architecture of chromosome ,morphology , description of chromosome and karyotype

2. Cont.
 Thetermchromosomeisderivedfromtwowordschroma=colour+ soma=bodydueto theirmarkedaffinityto basicdyes.
 Chromosomeswasfirst discoveredby Strausberger in1875 whenhe describedcelldivisioninplantcells.
 Thetermchromosomewasgivenby Waldeyerin1888 .
 .chromosomesaretherod shaped, darkstainedbodiesseenduringmetaphasestageof mitosisbecausechromosomes
containmaximumdegreeof condensationduringmetaphase.
 Chromosomesareclearlyvisibleasdistinctbodiesduringthe stagesof celldivisiononly,and theirnumbercanbe counted
withrelativeeaseonlyduringmitoticmetaphase.

3. Chromosome number
 Each chromosome has a definite and ,generally a constant somatic and gametic chromosome number.
 Somatic chromosome number is the chromosome number found in meristematic tissues of a species and is represented
by 2n because somatic cells contain two copies of each chromosome called homologous chromosomes.
 Two copies in homologous chromosomes are identical in morphology ,gene content and gene order .
 Gametic chromosome number is the chromosome number found in gametes and is precisely half of the somatic
chromosome number and is represented by n.

4. ENDOREDUPLICATION & Diminution
 In meristematic tissues of all species , one round of DNA synthesis is constantly followed by one cell division .But in differentiating cells of
most of most plant and animal species ,some cells undergo two or more rounds of successive DNA replication without cell division ; this is
called Endoreduplication.
 Endoreduplication leads to the production of endopolyploidy differentiated cells .
 Several plant species do not show endopolyploidy in their differentiated tissues e.g Crepis species , Lactuca sativa etc.
 In some animals, e.g., Parascaris equorum (syn., Ascaris megalocephala), the complete somatic complement may be present only in the
reproductive tissue, while some chromosomes portions of chromosomes may be eliminated in somatic tissues; this phenomenon is known
as chromosome diminution.

5. CHROMOSOME SIZE
 The size of chromosomes varies depending on the stage of cell division, with the longest and thinnest during interphase and the
smallest during anaphase. During mitotic metaphase, the size varies between 0.5 micron and 32 micron in length, and between 0.2
micron and 3.0 micron in diameter.
 The smallest known chromosome is only about 1/80,000 in length of the longest reported chromosome, and the longest metaphase
chromosomes are found in Trillium.
 Giant chromosomes are permanently in pre metaphase stage, and may be as long as 300 micron and up to 10 micron in diameter.
Plants have longer chromosomes than animals, and species having lower chromosome numbers have longer chromosomes.
 Different chromosomes of the haploid complement of a single species may be more or less of comparable size in many cases, while
in many other cases they may show considerable variation in length.

6. CHROMOSOME MORPHOLOGY
 As stated earlier, chromosome appearance (morphology) changes with the stage of cell division and
Mitotic metaphase chromosomes are the most suitable for studies on chromosome morphology
Further, some features of chromosome morphology are relatively more clear during mitotic
metaphase than during any other stage of cell division .
 In mitotic metaphase chromosomes, the following structural features (except chromomere) can be
seen under the light microscope (with appropriate staining): (1) chromatid, (2) centromere, (3)
telomere, (4) secondary construction and satellite, and (5) chromomere (seen during meiotic
prophase, particularly pachytene; not visible in metaphase chromosomes).

7. CHROMATID
 Chromatid Each metaphase chromosome appears to be longitudinally divided into two identical
parts each of which is known as chromatid.
 The two chromatids of a chromosome appear to be ‘joined’ or ‘fused’ (not really fused; they are only
‘held together’ rather closely) together at a point called centromere
 It is universally accepted that chromatid is the structural and functional unit of chromosomes
and that it is not further subdivisible into smaller subunits without adversely affecting its
structural integrity and functional capability.

8. Cont.
 Therefore, the position of centromere serves as an important landmark in the identification of different
chromosomes of a species.
 Each chromosome is divided into two transverse parts by its centromere; these parts are called }arms. In
most cases, one arm of the chromosome is longer than the other, hence they are termed as long arm and
short arm, respectively.
 The short arm is usually represented by the letter p and the long arm is denoted by q .
 Chromosomes may be divided into four classes on the basis of the position of their centromeres: (1)
metacentric, (2) submetacentric, (3) acrocentric and (4) telocentric
 . Metacentric Chromosome. In such a chromosome, the centromere is located in the centre of chromosome,
i.e., the centromere is median, so that the two arms of such a chromosome are equal, and the arm ratio is 1 .
Metacentric chromosomes appear ‘V’-shaped during anaphase

