Chromosomes are structures in the nucleus that contain genetic material. They are complexes of DNA and proteins. Chromosomes are visible during cell division and come in different sizes. The number and structure of chromosomes provide identifying characteristics for different organisms and species. Chromosomes contain both euchromatin and heterochromatin regions that are identifiable by their staining patterns.

2. 2

3. INTRODUCTIONINTRODUCTION
 ChromosomesChromosomes are the structures that contain theare the structures that contain the
genetic materialgenetic material
 They are complexes of DNA and proteinsThey are complexes of DNA and proteins
 TheThe genomegenome comprises all the genetic material that ancomprises all the genetic material that an
organism possessesorganism possesses
 In bacteria, it is typically a single circular chromosomeIn bacteria, it is typically a single circular chromosome
 In eukaryotes, it refers to one complete set ofIn eukaryotes, it refers to one complete set of nuclearnuclear
chromosomeschromosomes
 Note: Eukaryotes possess a mitochondrial genomeNote: Eukaryotes possess a mitochondrial genome
Plants also have a chloroplast genomePlants also have a chloroplast genome
3

4. What Exactly is a chromosome?What Exactly is a chromosome?
Chromosomes are theChromosomes are the rod-shapedrod-shaped,,
filamentous bodiesfilamentous bodies present in thepresent in the nucleusnucleus,,
which become visiblewhich become visible during cell divisionduring cell division..
They are theThey are the carriers of the genecarriers of the gene or unit ofor unit of
heredity.heredity.
Chromosome areChromosome are not visiblenot visible in active nucleusin active nucleus
due to theirdue to their high water contenthigh water content, but are, but are
clearly seen during cell division.clearly seen during cell division.
4

5. Diagram of a chromosomeDiagram of a chromosome
5

6.  Chromosomes were first described byChromosomes were first described by
StrausbergerStrausberger inin 18751875..
 The term “Chromosome”, however wasThe term “Chromosome”, however was
first used byfirst used by WaldeyerWaldeyer inin 18881888..
 They were given the name chromosomeThey were given the name chromosome
(Chromo = colour; Soma = body) due to(Chromo = colour; Soma = body) due to
their markedtheir marked affinity for basic dyesaffinity for basic dyes..
 Their number can be counted easily onlyTheir number can be counted easily only
duringduring mitotic metaphase.mitotic metaphase. 6

7.  Chromosomes are composed ofChromosomes are composed of thinthin
chromatinchromatin threads calledthreads called Chromatin fibersChromatin fibers..
 These fibers undergoThese fibers undergo foldingfolding,, coilingcoiling andand
supercoilingsupercoiling during prophase so that theduring prophase so that the
chromosomes become progressivelychromosomes become progressively
thicker and smaller.thicker and smaller.
 Therefore, chromosomes become readilyTherefore, chromosomes become readily
observable under light microscope.observable under light microscope.
7

8. Number of chromosomesNumber of chromosomes
 Normally, all the individuals of aNormally, all the individuals of a species havespecies have
the same numberthe same number of chromosomes.of chromosomes.
 Presence of a whole sets of chromosomes isPresence of a whole sets of chromosomes is
calledcalled euploidyeuploidy..
 It includes haploids, diploids, triploids,It includes haploids, diploids, triploids,
tetraploids etc.tetraploids etc.
 Gametes normally contain only one set ofGametes normally contain only one set of
chromosome – this number is calledchromosome – this number is called HaploidHaploid
 Somatic cells usually contain two sets ofSomatic cells usually contain two sets of
chromosome -chromosome - 2n : Diploid2n : Diploid
8

9. 3n – triploid3n – triploid
4n – tetraploid4n – tetraploid
The condition in which the chromosomes setsThe condition in which the chromosomes sets
are present in a multiples of “n” isare present in a multiples of “n” is PolyploidyPolyploidy
When a change in the chromosome number doesWhen a change in the chromosome number does
not involve entire sets of chromosomes, butnot involve entire sets of chromosomes, but
only a few of the chromosomes - isonly a few of the chromosomes - is
Aneuploidy.Aneuploidy.
 Monosomics (2n-1)Monosomics (2n-1)
 Trisomics (2n+1)Trisomics (2n+1)
 Nullisomics (2n-2)Nullisomics (2n-2)
 Tetrasomics (2n+2)Tetrasomics (2n+2)
9

