B.Sc.

Botany (H) Paper VI (Cytogenetics and Molecular Biology) Dr Kadambini Das

CHROMOSOME: STRUCTURE, TYPES AND FUNCTIONS

Chromosomes (Gr.,chrom=colour, soma=body) are the rod shaped, dark stained bodies seen
during the metaphase stage of mitosis when cells are stained with a suitable basic dye and
viewed under a light microscope. Chromosomes were first described by Strasburger (1815), and
the term ‘chromosome’ was first used by Waldeyer in 1888. Chromosomes are composed of thin
chromatin threads called chromonemata which undergo coiling and super coiling during
prophase so that the chromosomes become progressively thicker and smaller, and become
readily observable under light microscope.

Chromosome Number:
The number of the chromosomes is constant for a particular species. Therefore, these are of great
importance in the determination of the phylogeny and taxonomy of the species. The number or
set of the chromosomes of the gametic cells such as sperms and ova is known as the gametic,
reduced or haploid set of chromosomes. The haploid set of the chromosomes is also known as
the genome. The somatic or body cells of most organisms contain two haploid set or genomes
and are known as the diploid cells. The diploid cells achieve the diploid set of the chromosomes
by the union of the haploid male and female gametes in the sexual reproduction. The number of
chromosomes in each somatic cell is the same for all members of a given species. The organism
with the lowest chromosome number is the nematode, Ascaris megalocephalus univalens which
has only two chromosomes in the somatic cells (2n = 2). In the radiolarian protozoan Aulacantha
is found a diploid number of approximately 1600 chromosomes. Among plants, chromosome
number varies from 2n = 4 in Haplopappus gracilis (Compositae) to 2n=> 1200 in some
pteridophytes. Chromosome number of few common animals and plants is given below:

Animals Chromosome Number

1. Paramecium aurelia 30 – 40
2. Hydra vulgaris 32
3. Ascaris lumbricoides 24
4. Musca domestica 12
5. Homo sapiens 46
Plants Chromosome Number
1. Mucor sp. 2
2. Allium cepa 16

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 B.Sc. Botany (H) Paper VI (Cytogenetics and Molecular Biology) Dr Kadambini Das

3. Aspergillus nidulans 16

Autosomes and sex chromosomes: In a diploid cell there is two of each kind of chromosome
termed as homologous chromosomes, except for the sex chromosomes. For example, in human,
there are 23 pairs of homologous chromosomes (i.e., 2n = 46). The human male has 44 non-sex
chromosomes, termed autosomes and one pair of heteromorphic or morphologically dissimilar
sex chromosomes, i.e., one X chromosome and one Y chromosome. The human female has 44
non-sex chromosomes (autosomes) and one pair of homomorphic (morphologically similar) sex
chromosomes designated as XX.

Morphology:
Chromosome morphology changes with the stage of cell division, and mitotic metaphase
chromosomes are the most suitable for studies on chromosome morphology. In mitotic
metaphase chromosomes, the following structural feature (except chromomere) can be see under
light microscope:
(1) Chromatid,
(2) Chromonema,
(3) Chromomeres,
(4) Centromere
(5) Secondary constriction or Nucleolar organizer,
(6) Telomere and
(7) Satellite.

Structure and regions recognized in chromosomes: Structurally, each chromosome is

differentiated into three parts: (a) Pellicle, (b) Matrix, (c) Chromonemata.
a) Pellicle: It is the outer envelope around the substance of chromosome. It is very thin and
is formed of achromatic substances. Certain scientists Darlington (1935) and Ris (1940)
have denied its presence.
b) Matrix: It is the ground substance of chromosome which contains the chromonemata. It
is also formed of nongenic materials.
c) Chromonemata: Embedded in the matrix of each chromosome are two identical, spirally
coiled threads, the chromonemata. The two chromonemata are also tightly coiled together

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 B.Sc. Botany (H) Paper VI (Cytogenetics and Molecular Biology) Dr Kadambini Das

that they appear as single thread of about 800A thickness. Each chromonemata consists
of about 8 micro fibrils, each of which is formed of a double helix of DNA.

