Download Eukaryotic Chromosomes

Text
Historical Background
Chromosomes were first observed as rod-like structures
in plant cells by Karl Wilhelm von Nageli in 1842. E. Russow
in 1872 made the first serious attempt to describe
chromosomes. In 1873, A. Schneider published a most
significant paper dealing with the relation between
chromosomes and the stages of cell division. E. Strasburger
(1875) discovered thread-like structures which appeared
during cell division. Walther Flemming in 1879 introduced the
term chromatin to describe the thread-like material of the
nucleus that became intensely coloured after staining. The
name chromosome (Gr. chroma, colour+soma, body) was
coined by the German anatomist, W. Waldeyer in 1888 for
the darkly stained bodies of nucleus. In 1902 Sutton and
Boveri suggested that chromosomes were the physical
structures which acted as messengers of heredity. Morgan
(1933) discovered the function of chromosomes in
transmission of hereditary traits.
Viral chromosomes
The
chromosomes
of viruses are
called
viral
chromosomes. They occur singly in a viral species and
chemically may contain either DNA or RNA. It is a
distinguishing feature of viruses from all living organisms
which possess both DNA and RNA (Fig. 1). DNA-containing
viruses are called deoxyviruses and RNA-containing viruses
as riboviruses. The DNA-containing viral chromosomes may
be either of linear shape (e.g., T2, T3, T4, T5,
bacteriophages), or of circular shape (e.g., most
animal viruses and certain bacteriophages). The RNAcontaining viral chromosomes are composed of a linear,
single-stranded RNA molecule and occur in some animal
viruses (e.g., poliomyelitis virus, influenza virus, etc.);
most plant viruses (e.g., tobacco mosaic virus) and some
bacteriophages (Fig. 2). Both types of viral chromosomes are
either tightly packed within the capsids of mature virus
particles (virons) or occur freely inside the host cell. The
nucleic acid in a virus could be either single stranded or
double stranded. The genomes vary in size from around 1 kb
to nearly 30 kb, and replicate using combinations of viral and
cellular enzymes.
Fig. 1 Schematic diagram illustrating the components of
the complete virus particle (the virion). A- enveloped virus
with icosohedral symmetry, B-virus with helical symmetry.
Fig. 2 Schematic representation of tobacco mosaic
virus
Bacterial Chromosomes
In bacteria, which being prokaryotic organisms, the
entire hereditary material is packed into a single, irregularly
folded compact mass called nucleoid or genophore or
bacterial chromosome (Figure 3). It is short and simple
consisting of a single DNA molecule. A nucleoid has no
ribosome and nucleolus. The DNA is in the form of a double
helix which forms a closed ring or circle with no free ends. It
is permanently attached to a mesosome, an infolding of the
plasma membrane. The nucleoid has a very high DNA
concentration as well as containing all the proteins associated
with DNA, such as polymerases, repressors and others such
as HU and H-NS. The genome is organized into 50-100 large
loops or domains of 50-100 kb in length, which are
constrained by binding to a membrane-protein complex. The
genome is negatively supercoiled. Individual domains may be
topologically independent, that is they may be able to
support different levels of supercoiling. The DNA domains are
compacted by wrapping around non-specific DNA-binding
proteins, such as HU and H-NS (histone-like proteins). These
proteins constrict about half of the supercoiling of the DNA.
Other molecules such as integration host factor, RNA
polymerase and mRNA may help to organize the nucleoid.
In addition to the nucleoid, a bacterial cell may show the
presence of extra chromosomal DNA molecules, called
plasmids (Fig. 3). Like the bacterial chromosome, plasmids
are double stranded circular DNA, molecules which can
replicate and function independently. The plasmids mainly
carry genes responsible for characteristics like fertility,
antibiotic resistance, etc. The plasmids can be easily isolated
from or introduced into the bacterial cells. They can be
integrated with desired genes. Hence, plasmids are of
immense use in genetic engineering.
Fig. 3 Bacterial Chromosome
Eukaryotic Chromosomes
The
eukaryotes
usually
contain
much
more
genetic information than
the
viruses and
prokaryotes;
therefore, they contain a great amount of genetic material or
DNA molecule which may not occur as a single unit, but
many
units called chromosomes.