10. Organism No. chromosomes
 Human 46
 Chimpanzee 48
 Dog 78
 Horse 64
 Chicken 78
 Goldfish 94
 Fruit fly 8
 Mosquito 6
 Nematode 11(m), 12(f)
 Horsetail 216
 Sequoia 22
 Round wormRound worm 22
10

11. Organism No. chromosomes
 OnionOnion 1616
 MoldMold 1616
 CarrotCarrot 2020
 TomatoTomato 2424
 TobaccoTobacco 4848
 RiceRice 2424
 MaizeMaize 2020
 Haploppus gracilisHaploppus gracilis 44
 Crepis capillarisCrepis capillaris 6
11

12. Chromosome SizeChromosome Size
 In contrast to other cell organelles, the size of chromosomesIn contrast to other cell organelles, the size of chromosomes
shows a remarkable variation depending upon the stages ofshows a remarkable variation depending upon the stages of
cell division.cell division.
 Interphase:Interphase: chromosome are longest & thinnestchromosome are longest & thinnest
 Prophase:Prophase: there is a progressive decrease in their lengththere is a progressive decrease in their length
accompanied with an increase in thicknessaccompanied with an increase in thickness
 Metaphase:Metaphase: Chromosomes are the most easily observedChromosomes are the most easily observed
and studied during metaphase when they are very thick,and studied during metaphase when they are very thick,
quite short and well spread in the cell.quite short and well spread in the cell.
 Anaphase:Anaphase: chromosomes are smallest.chromosomes are smallest.
 TTherefore, chromosomes measurements are generallyherefore, chromosomes measurements are generally
taken during mitotic metaphase.taken during mitotic metaphase.
12

13.  In order to understand chromosomes and theirIn order to understand chromosomes and their
function, we need to be able to discriminate amongfunction, we need to be able to discriminate among
different chromosomes.different chromosomes.
 First, chromosomes differ greatly in sizeFirst, chromosomes differ greatly in size
 Between organisms the size difference can be overBetween organisms the size difference can be over
100-fold, while within a sp, some chromosomes are100-fold, while within a sp, some chromosomes are
often 10 times as large as others.often 10 times as large as others.
13

14.  KaryotypeKaryotype: is the general morphology of the: is the general morphology of the
somatic chromosome. Generally, karyotypessomatic chromosome. Generally, karyotypes
represent by arranging in the descending order ofrepresent by arranging in the descending order of
size keeping their centromeres in a straight line.size keeping their centromeres in a straight line.
 IdiotypeIdiotype: the karyotype of a species may be: the karyotype of a species may be
represented diagrammatically, showing all therepresented diagrammatically, showing all the
morphological features of the chromosome; suchmorphological features of the chromosome; such
a diagram is known asa diagram is known as Idiotype.Idiotype.
14

15.  Chromosomes may differ in the position of theChromosomes may differ in the position of the
CentromereCentromere, the place on the chromosome where, the place on the chromosome where
spindle fibers are attached during cell division.spindle fibers are attached during cell division.
 In general, if the centromere is near the middle, theIn general, if the centromere is near the middle, the
chromosome ischromosome is metacentricmetacentric
 If the centromere is toward one end, theIf the centromere is toward one end, the
chromosome ischromosome is acrocentricacrocentric oror submetacentricsubmetacentric
 If the centromere is very near the end, theIf the centromere is very near the end, the
chromosome ischromosome is telocentrictelocentric..
15