Chromomeres: In favourable preparations, chromomeres in the form of small dense masses are
observed at regular intervals on the chromonemata. These are more distinct in the prophase stage
when chromonemata are less coiled and most clearly visible during leptotene and zygotene
stages of meiotic prophase. The thin and lightly stained parts between the adjacent chromosomes
are termed as inter-chromomeres. The position of chromomeres on chromonemata is constant for
a given chromosome. While pairing during zygotene of meiotic prophase the homologous chro-
mosomes pair chromomere to chromomere. Chromomeres are regions of tightly folded DNA and
are believed to correspond to the units of genetic function in the chromosomes.

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 B.Sc. Botany (H) Paper VI (Cytogenetics and Molecular Biology) Dr Kadambini Das

Chromatid: At mitotic metaphase each chromosome consists of two symmetrical structures

called chromatids. Each chromatid contains a single DNA molecule. Both chromatids are
attached to each other only by the centromere and become separated at the beginning of
anaphase, when the sister chromatids of a chromosome migrate to the opposite poles.

Centromere: A part of the chromosome is recognised as permanent. It is a small structure in the

chromonema and is marked by a constriction. At this point the two chromonemata are joined
together. This is known as centromere or kinetochore or primary constriction. Its position is
constant for a given type of chromosome and forms a feature of identification. In thin electron
microscopic sections, the kinetochore shows a trilaminar structure, i.e., a 10 nm thick dense outer
protein aceous layer, a middle layer of low density and a dense inner layer tightly bound to the
centromere. The chromosomes are attached to spindle fibres at this region during cell division.
The part of the chromosome which lies on either side of the centromere represents arms which
may be equal or unequal depending upon the position of centromere.

Depending upon the number of centromeres, the chromosomes may be:

1. Monocentric with one centromere.
2. Dicentric with two centromeres.
3. Polycentric with more than two centromeres.
4. Acentric without centromere. Such chromosomes represent freshly broken segments of
chromosomes which do not survive for long.
5. Diffused or non-located with indistinct centromere diffused throughout the length of
chromosome.
Depending upon the location of centromere the chromosomes are categorised into:
1. Telocentric are rod-shaped chromosomes with centromere occupying the terminal
position, so that the chromosome has just one arm.
2. Acrocentric are also rod-shaped chromosomes with centromere occupying a sub-terminal
position. One arm is very long and the other is very short.
3. Submetacentric chromosomes are with centromere slightly away from the mid-point so
that the two arms are unequal.
4. Metacentric are V-shaped chromosomes in which centromere lies in the middle of chro-
mosome so that the two arms are almost equal.

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 B.Sc. Botany (H) Paper VI (Cytogenetics and Molecular Biology) Dr Kadambini Das

Centromere controls the orientation and movement of the chromosomes on the spindle. It is the
point where force is exerted when the chromosomes move apart during anaphase.

Secondary Constriction or Nucleolar Organiser: The chromosome besides having the primary
constriction or the centromere possesses secondary constriction at any point of the chromosome.
Constant in their position and extent, these constrictions are useful in identifying particular
chromosomes in a set. Secondary constrictions can be distinguished from primary constriction or
centromere, because chromosome bends only at the position of centromere during anaphase. The
chromosome region distal to the secondary constriction i.e., the region between the secondary
constriction and the nearest telomere is known as satellite. Therefore, chromosomes having
secondary constrictions are called satellite chromosomes or sat-chromosomes. The number of
sat-chromosomes in the genome varies from one species to the other. Nucleolus is always
associated with the secondary constriction of sat-chromosomes. Therefore, secondary
constrictions are also called nucleolus organiser region (NOR) and sat-chromosomes are often
referred to as nucleolus organiser chromosomes. NOR of each sat-chromosome contains several
hundred copies of the gene coding for ribosomal RNA (rRNA).