Different
species of
eukaryotes have different, but always constant and
characteristic number of chromosomes.
The eukaryotic chromosomes differ from the prokaryotic
chromosomes in morphology, chemical composition and
molecular structure. The shape of the eukaryotic
chromosomes is changeable from phase to phase in the
continuous process of the cell growth and cell division. They
are thin, coiled, elastic, thread-like structures during the
interphase and are called chromatin threads. The morphology
of the chromosome is best studied during metaphase and
anaphase, which are the periods of maximal contraction (Fig.
4). During these phases, the chromatin threads become
highly coiled and folded to form compact and individually
distinct ribbon-shaped chromosomes (Fig. 5). The study of
chromosomes is of utmost importance in biology, because it
allows one to follow the behaviour of DNA molecules and
genes in a visual way.
Fig. 4 Human chromosomes during metaphase
Fig. 5 Highly coiled chromosomes
A chromosome is usually composed of the following
parts:
-Pellicle and matrix
-Chromatid
-Chromonema
-Chromomeres
-Centromere
-Secondary constriction and
-Satellite bodies
Pellicle and matrix: Each chromosome is apparently
bounded by a thin membrane, called pellicle. The pellicle
encloses a jellylike substance called matrix. Both the pellicle
and matrix are composed of achromatic or non-genetic
material and appear only at metaphase, when the nucleolus
disappears. It is believed that nucleolar material and matrix
are interchangeable, i.e., when matrix disappears, nucleolus
appears and vice versa. Their true structure and function are
not known because they are not stainable with dyes. Most
probably, the matrix aids in keeping the chromonemata
within bounds, so that the manoeuvres of the chromosome
during cell division can take place unhindered. It may also
serve as an insulating sheath for the genes during cell
division. The recent cytological findings have rejected the
view that chromosomes have pellicle and matrix.
Chromatid: At metaphase, each chromosome consists of
two symmetrical structures, the chromatids, each one of
which contains a single DNA molecule. The chromatids are
attached to each other only by the centromere and becomes
separated at the start of anaphase, when the sister
chromatids migrate to opposite poles. Therefore, anaphase
chromosomes have only one chromatid, while metaphase
chromosomes have two (Fig. 6).
Fig. 6 Chromatids
Chromonemata: During prophase and sometimes during
interphase, the chromosomal material becomes visible as
very thin filaments, which are called chromonemata and
which represent chromatids in early stages of condensation.
They were first of all observed by Baranetzky in 1880 in the
pollen mother cells of Tradescantia, but the term
chromonema was coined by Vejdovsky in 1912. The terms
chromatid and chromonemata are two names for the same
structure, a single linear DNA molecule with its associated
proteins. The chromonemata form the gene-bearing portions
of the chromosomes, although each chromonema may
contain non-genic materials that serve to maintain its
integrity. The chromonemata are embedded in a
proteinaceous substance called matrix, which is covered by a
sheath called pellicle. Each chromonemata is 800Ǻ thick and
contains 8-microfibrils, each of which in its turn contains two
double helices of DNA. Both chromonematae remain
intimately coiled in spiral manner with each other and have a
series of microscopically visible, bead-like swellings along its
length, called chromomeres. The early geneticists have
attached great significance to the chromomeres and,
errorneously considered them as hereditary unit, hereditary
or Mendelian factors or genes; but modern cytological
investigations have confirmed that the chromomeres are not
genes but the regions of super-imposed coils.
Chromomeres: Chromomeres are bead-like bodies over the
chromonemata. The regions between the chromomeres are
called inter-chromomeres. Chromomeres are especially
obvious in polytene chromosomes, where they become
aligned side by side, constituting the chromosome bands.
These tightly folded regions of DNA are of considerable
interest, because they may correspond to the units of genetic
function in the chromosome. At metaphase, the chromosome
is tightly coiled and the chromomeres are no longer visible.