9. Cont.
 Submetacentric Chromosome. In most chromosomes, the centromere is located on one side of the central point, i.e., the centromere is sub median;
such chromosomes appear either similar to ‘V’ or J during anaphase.
 Acrocentric Chromosome. In these chromosomes centromeres are located close to one end of the chromosomes, i.e., they are subterminal.
Acrocentric chromosomes may appear either ‘j’ or ‘rod-shaped’ during anaphase.
 Telocentric Chromosomes. Occasionally, in some chromosomes, the centromere appears to be located at one end of the chromosome; such
centromeres are called terminal, and the chromosomes having them are known as telocentric chromosomes.
 Telecentric chromosomes always appear ‘rod- shaped’ during anaphase . Generally, telocentric chromosomes are unstable and most naturally
occurring telocentric chromosomes are believed to be in reality acrocentric.
 Telocentric chromosomes are believed to originate by a process called transverse division of a the centromere . Telocentric chromosomes have
been experimentally produced in wheat, and 42 different lines, are available in the variety Chinese Spring.

10. Types of Chromosomes

11. Telomeres
 The two ends of a chromosome are known as telomeres. Telomeres are highly stable and they do not
fuse or unite with telomeres of other chromosomes.
 But when telomeres are damaged or removed due to chromosome breakage, the damaged
chromosome ends are highly unstable and they readily fuse or unite with broken ends of other
chromosomes.
 it is generally accepted that the structural integrity and individuality of chromosomes is maintained to
the telomeres, and that all stable chromosome is attached with the nuclear envelope at the periphery
of an annulus by one of its telomeres.
 During interphase, chromosomes are fully uncoiled and extended, and are represented by chromatin
fibers of 300 A diameter.
 At the molecular level telomeres contain short, tandem repeat sequences, and the terminal single-
stranded region is folded to form a hairpin loop, which gives it stability.

12. Cont.
 The centromeres contains specific DNA sequences (centromeric sequences) and variant of H3 histone( CENH3) to which
kinetochore proteins bind.
 CENH3 is universal component of centromeres of all species. The presence of CENH3 in a location enables it to function
as a centromere.
 In Yeast centromeric function is confined to a stretch of 120 bp which has following three types of sequences.(1)At the left
boundary ,a 9 bp conserved sequence forms CDE1 (2) The next sequence is 80-90 bp long with more than 90 percent A;T
bp and is found in all centromeres CDE 2 *(3) CDE 3 element forms right boundary and is 11 bp long and is highly
conserved .
 In most cases, centromeric region is almost the last segment of chromosomes to replicate during late
S phase

13. Secondary constriction
 In some chromosomes, a second constriction, in addition to that due to centromere (the primary constriction), is also
present; this additional constriction is known as secondary constriction.
 Secondary constrictions may appear as constrictions or as gaps in chromosomes, but sometimes they may not be
observable. Generally, secondary constrictions are located in the short arm of chromosomes near one end, but in many
chromosomes they are located in the long arm and/or nearer to the centromere than to the telomere.
 The chromosome , the region between the secondary constriction and the nearest telomere, is known as satellite.
chromosomes having secondary constrictions are called satellite chromosomes or sat-chromosomes.
The position of secondary constriction in sat-chromosomes is fixed and remains constant. The number of sat-chromosomes in
the genome varies from one species to the other. In many species, somatic cells contain two (and gametes have one) sat-
chromosomes.
Nucleolus is always associated with the secondary constrictions of sat-chromosomes. Therefore, secondary constrictions are also
called nucleolus organiser regions (NORs) and sat-chromosomes are often referred to as nucleolus organiser chromosomes
(NOCs). NOR of each sat-chromosome contains several hundred copies of the gene coding for ribosomal RNA

14. Chromomere
 . Chromomere In some species, e.g., maize, amphibia etc., chromosomes during the first
prophase of meiosis, more particularly during pachytene, show small bead-like structures
called chromomeres.
 The distribution of chromomeres in a chromosome is-highly characteristic and constant, the
patterns of distribution being different for different chromosomes; homologous chromosomes
show an identical pattern.
 Chromomeres are most clearly visible in the dipteran giant salivary gland chromosomes as
dark staining bands. Chromomeres of a single chromosome show a considerable variation in
size.
 .chromatin appears to be present in a more highly condensed state in chromomeres than in the
neighbouring regions of the chromosomes; this may account for both their larger diameter and
stainability.

15. Types of DNA
 Repetitive DNA consists of DNA nucleotide or base sequences, which are few to several hundred base pairs (bp) long and
are present in several to a million copies per - genome.
 The proportion of repetitive DNA in the genome varies from one species to the other: human genome has only 30 per
cent repetitive DNA, while in rye it constitutes about 92 per cent of the genome.
 Repetitive DNA sequences are grouped into the following two classes mainly on the basis of the number of copies
present per genome: (1) highly repetitive DNA and (ii) moderately repetitive DNA
 Unique DNA consists of those DNA segments whose base sequences are present only in a single copy per genome. All the
structural genes, except those genes whose products are present in large quantities in cells (e.g., genes for histones,
ribosomal RNA, storage proteins etc.), are made up of unique DNA, that is, only a single copy of these genes is present in
a genome.