16. 16

17. Euchromatin and HeterochromatinEuchromatin and Heterochromatin
 Chromosomes may be identified by regions that stain inChromosomes may be identified by regions that stain in
a particular manner when treated with various chemicals.a particular manner when treated with various chemicals.
 Several different chemical techniques are used toSeveral different chemical techniques are used to
identify certain chromosomal regions by staining then soidentify certain chromosomal regions by staining then so
that they formthat they form chromosomal bands.chromosomal bands.
 For example, darker bands are generally found near theFor example, darker bands are generally found near the
centromeres or on the ends (telomeres) of thecentromeres or on the ends (telomeres) of the
chromosome, while other regions do not stain aschromosome, while other regions do not stain as
strongly.strongly.
 The position of the dark-staining areThe position of the dark-staining are heterochromaticheterochromatic
regionregion oror heterochromatinheterochromatin..
 Light staining areLight staining are euchromatic regioneuchromatic region oror euchromatineuchromatin..
17

18.  Heterochromatin is classified into two groups:Heterochromatin is classified into two groups:
(i)(i) ConstitutiveConstitutive and (ii)and (ii) Facultative.Facultative.
 Constitutive heterochromatin remainsConstitutive heterochromatin remains
permanently in the heterochromatic stage, i.e., itpermanently in the heterochromatic stage, i.e., it
does not revert to the euchromatic stage.does not revert to the euchromatic stage.
 In contrast, facultative heterochromatin consistsIn contrast, facultative heterochromatin consists
of euchromatin that takes on the staining andof euchromatin that takes on the staining and
compactness characteristics of heterochromatincompactness characteristics of heterochromatin
during some phase of development.during some phase of development.
18

19. ChromatinChromatin
 The complexes between eukaryotic DNA and proteins areThe complexes between eukaryotic DNA and proteins are
calledcalled ChromatinChromatin, which typically contains about twice as, which typically contains about twice as
much protein as DNA.much protein as DNA.
 The major proteins of chromatin are theThe major proteins of chromatin are the histoneshistones – small– small
proteins containing a high proportion of basic aminoacidsproteins containing a high proportion of basic aminoacids
((arginine and lysinearginine and lysine) that facilitate binding negatively) that facilitate binding negatively
charged DNA molecule .charged DNA molecule .
 There areThere are 5 major types5 major types of histones:of histones: H1, H2A, H2B, H3,H1, H2A, H2B, H3,
andand H4H4 – which are very similar among different sp of– which are very similar among different sp of
eukaryotes.eukaryotes.
 The histones are extremely abundant proteins in eukaryoticThe histones are extremely abundant proteins in eukaryotic
cells.cells.
19

20. The major histone proteins:The major histone proteins:
Histone Mol. WtHistone Mol. Wt No. ofNo. of PercentagePercentage
Amino acidAmino acid Lys + ArgLys + Arg
H1H1 22,50022,500 244244 30.830.8
H2AH2A 13,96013,960 129129 20.220.2
H2BH2B 13,77413,774 125125 22.422.4
H3H3 15,27315,273 135135 22.922.9
H4H4 11,23611,236 102102 24.524.5
The DNA double helix is bound to proteins called histones. The
histones have positively charged (basic) amino acids to bind the
negatively charged (acidic) DNA. Here is an SDS gel of histone
proteins, separated by size
20

21.  The basic structural unit of chromatin, theThe basic structural unit of chromatin, the nucleosomenucleosome, was, was
described bydescribed by Roger KornbergRoger Kornberg inin 1974.1974.
 The binding of proteins to DNA inThe binding of proteins to DNA in chromatin protectschromatin protects the regions ofthe regions of
DNA from nuclease digestion.DNA from nuclease digestion.
21

22.  Electron microscopy revealed that chromatinElectron microscopy revealed that chromatin
fibers have a beaded appearance, with the beadsfibers have a beaded appearance, with the beads
spaced at intervals of approximately 200 basespaced at intervals of approximately 200 base
pairs.pairs.
 Thus, both nuclease digestion and the electronThus, both nuclease digestion and the electron
microscopic studies suggest that chromatin ismicroscopic studies suggest that chromatin is
composed of repeating 200 base pair unit, whichcomposed of repeating 200 base pair unit, which
were calledwere called nucleosome.nucleosome.
22