Centromere: The centromere is the constricted region
where the two sister chromatids are joined in the metaphase
chromosome. This is the site of assembly of the kinetochore,
a protein complex which attaches to the microtubule of the
mitotic spindle. The microtubules act to separate the
chromatids at anaphase. The DNA of the centromere has
been shown in yeast to consist merely of a short AT-rich
sequence of 88 bp, flanked by two very short conserved
regions, although in mammalian cells, centromeres seem to
consist of rather longer sequences, and are flanked by a
larger quantity of repeated DNA, known as satellite DNA.
Within the centromere region, most species have several
locations where spindle fibers attach, and these sites consist
of DNA as well as protein. The actual location where the
attachment occurs is called the kinetochore and is composed
of both DNA and protein (Fig. 7). The DNA sequence within
these regions is called CEN DNA. Because CEN DNA can be
moved from one chromosome to another and still provide the
chromosome with the ability to segregate, these sequences
must not provide any other function.
Fig. 7 Position of centromere and kinetochore in the
chromosomes
The number and position of centromeres is variable, but
is definite in a specific chromosome of all the cells and in all
the individuals of the same species. Thus, according to the
number of the centromere, the eukaryotic chromosomes may
be acentric (without any centromere and these represent
freshly broken segments of chromosomes which do not
survive for long and are left in the cell cytoplasm during cell
division); monocentric (with one centromere); dicentric (with
two centromeres) or polycentric (with more than two
centromeres). Chromosomes from organisms, such as some
plants, in which spindle microtubules attach along the
chromosome length rather than at a specific site, are termed
holocentric. Occasionally, dicentric chromosomes arise. If
both of these centromeres function, this can lead to errors in
chromosome segregation. However, in such cases only one of
the two centromeres is functional while the other is
inactivated. The centromere has small granules or spherules
and divides the chromosomes into two or more equal or
unequal chromosomal arms.
A chromosome may be characterized by its total
length and the position of its centromere. The position of
centromere varies from chromosome to chromosome and it
provides following different shapes to the chromosomes
(Fig.8):
Telocentric:When the centromere is at the terminal
or proximal position, the chromosome is called telocentric.
The chromosomes are rod-shaped. Telocentric chromosomes
are very rare. In telocentric chromosomes, one arm is very
long and the other one is very short. The short chromosome
arm is designated p (petite) and the long arm q (one letter
after p).
Acrocentric: When the centromere is situated near
one end forming a long arm and a very short arm or even
imperceptible short arm and the chromosome appears rodshaped.
Submetacentric: When the centromere is situated
slightly away from the middle and one arm of the
chromosome will be shorter than the other. Such a
chromosome will appear L-shaped during anaphasic
movement.
Metacentric: When the centromere lies at or near
the middle of the chromosome and the two arms of the
chromosome are almost equal, then it is called as
metacentric.
During
anaphasic
movements,
the
chromosomes bend at the centromere, so that the
metacentric chromosomes are V-shaped.
Telocentric
Metacentric
Acrocentric
Submetacentric
Fig.8 Shapes of chromosomes on the basis of position of the
centromere
Telomere: Telomeres are the specialized structures at
the ends of the linear DNA molecules of eukaryotes (Fig. 9).
Telomeres ‘hide’ the ends of the chromosome from the
mechanisms within the cell that monitor DNA damage. They
are also needed to overcome the problem of end replication.
The end-replication problem arises because all known DNA
polymerases add nucleotides to a free 3´ OH, i.e. they work
only in the 5´ to 3´ direction. Replicative DNA synthesis is
primed from an RNA primer that is subsequently removed. At
the extreme 3´ end of a linear DNA strand being copied by
lagging strand synthesis, removal of the RNA primer and
ligation of Okazaki fragments leaves a gap at the end of the
new strand. Because this is at the end of the DNA molecule
or chromosome, there is no DNA template beyond this from
which to prime synthesis of DNA across this gap. Hence,
without a mechanism to counteract this, the ends of linear
DNA molecules would get progressively shorter through
subsequent rounds of conventional DNA replication.