Telomeres: These are specialized ends of a chromosome which exhibits physiological

differentiation and polarity. Each extremity of the chromosome due to its polarity prevents other

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 B.Sc. Botany (H) Paper VI (Cytogenetics and Molecular Biology) Dr Kadambini Das

chromosomal segments to be fused with it. The chromosomal ends are known as the telomeres.
If a chromosome breaks, the broken ends can fuse with each other due to lack of telomeres.

Karyotype and Idiogram: A group of plants and animals comprising a species is characterized
by a set of chromosomes, which have certain constant features such as chromosome number, size
and shape of individual chromosomes. The term karyotype has been given to the group of
characteristics that identifies a particular set of chromosomes. A diagrammatic representation of
a karyotype of a species is called idiogram. Generally, in an idiogram, the chromosomes of a
haploid set of an organism are ordered in a series of decreasing size.

Uses of Karyotypes:
1. The karyotypes of different groups are sometimes compared and similarities in
karyotypes are presumed to present evolutionary relationship.
2. Karyotype also suggests primitive or advanced feature of an organism. A karyotype
showing large differences between smallest and largest chromosome of the set and
having fewer metacentric chromosomes, is called asymmetric karyotype, which is
considered to be a relatively advanced feature when compared with symmetric karyotype
which has all metacentric chromosomes of the same size. Flowering plants there is a
prominent trend towards asymmetric karyotypes.

Material of the Chromosomes:

The material of the chromosomes is the chromtin. Depending on their staining properties with
basic dyes (particularly the Feulgen reagent), the following two types of chromatin may be
distinguished in the interphase nucleus.
1. Euchromatin: Portions of chromosomes that stain lightly are only partially condensed;
this chromatin is termed euchromatin. It represents most of the chromatin that disperse
after mitosis has been completed. Euchromatin contains structural genes which replicate
and transcribe during G1 and S phase of interphase. It is considered genetically active
chromatin, since it has a role in the phenotype expression of the genes. In euchromatin,
DNA is found packed in 3 to 8 nm fibre.
2. Heterochromatin: In the dark-staining regions, the chromatin remains in the condensed
state and is called heterochromatin. The regions of the chromosome that remains

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 B.Sc. Botany (H) Paper VI (Cytogenetics and Molecular Biology) Dr Kadambini Das

condensed during interphase and early prophase and form the so-called chromocentre.
Heterochromatin is characterized by its especially high content of repetitive DNA
sequences and contains very few, if any, structural genes. It is late replicating (i.e., it is
replicated when the bulk of DNA has already been replicated) and is not transcribed. It is
thought that in heterochromatin the DNA is tightly packed in the 30 nm fibre. It is
established now that genes in heterochromatic region are inactive. During early and mid-
prophase stages, the heterochromatic regions are constituted into three structures namely
chromomeres, centromeres and knobs. Chromomeres may not represent true
heterochromatin since they are transcribed. Centromeric regions invariably contain
heterochromatin; in salivary glands, these regions of all the chromosomes fuse to form a
large heterochromatic mass called chromocentre. Knobs are spherical heterochromatin
bodies, usually several times the diameter of the concerned chromosomes, present in
certain chromosomes of some species, e.g. Maize; knobs are more clearly observable
during pachytene stage in maize. Where present, knobs serve as valuable chromosome
markers. Heterochromatin is classified into two groups: (i) Constitutive and (it)
Facultative.
(i) Constitutive heterochromatin remains permanently in the heterochromatic state,
i.e., it does not revert to euchromatic state, e.g., centromeric regions. It contains
short repeated sequences of DNA, called satellite DNA.
(ii) Facultative heterochromatin is essentially euchromatin that has undergone
heterochromatinization which may involve a segment of chromosome, a whole
chromosome (e.g. one X chromosome of human females and females of other
mammals), or one whole haploid set of chromosomes (e.g., in some insects, such
as mealy bugs).