23. individual nucleosomes = “beads on a string”
23

24. 24

25. 25

26. Centromeres and TelomeresCentromeres and Telomeres
 CentromeresCentromeres andand telomerestelomeres are two essentialare two essential
features of all eukaryotic chromosomes.features of all eukaryotic chromosomes.
 Each provide a unique function i.e.,Each provide a unique function i.e., absolutelyabsolutely
necessary for the stability of the chromosomenecessary for the stability of the chromosome..
 Centromeres are required for the segregation ofCentromeres are required for the segregation of
the centromere during meiosis and mitosis.the centromere during meiosis and mitosis.
 Telomeres provide terminal stability to theTelomeres provide terminal stability to the
chromosome and ensure its survivalchromosome and ensure its survival
26

27. CentromereCentromere
 The region where two sister chromatids of a chromosomeThe region where two sister chromatids of a chromosome
appear to be joined or “appear to be joined or “held togetherheld together” which is called” which is called
CentromereCentromere
 When chromosomes are stained they typically show aWhen chromosomes are stained they typically show a dark-dark-
stainedstained region that is the centromere.region that is the centromere.
 Also termed asAlso termed as Primary constrictionPrimary constriction
 DuringDuring mitosismitosis, the centromere that is shared by the sister, the centromere that is shared by the sister
chromatids must divide so that the chromatids can migrate tochromatids must divide so that the chromatids can migrate to
opposite poles of the cell.opposite poles of the cell.
 Therefore the centromere is an important component ofTherefore the centromere is an important component of
chromosome structure and segregation.chromosome structure and segregation.
27

28.  As a result, centromeres are the first partsAs a result, centromeres are the first parts
of chromosomes to be seen movingof chromosomes to be seen moving
towards the opposite poles duringtowards the opposite poles during
anaphase.anaphase.
 The remaining regions of chromosomes lagThe remaining regions of chromosomes lag
behind and appear as if they were beingbehind and appear as if they were being
pulled by the centromere.pulled by the centromere.
28

29. TelomereTelomere
 The two ends of a chromosomeThe two ends of a chromosome are known asare known as
telomeres.telomeres.
 It required for theIt required for the replication and stabilityreplication and stability of theof the
chromosome.chromosome.
 When telomeres are damaged or removed due toWhen telomeres are damaged or removed due to
chromosome breakage, the damaged chromosomechromosome breakage, the damaged chromosome
ends can readily fuse or unite with broken ends ofends can readily fuse or unite with broken ends of
other chromosome.other chromosome.
 Thus it is generally accepted that structuralThus it is generally accepted that structural
integrity and individuality of chromosomes isintegrity and individuality of chromosomes is
maintained due to telomeres.maintained due to telomeres. 29

30.  McClintockMcClintock noticed that if two chromosomes werenoticed that if two chromosomes were
broken in a cell, the end of one could attach to thebroken in a cell, the end of one could attach to the
other and vice versa.other and vice versa.
 What she never observed was the attachment of theWhat she never observed was the attachment of the
broken end to the end of an unbrokenbroken end to the end of an unbroken
chromosome.chromosome.
 Thus the ends ofThus the ends of broken chromosomes are stickybroken chromosomes are sticky,,
whereas thewhereas the normal end is not stickynormal end is not sticky, suggesting, suggesting
the ends of chromosomes have unique features.the ends of chromosomes have unique features.
30

31. Staining and Banding chromosomeStaining and Banding chromosome
Staining procedures have been developed in the past twoStaining procedures have been developed in the past two
decades and these techniques help to study the karyotype indecades and these techniques help to study the karyotype in
plants and animals.plants and animals.
1. Feulgen Staining1. Feulgen Staining
22. Q banding. Q banding
3. R banding3. R banding
4. G banding4. G banding
5. C banding5. C banding
31