Most eukaryotes overcome the end-replication problem
with an enzyme called telomerase (a ribonucleoprotein) that
uses its own RNA template to add on simple repeats to the
3´ ends of chromosomes elongating them. Conventional DNA
polymerases can use this extended DNA strand as a template
on which to synthesize the complementary strand. The
telomere sequence added is very similar in a wide variety of
eukaryotes. A telomere consists of up to hundreds of copies
of a repeated sequence of (5´-TTAGGG-3´ in humans),
which is synthesized by the enzyme telomerase, a
ribonucleoprotein. Large stretches of telomere-like repeats
are found at the ends of chromosomes. The telomeres of
Drosophila melanogaster are unusual in that mobile
repetitive sequences (non-long terminal repeat (LTR)
retroposons) are found at the ends of the chromosomes
rather than tandem repeats of TTAGGG.
Telomerase activity is highest in the germline cells.
Somatic cells have little telomerase and they have shorter
telomeres than cells of the germline. It is estimated that 50–
200 bp of DNA is lost from the end of the chromosome per
cell division in the absence of telomerase activity. Telomere
length also decreases with age and this has led to
speculation that telomere shortening may play some role in
ageing. Indeed, in the human syndrome of premature ageing
telomeres are excessively short.
Centromeres and telomeres are two essential features
of all eukaryotic chromosomes. Each provides a unique
function that is absolutely necessary for the stability of the
chromosome. Centromeres are required for the segregation
of the chromosome during meiosis and mitosis, and
teleomeres provide terminal stability to the chromosome and
ensure its survival.
Fig. 9 Telomere-the ends of linear chromosomes
Secondary Constrictions: Sometimes, a chromosome may
have an additional constriction apart from the centromere,
called
secondary
constriction
(Fig.
10).
Secondary
constrictions mark the locations at which nucleoli are
assembled. The chromosomes possess secondary constriction
at any point. These contain the genes coding for 18S and
28S ribosomal RNA and induce the formation of nucleoli. The
secondary constrictions may arise because the rRNA genes
are transcribed very actively, interfering with chromosomal
condensation. It is often associated with the nucleolus during
interphase and may take part in the reorganization of the
nucleolus at the end of cell division and because of this
reason secondary constriction may also be called a nucleolusorganizing region (Fig.11). It appears as a light staining
region with an additional segment of the chromosome
beyond it, called satellite body or trabant. Chromosomes
having a satellite are marker chromosomes and are called
SAT-chromosomes.
Secondary
constriction
can
be
distinguished from primary constriction or centromere,
because chromosome bends or shows angular deviation only
at the position of centromere.
Satellite bodies: The part of the chromosome which is
present beyond the secondary constriction is called satellite
body. It appears as rounded or knob-like appendage and
varies in size according to the position of the secondary
constriction. The satellites remains connected with the rest of
the chromosome by a thin chromatin filament. Chromosomes
bearing satellites are called SAT-chromosomes (sine
acidothymonucleinico). The shape and size of the satellites
remain
constant
for
each
particular
chromosome.
Chromosome satellites are a morphological entity and should
not be confused with satellite DNAs, which are highly
repeated DNA sequences.
A-external structure
B-
internal structure
1. Two chromonemata
1. Secondary constriction
2. Satellite
2. Satellite
3. Centromere
3. Centromere
Fig. 10 Internal and external structure of the chromosome
showing its various components
Fig. 11 Figure showing the position of nucleolar organizer in
the chromosome: A. Nucleolar organizer, B. Chromosome, C.
Nucleolus
Chromosome Number
The number of chromosomes is fixed for a given
species. It varies from a minimum of two to several hundred
in different species of plants and animals. All individuals of a
given species show the same chromosome number in all their
body cells. The number of chromosomes in a species has no
specific significance, nor does it indicate any relationship
between two species which may have the same chromosome
number. It is used in the identification of species and in
tracing the relationship within the species. The chromosome
number of a given species is generally represented as the
diploid number (2n) since chromosomes occur in pairs.
