5. Architecture of chromosome.pdf

Detail study about Cromosome Structure, function of chomoses and types of chromosome

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

2. Introduction
Chromosome,themicroscopicthreadlikepartof
thecellthatcarrieshereditaryinformationintheform
ofgenes.
Chromosome=chroma(colour)+some(body)
In1842,KarlWilhelmvonNageli,aswissbotanist
discoveredastructurewhichlaterknownas
chromosomes.
ChromosomeswerefirstdiscoveredbyStrasburger
in1815.
Term‘chromosome’wasfirstusedbyWaldeyerin
1888.

3. Organism
Number of chromosomes
(2n)
Human 46
Potato 48
RIce 24
Monkey 42
Mouse 40
“Anessentialelementoflife-chromosomesarelocatedinthe
nucleusofacellonwhichtheentiregenomicDNAofanorganismis
arranged.”
Different species have a different number of chromosome based on
their genome size.
Organism
Number of chromosomes
(2n)
Human 46
Potato 48
RIce 24
Monkey 42
Mouse 40

4. Structure of Chromosome

5. CentromereorKinetochore:Itistheprimaryconstrictionatthecenter
towhichthechromatidsorspindlefibersareattached.Itsfunctionisto
enablemovementofthechromosomeduringtheanaphasestageofcell
division.
Chromatid:Duringcelldivision,achromosomeisdividedinto2
identicalhalfstrandsjoinedbyacentromere.Achromatidiseachhalfof
thechromosomejoined.EachchromatidcontainsDNAandseparatesat
Anaphasetoformaseparatechromosome.Bothchromatidsareattachedto
eachotherbythecentromere.

6. Chromatin:ItisacomplexofDNAandproteinsthatformschromosomes
withinthenucleusofeukaryoticcells.NuclearDNAishighlycondensedand
wrappedaroundnuclearproteinsinordertofitinsidethenucleus.Inother
words,itisnotpresentasfreelinearstrands.ThechromatinconsistsofDNA,
RNA,andprotein.

7. SecondaryConstriction:Itisgenerallypresentforthenucleolar
organization.

8. Chromonema:Itisathreadlikecoiledfilamentousstructurealongwhich
chromomeresarearranged.Chromonemacontrolsthesizeofthe
chromosomeanditactsasasiteofgenebearing.

9. Chromomeres:Thesearethebead-likestructurespresentonthreadsor
chromonema.Thesearearrangedinarowalongthelengthofchromonema.The
numberofchromosomesisconstantanditisresponsibleforcarryingthegenes
duringcelldivisiontothenextgeneration.
Matrix:Pellicleisthemembranesurroundingeachofthechromosomes.
Matrixisthejelly-likesubstancepresentinsidepellicle.Itisformedofnon-
geneticmaterials

10. Telomere:Telomereistheterminal
regionofeachsideofthechromosome.
Telomeresaretheendof
chromosomeswhichprotectsgene-rich
region-andaremadeupofthe
repetitiveDNAsequence,thenon-
codingDNAsequencesonthe
telomeresarecategorisedin
microsatelliteandminisatellite.
Therepetitivesequencesarehighly
packedandthusdoesnotencodesany
proteinhowever,itprotectsothergenes
fromthecellsownmistakecalled“end
replicationproblem.”

11. Eachchromosomehasaconstriction
pointcalledthecentromere,whichdivides
thechromosomeintotwosections,or
“arms.”
Theshortarmofthechromosomeis
labeledthe“parm.”
Thelongarmofthechromosomeis
labeledthe“qarm.”
Thelocationofthecentromereoneach
chromosomegivesthechromosomeits
characteristicshape,andcanbeusedto
helpdescribethelocationofspecific
genes.

12. Armsarethecomplexnetworkofprotein
andDNAwheregenesarelocated.
Thedenselypackedareaofarmsiscalled
heterochromatinregion-agenelessregion,
richinnon-codingDNA.
Ontheotherhand,thelooselypacked
regionisknownaseuchromatinregion-a
gene-richregion.Thetipofeacharmis
protectedbythestructurecalledtelomeres.
“Higherthelengthofarms,moregenesit
contains.”