32. VARIATION IN STRUTURE OFVARIATION IN STRUTURE OF
CHROMOSOMECHROMOSOME
32

33. Chromosomal AberrationsChromosomal Aberrations
 The somatic (2n) and gametic (n) chromosomeThe somatic (2n) and gametic (n) chromosome
numbers of a species ordinarily remain constant.numbers of a species ordinarily remain constant.
 This is due to the extremely precise mitotic and meioticThis is due to the extremely precise mitotic and meiotic
cell division.cell division.
 Somatic cells of a diploid species contain two copies ofSomatic cells of a diploid species contain two copies of
each chromosome, which are called homologouseach chromosome, which are called homologous
chromosome.chromosome.
 Their gametes, therefore contain only one copy of eachTheir gametes, therefore contain only one copy of each
chromosome, that is they contain one chromosomechromosome, that is they contain one chromosome
complement or genome.complement or genome.
 Each chromosome of a genome contains a definiteEach chromosome of a genome contains a definite
numbers and kinds of genes, which are arranged in anumbers and kinds of genes, which are arranged in a
definite sequence.definite sequence.
33

34. Chromosomal AberrationsChromosomal Aberrations
 Sometime due to mutation or spontaneousSometime due to mutation or spontaneous
(without any known causal factors), variation in(without any known causal factors), variation in
chromosomal number or structure do arise inchromosomal number or structure do arise in
nature. - Chromosomal aberrations.nature. - Chromosomal aberrations.
 Chromosomal aberration may be grouped intoChromosomal aberration may be grouped into
two broad classes:two broad classes:
1. Structural and 2. Numerical1. Structural and 2. Numerical
34

35.  There areThere are fourfour common type of structuralcommon type of structural
aberrations:aberrations:
1. Deletion or Deficiency1. Deletion or Deficiency
2. Duplication or Repeat2. Duplication or Repeat
3. Inversion, and3. Inversion, and
4. Translocation.4. Translocation.
35

36.  Consider a normal chromosome with genes inConsider a normal chromosome with genes in
alphabetical order:alphabetical order: a b c d e f g h ia b c d e f g h i
1. Deletion1. Deletion:: part of the chromosome has beenpart of the chromosome has been
removed:removed: a b c g h ia b c g h i
2. Dupliction2. Dupliction:: part of the chromosome is duplicated:part of the chromosome is duplicated:
a b c d e fa b c d e f d e fd e f g h ig h i
3. Inversion3. Inversion:: part of the chromosome has been re-part of the chromosome has been re-
inserted in reverse order:inserted in reverse order: a b c fa b c f ee d g h id g h i
ringring:: the ends of the chromosome are joinedthe ends of the chromosome are joined
together to make a ringtogether to make a ring 36

37. 4. translocation4. translocation:: parts of two non-homologousparts of two non-homologous
chromosomes are joined:chromosomes are joined:
If one normal chromosome isIf one normal chromosome is a b c d e f g h ia b c d e f g h i
and the other chromosome isand the other chromosome is u v w x y z,u v w x y z,
then a translocation between them would bethen a translocation between them would be
a b c d e f x y za b c d e f x y z andand u v w g h i.u v w g h i.
37

38. 38

39. TranslocationTranslocation
 Integration of a chromosome segment into aIntegration of a chromosome segment into a
nonhomologous chromosome is known asnonhomologous chromosome is known as
translocationtranslocation..
 Three types:Three types:
1. simple translocation1. simple translocation
2. shift2. shift
3. reciprocal translocation.3. reciprocal translocation.
39

40.  Simple translocationSimple translocation: In this case,: In this case, terminalterminal
segmentsegment of a chromosome isof a chromosome is integratedintegrated at oneat one
end of a non-homologous region. Simpleend of a non-homologous region. Simple
translocations are rathertranslocations are rather rarerare..
 ShiftShift: In shift, an: In shift, an intercalary segmentintercalary segment of aof a
chromosome ischromosome is integratedintegrated within a non-within a non-
homologous chromosome. Such translocationshomologous chromosome. Such translocations
are known in the populations ofare known in the populations of DrosophilaDrosophila,,
NeurosporaNeurospora etc.etc.
 Reciprocal translocationReciprocal translocation: It is produced when: It is produced when
two non-homologous chromosomes exchangetwo non-homologous chromosomes exchange
segments – i.e., segmentssegments – i.e., segments reciprocallyreciprocally
transferred.transferred.
 Translocation of this type is most commonTranslocation of this type is most common
40