In some cases, males have one chromosome less
than females. Male honey bees are haploid (monoploid)
whereas females are diploid. In some cases, the chromosome
number will be haploid (n) indicating the presence of
unpaired chromosomes. Haploid number is a characteristic
feature of gametes, the sperm and the ovum in animals or
the gametophytic generation in plants. Haploid chromosome
number may be seen in some adult organisms also. Yeasts
show both haploid and diploid cells. In animals, the lowest
chromosome number is found in Ascaris megalocephala
(2n=2), while the highest number of chromosomes is found
in a radiolarian protozoan-Aulocantha (2n=1600). In plants,
lowest chromosome number is found in Mucorheimalis, a
bread mold fungus (2n=2), and the highest chromosome
number has been reported in Ophioglossum (2n=1262).
Size of Chromosomes
The genome size in eukaryotes varies widely, from those
of the yeasts (17 million base pairs) to those of vertebrates
(3000 million base pairs or more). The genomes of some
plants are huge. In Lilium longiflorum, for example, the
genome consists of 300 000 million base pairs. Similarly, the
size and number of chromosomes in any particular species
varies widely. A minimum size is required for a stable
eukaryotic chromosome. Small yeast artificial chromosomes
(YACs), based on Saccharomyces cerevisiae, are stabilized by
the presence of additional DNA between the centromere and
telomere, and the minimal size appears to be 50 kb. A
maximum limit also exists for chromosome size. It has been
suggested that the longest chromosome arm must not be
longer than half the length of the spindle axis at telophase.
In most organisms, chromosomes fall in the size
range of 0.1 to 30 μm in length and 0.2 to 2.0 μm in
thickness. Chromosomes are relatively larger in size where
the number is less. In general, chromosomes are larger in
size in plants than in animals. Among plants, monocots
contain larger chromosomes than dicots.
Chemical composition
The chromosomes are generally composed of chromatin,
a complex of DNA and proteins; most are about 40% DNA
and 60% protein. A significant amount of RNA is also
associated with chromosomes because chromosomes are the
sites of RNA synthesis. The DNA of a chromosome is one
very long, double-stranded fiber that extends unbroken
through the entire length of the chromosome. The most
convenient measurement of DNA is pictogram (10-12 gm).
The proteins are of two major classes-histone and nonhistone proteins. Histones are very basic proteins, because
they have a high content (20-30%) of the basic amino acids
arginine and lysine and having high molecular weight. Being
basic, histones bind tightly to DNA, which is acidic. As many
as five histones are found in the chromosomes namely H1,
H2A, H2B, H3 and H4. Histones play a major structural role in
chromatin. The histones have been highly conserved during
evolution, four of the five types of histones being very similar
in all higher eukaryotes. The non-histone protein fraction of
chromatin consists of a large number of very heterogenous
proteins. Heterogeneity of these non-histone proteins
suggested that these proteins are not as conserved in
evolution as histones. The non-histone proteins differ even
between different tissues of the same organism suggesting
that they regulate the activity of specific genes. The nonhistone
proteins
are
mostly
acidic
and
have
been considered more important than histones as regulatory
molecule. Some non-histone proteins also have enzymatic
activities. The most important enzymatic proteins of
chromosomes are phosphoproteins, DNA polymerase, RNApolymerase,
DPN-pyrophosphorylase,
and
nucleosidetriphosphatase. The metal ions as Ca+ and Mg+
are supposed to maintain the oragnization of chromosomes
intact. In spermatozoa of fishes, protamines are found. The
protamines are low molecular weight basic proteins rich in
the amino acid arginine.
Karyotype
The name karyotype is given to the whole group of
characteristics that allows the identification of a particular
chromosomal set, i.e., the number of chromosomes, relative
size, position of the centromere, length of the arms,
secondary constrictions and satellites. It is characteristic of
an individual, species, genus, or larger grouping, and may be
represented by a diagram called karyogram or idiogram (Gr.,
idios, distinctive+gramma, something written). Generally, in
an ideogram, the chromosomes of a haploid set of an
organism are ordered in a series of decreasing order (Fig.
13). Sometimes, an ideogram is prepared for the diploid set
of chromosomes, in which the pairs of homologues are
ordered in a series of decreasing order. Some species may
have special characteristics, e.g., the mouse has acrocentric
chromosomes, many amphibian have only metacentric
chromosomes, and plants frequently have heterochromatic
regions at the telomeres.