16. Heterochromatin and Euchromatin
The material of which chromosomes are composed is called chromatin. Chromatin was classified into two groups by cytologists on the basis of
its stainability with basic dyes, more particularly the Feulgen reagent, during the various stages of cell cycle.
Heterochromatin takes up deep stain during interphase and prophase, while during metaphase it is stained lightly. In contrast, euchromatin
takes up little stain during interphase, stains only lightly during prophase, particularly early prophase, but is deeply stained during metaphase.
In contrast, euchromatin takes up little stain during interphase, stains only lightly during prophase, particularly early prophase, but is deeply
stained during metaphase.
. In contrast, facultative heterochromatin is essentially euchromatin that has undergone heterochromatinization (conversion into
heterochromatin). Heterochromatinization may involve a segment chromosome, a whole chromosome or one whole haploid set of chromosomes
(e.g., in insects like mealy bugs).
The distribution of constitutive heterochromatin within a chromosome is highly specific and characteristic. In general, heterochromatin is found
in the centromeric and telomeric regions, but other regions may also contain heterochromatin.

17. Cont.
Heterochromatic chromosomes and chromosome regions ordinarily replicate later than
euchromatic regions of the chromosomes.
Genetic studies have revealed that constitutive heterochromatin is largely genetically
inactive.
Heterochromatin appears to be composed of more densely packed 300 A chromatin fibers,
While In euchromatin these fibers are less tightly packed.
However, heterochromatin remains highly condensed throughout the cell cycle, while
euchromatin becomes fully extended and is not visible with light microscope during
interphase.

18. Karyotype and Ideotype
 General morphology of the somatic chromosome complement of an individual constitutes its karyotype.
 Ordinarily, karyotypes are presented by arranging the chromosomes of somatic complement in a descending order of size keeping their
centromeres in a straight line; thus the longest chromosomes is placed on the extreme left and the smallest one on the extreme right .
 All the normal members of a species have an identical karyotype. Therefore, the karyotype of a normal somatic cell of a normal
individual represents the karyotype of the concerned species.
 A perfectly symmetrical karyotype has all metacentric chromosomes of the same size; karyotypes showing a deviation from this state are
called asymmetrical.
 It is believed that perfectly symmetrical karyotypes represent a primitive state from which more advanced asymmetrical karyotypes have
evolved through structural changes in chromosomes .
 The karyotype of a species may be represented diagrammatically showing all the morphological features of chromosomes ; such a
diagram is known as Ideotype.

19. Chemical composition
 Chromatin is composed of DNA, protein and RNA. chromatin isolated from interphase nuclei consists of about 30-40 per cent DNA, 50-65 per cent
protein and 0.5-10 per cent RNA . there is considerable variation due to species and tissues of the same species
 . The chemical composition of metaphase chromosomes differs markedly from that of interphase chromatin; they contain relatively less DNA and
more protein and RNA than does the latter. Metaphase chromosomes contain, on an average, 15-20 per cent DNA, 10-15 per cent RNA and 65-70
percent protein.
 DNA is the only component of chromosomes the amount of which exhibits a constant relationship with the stage of chromosome replication and the
increase or decrease in chromosome number. The DNA found in chromosomes is mainly of two types: (1) unique DNA (2) Repetitive DNA.
 . Unique DNA. Unique DNA consists of those DNA segments whose base sequences are present only in a single copy per genome. All the structural
genes, except those genes whose products are present in large quantities in cells .
 The proportion of unique DNA in cells of various species may vary from 70 % (man) to only 8 % (rye) but values between 40-70 per cent are more
common. In contrast, 98.5 % of the bacterial DNA is unique, and the 0.3 per cent repetitive DNA is due to reiteration (presence in several copies or
amplification) of ribosomal RNA genes.

20. Proteins
 . Proteins associated with chromosomes may be classified into two broad groups: (1) basic proteins or histones and (2)
non-histone proteins
 . Non-histone proteins are mainly acidic in nature, but a small fraction of these proteins may be neutral. Histones
constitute about 80 per cent of the total chromosomal protein; they are present in an almost 1 : 1 ratio with DNA
 The molecular weight of histones ranges from 10,000 to 30,000 and they are completely devoid of tryptophan. Histones
are a highly heterogeneous class of proteins and they are separable in 5 different fractions through electrophoresis.
These fractions are designated as H1, H2a, H2b, H3; H4 after Lewin (1975).
 . These five fractions are present in all cell types of eukaryotes, except in yeast (S. cerevisiae), which does not contain H1
histone and in the sperm of some animal species where histones are replaced by another class of smaller molecule basic
proteins called prot-amines.
 The approximate molar ratio of these fractions is 1 HI : 2H2a : 2H2b: 2H3; : 2H4.