41. Variation in chromosome number
 Organism with one complete set of chromosomes
is said to be euploid (applies to haploid and diploid
organisms).
 Aneuploidy - variation in the number of individual
chromosomes (but not the total number of sets of
chromosomes).
 The discovery of aneuploidy dates back to 1916The discovery of aneuploidy dates back to 1916
whenwhen BridgesBridges discovered XO male and XXYdiscovered XO male and XXY
femalefemale DrosophilaDrosophila, which had 7 and 9, which had 7 and 9
chromosomes respectively, instead of normal 8.chromosomes respectively, instead of normal 8.
41

42.  Nullisomy - loss of one
homologous chromosome
pair. (e.g., Oat )
 Monosomy – loss of a
single chromosome
(Maize).
 Trisomy - one extra
chromosome. (Datura)
 Tetrasomy - one extra
chromosome pair.
More about Aneuploidy
42

43. Trisomy in HumansTrisomy in Humans
Down SyndromeDown Syndrome
43

44. Other SyndromesOther Syndromes
Chromosome NomenclatureChromosome Nomenclature: 47, 46 +1 (13): 47, 46 +1 (13)
Chromosome formulaChromosome formula: 2n+1: 2n+1
Clinical SyndromeClinical Syndrome: Patau’s: Patau’s
Estimated Frequency BirthEstimated Frequency Birth: 1/20,000: 1/20,000
Main Phenotypic CharacteristicsMain Phenotypic Characteristics::
Mental deficiency and deafness, minorMental deficiency and deafness, minor
muscle seizures, cleft lip, cardiac anomaliesmuscle seizures, cleft lip, cardiac anomalies
44

45. Other SyndromesOther Syndromes
ChromosomeChromosome NomenclatureNomenclature: 47, 46+1 (18): 47, 46+1 (18)
Chromosome formulaChromosome formula: 2n+1: 2n+1
Clinical SyndromeClinical Syndrome: Edward’s: Edward’s
Estimated Frequency BirthEstimated Frequency Birth: 1/8,000: 1/8,000
Main Phenotypic CharacteristicsMain Phenotypic Characteristics::
Multiple congenital malformation of manyMultiple congenital malformation of many
organs, malformed ears, small mouth and noseorgans, malformed ears, small mouth and nose
with general elfin appearance.with general elfin appearance.
90% die in the first 6 months.90% die in the first 6 months.
45

46. Other SyndromesOther Syndromes
ChromosomeChromosome NomenclatureNomenclature: 45, 46-1 (X): 45, 46-1 (X)
Chromosome formulaChromosome formula: 2n - 1: 2n - 1
Clinical SyndromeClinical Syndrome: Turner: Turner
Estimated Frequency BirthEstimated Frequency Birth: 1/2,500 female: 1/2,500 female
Main Phenotypic CharacteristicsMain Phenotypic Characteristics::
Female with retarded sexual development,Female with retarded sexual development,
usually sterile, short stature, cardiovascularusually sterile, short stature, cardiovascular
abnormalities, hearing impairment.abnormalities, hearing impairment.
46

47. Other SyndromesOther Syndromes
Chromosome NomenclatureChromosome Nomenclature:: 47-XXY, 48-XXXY,47-XXY, 48-XXXY,
48-XXYY, 49- XXXXY, 50-XXXXXY.48-XXYY, 49- XXXXY, 50-XXXXXY.
Chromosome formulaChromosome formula: 2n+1; 2n+2; 2n+2; 2n+3; 2n+4: 2n+1; 2n+2; 2n+2; 2n+3; 2n+4
Clinical SyndromeClinical Syndrome: Klinefelter: Klinefelter
Estimated Frequency BirthEstimated Frequency Birth: 1/500 male borth: 1/500 male borth
Main Phenotypic CharacteristicsMain Phenotypic Characteristics::
Pitched voice, Male, subfertile with smallPitched voice, Male, subfertile with small
testes, developed breasts, feminine, long limbs.testes, developed breasts, feminine, long limbs.
47