Chemical Composition:
Chromatin is composed of DNA, RNA and protein. The protein of chromatin is of two types: the
histones and the non-histones. Purified chromatin isolated from interphase nuclei consists of
about 30-40% DNA, 50-65% protein and 0.5-10% RNA: but there is a considerable variation due
to species and tissues of the same species.
DNA: The amount of DNA present in normal somatic cells of a species is constant for that
species; any variation in DNA from this value is strictly correlated with a corresponding

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 B.Sc. Botany (H) Paper VI (Cytogenetics and Molecular Biology) Dr Kadambini Das

variation at the chromosome level. Gametes of a species contain only half of the amount of DNA
present in its somatic cells. The amount of DNA present in somatic cells also depends on the
phase of cell cycle.
Protein: Proteins associated with chromosomes may be classified into two broad groups: (/)
basic proteins or histones and (ii) non-histone proteins. Histones constitute about 80% of the
total chromosomal protein; they are present in an almost 1:1 ratio with DNA (weight/weight).
Their molecular weight ranges from 10,000-30,000 and they are completely devoid of
tryptophan. Histones are a highly heterogenous class of proteins separable in 5 different fractions
designated as H1 H2a, H2b, H3 and H4. Fraction H1 is lysin-rich, H2a and H2b are slightly lysine
rich, while H3 and H4 are arginine-rich. These five fractions are present in all cell types of
eukaryotes, except in the sperm of some animal species where they are replaced by another class
of smaller molecule basic proteins called protamines. Histones play a primary function in
chromosome organisation where H2a, H2b, H3 and H4 are involved in the structural organization
of chromatin fibres, while fraction H1 holds together the folded chromatin fibres of
chromosomes. Non-histone proteins make up about 20% 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 12 to more than 20 different types of non-histone
proteins which show variation from one species to the other and even in different tissues of the
same organism. This class of proteins includes many important enzymes, such as DNA and RNA
polymerases etc.

Ultrastructure of Chromosomes:
Electron microscopic studies have demonstrated that chromosomes have very fine fibrils having
a thickness of 2nm-4nm. Since DNA is 2 nm wide, there is possibility that a single fibril
corresponds to a single DNA molecule. Several models of chromosome structure have been
proposed from time to time based on various types of data on chromosomes.

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 B.Sc. Botany (H) Paper VI (Cytogenetics and Molecular Biology) Dr Kadambini Das

Folded Fibre Model of Chromosomes:

This model was proposed by Du Praw in
1965 and is widely accepted. According
to this model, chromosomes are made up
of chromatin fibres of about 230A°
diameter. Each chromatin fibre 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 230A° chromatin fibre is produced by
coiling of a single DNA double helix, the
coils of which are stabilized by proteins
and divalent cations (Ca++ and Mg++).
Each chromatid contains a single long
chromatin fibres; the DNA of this fibre
replicates during interphase producing
two sister chromatin fibres, it remains
unreplicated in the centromeric region so
that the two sister fibres remain joined in
the region. Subsequently, the chromatin fibre undergoes replication in the centromeric region as
well so that the sister chromatin fibre are separated in this region also. During cell division the
two sister chromatin fibres undergo extensive folding separately in an irregular manner to give
rise to two sister chromatids. Folding of the chromatin fibres drastically reduces their length and
increases their stainability and thickness. This folded structure normally undergoes supercoiling
which further increases the thickness of chromosomes and reduces the length. Most of the
available evidence supports this model. Each chromatid contains a single giant DNA molecule.
The strongest evidence in the support of the unineme model (single stranded chromatid) is
provided by studies on lamp-brush chromosomes.