21. cont.
 . Non-histone proteins make up about 20 per cent of the total chromosome mass, but their amount is variable and there is
no definite ratio between the amounts of DNA and non-histones present in chromosomes.
 There may be a large number of different types of non-histone proteins. These proteins show variation from one species to
the other and even in different tissues of the same organism.
 Clearly, they have not been as conserved during evolution as the histone proteins. This class of proteins includes many
important enzymes, such as, DNA and RNA polymerases (the latter being a prominent component) and transcription
factors (belonging to the high mobility group, HMG, of proteins).

22. FOLDED FIBER MODEL
 The most widely accepted model of chromosome structure, the folded-fibre model, was proposed by Dupraw in 1965. According to this model, chromosomes are
made up of chromatin fibres of about 230 A diameter .
 Each chromatin fibres contains only one DNA double helix, which is in a coiled state; this DNA coil is coated with histone and non-histone proteins. Thus the 230
A chromatin fibres is produced by coiling of a single DNA double helix; the coils of DNA molecule are stabilized by proteins and divalent cations (Ca** and Mg**).
 Each chromatid contains a single enormously long chromatin fibres. The DNA of this chromatin fibres replicates during interphase producing two sister
chromatin fibres; it remains un- replicated in the centromeric region so that the two sister chromatin fibres remain joined in this region .
 Subsequently, the chromatin fibres Chromosomes undergoes replication in the centromeric region as well so that the sister chromatin fibres are separated in
this region also.
 During cell division, the two sister chromatin fibres undergo extensive folding separately in an irregular (but definitely reproducible) manner to give rise to the
two sister chromatids. Folding of the chromatin fibres drastically reduces their length and at the same time markedly increases their thickness and stainability.
This folded structure normally undergoes supercoiling, which further reduces the length and increases the thickness of chromosomes.


23. Nucleosome Solenoid Model
 The nucleosome solenoid model of chromatin fiber was proposed by Kornberg and Thomas in 1974 and
is universally accepted model
 According to this model, chromatin is composed of a repeating unit called nucleosome. One complete
nucleosome, a complete disc of 11 nm diameter and 6 nm height, consists of (1) a nucleosome core, (2)
linker DNA, (3) an average of one molecule of H/ histone and (4) the other associated chromosomal
proteins
 . A nucleosome contains an average of ~200 bp DNA, which usually ranges between 180 and 200 bp
 . Nucleosome Core It consists of a histone octamer composed of two molecules each of histones H2a,
H2b, H3 and H4
 . In addition, a 146 bp long DNA molecule is wound round this histone octamer in 13/4 turns; this
segment of DNA in chromatin fibers are nuclease resistant..
 The structure of nucleosome core is essentially invariable in all the eukaryotes.

24. Linker DNA
 . The size of linker DNA varies from 8 bp to 114 bp depending on the species.
 This DNA forms the string part of the heads-on-a-string structure seen on isolation of chromatin fiber, and is nuclease
susceptible.
 Each nucleosome contains, on an average, one molecule of H1 histone.
 Complete removal of H1 histone does not affect the structure of nucleosome core, indicating that H1is located outside the
Chromosomes nucleosome core
. H1 is most likely located at the linker DNA and could ‘seal’ the DNA in the nucleosome by binding at the sites where
DNA enters and leaves the nucleosomes.

25. Cont.
 The linker DNA and nucleosome are associated with other chromosomal proteins
 . In native chromatin, the beads are about 110 A in diameter, Ca. 60 A high, and ellipsoidal in shape. Each bead corresponds to a single nucleosome core.
 The nucleosome fiber is also called 10 nm fiber; it is seen in chromatin under low ionic strength, and H1 histone is not needed for its organization.
 The nucleosomes seem to be arranged edge to edge with their flat faces (11 nm diameter) parallel to the axis of the fiber. The nucleosome fiber may then
supercoil to give rise to the 300 A chromatin fiber seen in electron micrographs of metaphase chromosomes. The supercoiled nucleosome fiber is called 3
nm fiber, which most likely has solenoid organization of nucleosomes. This fiber has a coiled structure having ~6 nucleosomes/turn and a packing ratio
of 40. Presence of H1 histone is required for its organization. The 30 nm fiber is seen in chromatin under conditions of greater ionic strength. The 30 nm
and 10 nm fibers can be reversibly produced from each other by manipulating the ionic strength. The 30 nm fiber is the basic constituent of both
interphase and mitotic chromosomes. The nucleosome model of chromatin fiber structure is consistent and is universally accepted.

26. THANK YOU