48.  Found in certain tissues e.g.,Found in certain tissues e.g.,
salivary glands of larvae, gutsalivary glands of larvae, gut
epithelium, Malphigianepithelium, Malphigian
tubules and some fat bodies,tubules and some fat bodies,
of some Diptera (of some Diptera (Drosophila,Drosophila,
Sciara, RhyncosciaraSciara, Rhyncosciara))
 These chromosomes are veryThese chromosomes are very
long and thick (uptolong and thick (upto 200200
times their sizetimes their size duringduring
mitotic metaphase in themitotic metaphase in the
case of Drosophila)case of Drosophila)
 Hence they are known asHence they are known as
Giant chromosomesGiant chromosomes..
Giant chromosomesGiant chromosomes
48

49.  They are first discovered byThey are first discovered by BalbianiBalbiani inin 18811881 inin
dipteran salivary glands and thus also known asdipteran salivary glands and thus also known as
salivary gland chromosomessalivary gland chromosomes..
 But their significance was realized only after theBut their significance was realized only after the
extensive studies byextensive studies by PainterPainter during 1930’s.during 1930’s.
 Giant chromosomes have also been discoveredGiant chromosomes have also been discovered
in suspensors of young embryos of many plants,in suspensors of young embryos of many plants,
but these do not show the bands so typical ofbut these do not show the bands so typical of
salivary gland chromosomes.salivary gland chromosomes.
49

50.  During certain stages of development, specificDuring certain stages of development, specific
bands and inter band regions are associated withbands and inter band regions are associated with
them greatly increase in diameter and producedthem greatly increase in diameter and produced
a structure calleda structure called PuffsPuffs oror Balbiani ringsBalbiani rings..
 Puffs are believed to be produced due toPuffs are believed to be produced due to
uncoiling of chromatin fibers present in theuncoiling of chromatin fibers present in the
concerned chromomeres.concerned chromomeres.
 The puffs are sites of activeThe puffs are sites of active RNA synthesisRNA synthesis..
50

51. 51

52. Lampbrush ChromosomeLampbrush Chromosome
 ItIt was given this name because it is similar inwas given this name because it is similar in
appearance to the brushes used to clean lampappearance to the brushes used to clean lamp
chimneys in centuries past.chimneys in centuries past.
 First observed byFirst observed by FlemmingFlemming in 1882.in 1882.
 The name lampbrush was given byThe name lampbrush was given by RuckertRuckert in 1892.in 1892.
 These are found inThese are found in oocyticoocytic nuclei of vertebratesnuclei of vertebrates
(sharks, amphibians, reptiles and birds)as well as in(sharks, amphibians, reptiles and birds)as well as in
invertebrates (Sagitta, sepia, Ehinaster and severalinvertebrates (Sagitta, sepia, Ehinaster and several
species of insects).species of insects).
 Also found in plants – but most experiments inAlso found in plants – but most experiments in
oocytes.oocytes.
52

53.  One loop represent oneOne loop represent one
chromatid, i.e., onechromatid, i.e., one
DNA molecule.DNA molecule.
 The size of the loopThe size of the loop
may be ranging themay be ranging the
average of 9.5average of 9.5 µmµm toto
about 200about 200 µmµm
 The pairs of loops areThe pairs of loops are
produced due toproduced due to
uncoiling of the twouncoiling of the two
chromatin fiberschromatin fibers
present in a highlypresent in a highly
coiled state in thecoiled state in the
chromomeres.chromomeres. 53

54.  One end of each loop is thinner (thin end) thanOne end of each loop is thinner (thin end) than
the other end (thick end).the other end (thick end).
 There is extensive RNA synthesis at the thin endThere is extensive RNA synthesis at the thin end
of the loops, while there is little or no RNAof the loops, while there is little or no RNA
synthesis at the thick end.synthesis at the thick end.
54

55. Phase-contrast and fluorescent micrographs of
lampbrush chromosomes
55

56. 56