculture, Balaghat
JNKVV Jabalpur (M.P.)

2. Cell Division
Inunicellularorganisms,celldivisionisthemeansofreproduction;in
multicellularorganisms,itisthemeansoftissuegrowthandmaintenance.
Everyday,everyhour,everysecondoneofthemostimportanteventsin
lifeisgoingoninyourbody—cellsaredividing.
Asinglecelldividestomaketwocellsandthesetwocellsthendivideto
makefourcells,andsoon.Wecallthisprocess"celldivision"and"cell
reproduction,"becausenewcellsareformedwhenoldcellsdivide.

3. Mitosis Cell Division
Theprocessofmitosis,orcelldivision,isalsoknownastheMphase.Thisis
wherethecelldividesitspreviously-copiedDNAandcytoplasmtomaketwo
new,identicaldaughtercells.
Mitosisishowsomatic—ornon-reproductivecells—divide.Somaticcells
makeupmostofyourbody'stissuesandorgans,includingskin,muscles,lungs,
gut,andhaircells.Reproductivecells(likeeggs)arenotsomaticcells.
Inmitosis,theimportantthingtorememberisthatthedaughtercellseach
havethesamechromosomesandDNAastheparentcell.
Thedaughtercellsfrommitosisarecalleddiploidcells.Diploidcellshavetwo
completesetsofchromosomes.
Sincethedaughtercellshaveexactcopiesoftheirparentcell'sDNA,no
geneticdiversityiscreatedthroughmitosisinnormalhealthycells.

4. Prophase Metaphase
Anaphase Telophase
Mitosis
Phases of Mitosis

5. The Mitotic Spindle
Spindlefibersformaproteinstructurethatdividesthegeneticmaterialin
acell.
Thespindleisnecessarytoequallydividethechromosomesinaparental
cellintotwodaughtercellsduringbothtypesofnucleardivision:mitosis
andmeiosis.
Duringmitosis,thespindlefibersarecalledthemitoticspindle.
Meanwhile,duringmeiosis,thespindlefibersarereferredtoasthemeiotic
spindle.
Atthebeginningofnucleardivision,twowheel-shapedproteinstructures
calledcentriolespositionthemselvesatoppositeendsofthecellforming
cellpoles.
Longproteinfiberscalledmicrotubulesextendfromthecentriolesinall
possibledirections,formingwhatiscalledaspindle.
Someofthemicrotubulesattachthepolestothechromosomesby
connectingtoproteincomplexescalledkinetochores.

6. The Mitotic Spindle
Kinetochoresareproteinformationsthatdeveloponeachchromosome
aroundthecentromere,whichisaregionlocatednearthemiddleofa
chromosome.Othermicrotubulesbindtothechromosomearmsorextend
totheoppositeendofthecell.

7. Prophase
TheformationofspindleandchromosomescondenseintoX-shaped
structuresthatcanbeeasilyseenunderamicroscope.
Eachchromosomeiscomposedoftwosisterchromatids,containing
identicalgeneticinformation.
Thechromosomespairupsothatbothcopiesofchromosome1are
together,bothcopiesofchromosome2aretogether,andsoon.
Attheendofprophasethemembranearoundthenucleusinthecell
dissolvesawayreleasingthechromosomes.
Themitoticspindle,consistingofthemicrotubulesandotherproteins,
extendsacrossthecellbetweenthecentriolesastheymovetoopposite
polesofthecell.

9. Metaphase
Thechromosomeslineupneatlyend-to-endalongthecentre
(equator)ofthecell.
Thecentriolesarenowatoppositepolesofthecellwiththemitotic
spindlefibresextendingfromthem.
Themitoticspindlefibresattachtoeachofthesisterchromatids.

11. Anaphase
Thesisterchromatidsarethenpulledapartbythemitoticspindlewhichpulls
onechromatidtoonepoleandtheotherchromatidtotheoppositepole.

12. Telophase
Ateachpoleofthecellafullsetofchromosomesgathertogether.
Amembraneformsaroundeachsetofchromosomestocreatetwo
newnuclei.
Thesinglecellthenpinchesinthemiddletoformtwoseparate
daughtercellseachcontainingafullsetofchromosomeswithina
nucleus.Thisprocessisknownascytokinesis.