13. Classification of Chromosome on the
base of centromerelocation
Metacentric

14. Classification of Chromosome on the
base of centromerelocation
Metacentric
Sub
Metacentric

15. Classification of Chromosome on the
base of centromerelocation
Metacentric
Sub
Metacentric
Acrocentric

16. Classification of Chromosome on the
base of centromerelocation
Metacentric
Sub
Metacentric
Acrocentric
Telocentric

18. TheDNAofthechromosomescarrygenes,whichareresponsible
fortransferofhereditarycharactersfromparentstooffspring.Thus,
chromosomeisalsotermedas“ThreadofLife.”
Function of chromosomes

19. Chromosomesfacilitatepropercelldivisionandreplication.
ThemainfunctionofthechromosomeistofittheDNAinsidethenucleus.
Asweallknow,thatourDNAistoolong,ifweunwindalltheDNAofacell,
itisupto2metersinlength.
Henceitisveryimportanttofititinsidethenucleuswhichisfacilitatedby
chromosomes.ByinteractingwithproteinsDNAformsacoiledstructure-
chromosome.
Furthermore,sexchromosomesdecidethesexoftheembryo.
Theprocessofsexdeterminationandsexdifferentiationisgovernedby
geneslocatedonautosomesandsexchromosomes.
Function of chromosomes

20. Important terms
Autosomes:
Autosomesarechromosomesapartfromthesexchromosomesina
eukaryoticcell.Inhumans,theXandYchromosomesarethesex
chromosomes.Allthechromosomesotherthanthesexchromosomesare
autosomes.
Allosomes
Anallosomeisasexchromosomethatdiffersinsize,formandbehaviour
fromanautosome.HumanshaveonepairofallosomesThesechromosomes
containgenesthatdeterminethebiologicalsexofanorganism.
Thesechromosomesformpairs.TheXandtheYchromosomespair
togetherduringmeiosisandthispairhelpsinsexdetermination.

21. Homologous Chromosomes
Homologouschromosomesconsistofallelesofthesametypeofgenesinthe
samelocithatarepairedduringmeiosis.

22. Non-homologous chromosomes
Non-homologousChromosomesarechromosomesthatdonot
belongtothesamepair.Generally,theshapeofthechromosome,
thatis,thelengthofthearmsandthepositionofthecentromere,is
differentinnon-homologouschromosomes.
Therefore,non-homologouschromosomesdonotpairduring
meiosis.Eachchromosomeofaparticularorganismonlypairswith
itshomologue.
Thismeanshomologouspairssegregatefromotherchromosomes
ofthenucleusasdescribedbytheLawofsegregation.

23. Karyotype
Akaryotypeisadiagramthatshowsthechromosomalnumberand
constitutioninthecells’nucleus.
Itconsistsofawholesetofhomologouschromosomepairs,arranged
indecreasingseriesoftheirsize.
Moreover,itrevealsinformationregardingthenumber,size,shape,
positionofcentromeresofeachchromosome,lengthofchromosomal
arms,presenceofsecondaryconstrictionsandsatellites,etc.

24. Idiogram
•Idiogramisadiagrammaticrepresentationoraschematicdiagramofa
karyotypeofaspecies.
•Idiogramshowsthechromosomemapsindicatingthelocationsofgenes
asbands.
•Itisnotanactualpictureoftotalchromosomesofacell.However,
ideogramprovidesmuchinformationabouteachchromosome.
•Mostimportantly,itprovideslocationsofindividualgenespresentina
chromosome.

25. Special types of Chromosomes
Giant
chromosomes
Somecellsatcertainparticular
stageoftheirlifecyclecontain
largenucleiwithgiantorlarge
sizedchromosomes.
Giantchromosomeswerefirst
timeobservedbyE.G.Balbiani
intheyear1881innucleiof
certainsecretorycells(salivary
glands)ofChironomaslarvae
(Diptera).