Organization of Chromatin Fibres:

Any model of chromatin fibre structure has to account for (i) packaging of a very long DNA
molecule into a unit length of fibre; (ii) production of very thick (230-300A0) fibres from very

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 B.Sc. Botany (H) Paper VI (Cytogenetics and Molecular Biology) Dr Kadambini Das

thin (20A°) DNA molecules and (iii) the beads-on-a-string ultrastructure of chromatin fibres
observed particularly during replication. Two clearly different models of chromatin fibre
structure have been proposed:
1. Coiled DNA Model: This is the simplest model of chromatin fibre organization and was
given by Du Praw. According to this model, the single DNA molecule of a chromatin
fibre is coiled in a manner similar to the wire in a spring; the coils being held together by
histone bridges produced by binding histone molecules in the large groove of DNA
molecules. Such a coiled structure that would be stabilized as a single histone molecule
would bind to several coils of DNA. This coiled structure is coated with chromosomal
proteins to yield the basic structure of chromatin fibres (type A fibre) which may undergo
supercoiling to produce the type B fibre of DuPraw which is akin to the beads seen in
electron micrographs of chromatin fibres.
2. Nucleosome-Solenoid Model: This model was proposed by Romberg and Thomas
(1974) and is the most widely accepted. According to this model, chromatin is composed
of a repeating unit called nucleosome. Nucleosomes are the fundamental packing unit
particles of the chromatin and give chromatin a “beads-on-a string” appearance in
electron micrographs that unfold higher-order packing (Olins and Olins, 1974). One
complete nucleosome consists of a nucleosome core, linker DNA, an average of one
molecule of H1 histone and other associated chromosomal proteins.

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 is nuclease resistant.

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 B.Sc. Botany (H) Paper VI (Cytogenetics and Molecular Biology) Dr Kadambini Das

Linker DNA: Its size varies from 8bp to 114 bp depending on the species. This DNA forms the
string part of the beads-on-a string chromatin fibre, and is nuclease susceptible; and the beads are
due to nucleosome cores. Thus, linker DNA joins two neighbouring nucleosomes.

H1 Histone: Each nucleosome contains, on an average, one molecule of HI histone, although its
uniform distribution throughout the length of chromatin fibres is not clearly know. Some studies
suggest that the molecules of H1 histone are involved in stabilizing the supercoils of nucleosome
chromatin fibres. Other studies suggest that HI is associated on the outside of each nucleosome
core, and that one H1 molecule stabilizes about 166 bp long DNA molecule.

Other Chromosomal Proteins: Both linker DNA and nucleosome are associated with other
chromosomal proteins. In native chromatin, the beads are about 110A° in diameter, 60A° high
and ellipsoidal in shape. Each bead corresponds to a single nucleosome core. Under some
conditions, nucleosomes pack together without any linker DNA, which produces the 100A° thick

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 B.Sc. Botany (H) Paper VI (Cytogenetics and Molecular Biology) Dr Kadambini Das

chromatin fibre called nucleosome fibre which may then supercoil to give rise to the 300A°
chromatin fibre called solenoid. The nucleosome model of chromatin fibre structure is consistent
with almost all of the evidence accumulated so far.

Functions of Chromosomes:
The role of chromosomes in heredity was suggested independently by Sutton and Bover in 1902.
This and various other functions of chromosomes may be summarised as under.
1. It is universally accepted that DNA is the genetic material, and that in eukaryotes almost
all the DNA is present in chromosomes. Thus, the most important function of

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B.Sc. Botany (H) Paper VI (Cytogenetics and Molecular Biology) Dr Kadambini Das

chromosomes is to provide the genetic information for various cellular functions essential
for growth, survival, development, reproduction, etc., of organisms.
2. Another very important function of chromosomes is to protect the genetic material
(DNA) from being damaged during cell division. Chromosomes are coated with histones
and other proteins which protect it from both chemical (e.g., enzymes) and physical
forces.
3. The properties of chromosomes ensure a precise distribution of DNA (genetic material)
to the daughter nuclei during cell division. Centromeres of chromosomes perform an
important function in chromosome movements during cell division which is due to the
contraction of spindle fibres attached to the centromeric regions of chromosomes.
4. Gene action in eukaryotes is believed to be regulated through histone and non-histone
proteins associated with chromosomes.

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