26. Special types of Chromosomes
Giant
chromosomes
Polytene
Chromosome

27. Special types of Chromosomes
Giant
chromosomes
Polytene
Chromosome
Lampbrush
chromosome

28. Special types of Chromosomes
Giant
chromosomes
Polytene
Chromosome
Lampbrush
chromosome
B chromosomes

29. PolyteneChromosome
ThepolytenechromosomewasproposedbyKollarduetotheoccurrence
ofmanychromonemata(DNA)inthem.
CellsinthelarvalsalivaryglandofDrosophila,mosquitoandChironema
containchromosomeswithhighDNAcontent.
Theymayalsooccurinmalphigiantubules,rectum,gut,footpads,fat
bodies,ovariannursecellsetc.
Polyteneyofgiantchromosomeshappensbyreplicationofthe
chromosomalDNAseveraltimeswithoutnucleardivision(endomitosis)
andtheresultingdaughterchromatidsdonotseparatebutremainaligned
sidebyside.
Thepolytenechromosomesarevisibleduringinterphaseandprophase
ofmitosis.

30. Polytenechromosome with puff
Thesechromosomesarenot
inertcellularobjectsbutdynamic
structuresinwhichcertain
regionsbecome“puffedout”due
toactiveDNAtranscriptionat
particularstagesofdevelopment.
Thesechromosomepuffsare
alsotermedBalbianirings.
Puffsmayappearand
disappeardependingonthe
productionofspecificproteins
whichneedstobesecretedin
largeamountsinthelarval
saliva.

31. Lampbrushchromosome
LampbrushchromosomeswerefirstobservedbyFlemmingin1882in
sectionsofSalamanderoocytesandlaterdescribedbyRuckertintheyear
1892.
ObservedduringthediplotenestageofprophaseIinmeiosisinthe
oocytesofallanimalspeciesbothvertebratesandinvertebrates.
Generallytheyaresmallerininvertebratesthanvertebrates.
Theyappearedlikebrushesusedforcleaninglamps,hencethename
lampbrushchromosome.
TheyareformedduetotheactivesynthesisofmRNAmolecules.
Lampbrushchromosomesareclearlyvisibleinthelightmicroscopethey
areorganizedintoaseriesofchromomereswithlargechromatin
symmetricalloopsextendinglaterally.
Eachloopappearsataconstantpositioninthechromosome(10,000
loopsperchromosomesetorhaploidset).

32. Lampbrushchromosome

33. B chromosomes
Manyplant(maize,etc.)andanimal(suchasinsectsandsmallmammals)
species,besideshavingautosomes(A-chromosomes)andsex-chromosomes
possessaspecialcategoryofchromosomescalledB-chromosomeswithout
obviousgeneticfunction.
TheseB-chromosomes(alsocalledsupernumerarychromosomes,accessory
chromosomes,accessoryfragments,etc.)usuallyhaveanormalstructure,are
somewhatsmallerthantheautosomesandcanbepredominantly,
heterochromatic(manyinsects,maize,etc.)orpro-dominantlyeuchromatic
(rye).
Inmaize,theirnumberpercellcanvaryfrom0to30andtheyadversely
affect,developmentandfertilityonlywhenoccur,inlargeamount.
Inanimals,theB-chromosomesdisappearfromthenon-reproductive
(somatic)tissueandaremaintainedonlyinthecell-linesthatleadtothe
reproductiveorgans.

34. B-chromosomeshavenegativeconsequencesfortheorganism,astheyhave
deleteriouseffectbecauseofabnormalcrossingoverduringthemeiosisof
animalsandabnormalnucleusdivisionsofthegametoophyteplants.
TheoriginoftheB-chromosomesisuncertain.Insomeanimalstheymaybe
derivativesofsexchromosomes,butthisisnottherule.
Theygenerallydonotshowanypairingaffinitywiththe'A-chromosomes.