The study of karyotypes (karyology) is made by
staining the chromosomes with a suitable dye, such as
Giemsa, after cells have been arrested during cell division by
colchicine. Sometimes observations may be made on nondividing cells. Karyotypes can be used for many purposes,
e.g., to study chromosomal aberrations, cellular function,
taxonomic relationships, and to gather information about
evolutionary events.
Fig. 13 Karyogram of human male using Giemsa staining (A)
the chromosomes as visualized as they originally spilled from
the lysed cell. (B) the same chromosomes artificially lined up
in order. In this karyotype, the homologous chromosomes
are numbered and arranged in pairs; the presence of a Y
chromosome indicates that the DNA was isolated from a
male. (From E. Schröcketal., 1996, Science 273:494–497).
Special Types of Chromosomes
The eukaryotes besides possessing the usual type of
chromosomes in their body cells contain some unusual and
special types of chromosomes in some body cells or at some
particular stage of their life cycle. The special types of
chromosomes are:
1. Lamp Brush Chromosomes
These chromosomes occur in the oocytes (germ cells
in the ovary) of amphibians and in some insects. They are
extremely large synapsed homologous chromosomes which
can be seen in the diplotene stage of prophase-I in meiosis.
They measure about 1500 to 2000 µm in length (Fig. 14).
The lampbrush chromosomes were discovered by Ruckert in
1892. They are formed during the active synthesis of mRNA
molecules for the future use by the egg during cleavage
when no synthesis of mRNA molecules is possible due to
active involvement of chromosomes in the mitotic cell
division. A lampbrush chromosome consists of an axis from
which paired loops extend in opposite directions, giving the
appearance of a lamp brush (Fig.15). The axis consists of
chromomeres and interchromomere regions. The loops
consist of transcriptionally active DNA which can synthesize
large amount of mRNA, necessary for the synthesis of yolk.
Fig. 14 Lampbrush Chromosomes
Fig. 15 (a) Lampbrush chromosomes at low magnification: ALoop, B-Chromosome axis. (b) Lampbrush chromosomes at a
loop magnified:1-Chromosome axis, 2-Chromosome fibers,3Matrix (RNA and proteins).
2.
Polytene
Chromosomes
(Salivary
gland
chromosomes)
These are giant chromosomes found in the salivary
gland cells of the fruitfly Drosophila and Chironomus (Fig.
16). These were first observes by E. G. Balbiani, an Italian
cytologist in 1881 in the salivary glands of Chironomus and
hence are called salivary gland chromosomes. They are many
times larger than the normal chromosomes reaching a length
of 2000µm and are visible even under a compound
microscope (Fig. 17). This is particularly surprising, since the
salivary gland cells do not divide after the glands are formed,
yet their chromosomes replicate several times (a process
called endomitosis) and become exceptionally giant-sized to
be called polytene or multistranded chromosomes. The
polytene chromosomes of the salivary gland cells of
Drosophila melanogaster are formed by nine or ten
consecutive multiplication cycles and remain associated
parallel to each other. Further, the polytene chromosomes
have alternating dark and light bands along their length. The
dark bands are comparable with the chromomeres of simple
chromosomes and are disc-shaped structures occupying the
whole diameter of chromosome. They contain euchromatin.
The light bands or inter bands are fibrillar and composed of
heterochromatin.
If the polytene chromosomes of dipteran larval
salivary glands are examined at several stages of
development, it is seen that specific areas (sets of bands)
enlarge or "puff". These enlargements are called
chromosomal puffs or Balbiani rings (Fig. 18). These regions
contain actively transcribing DNA involved in the synthesis of
RNA types. Such puffs change location as development
proceeds, those at specific locations being correlated with
particular developmental stages. This temporal puffing
indicates changes in gene activity and involves several
processes such as the accumulation of acidic proteins,
despiralization of DNA, formation of chromonemal loops
called Balbiani rings at the lateral sides of dark bands,
synthesis of mRNA and storage of newly synthesized mRNA
around the Balbiani rings.
Fig. 16 Drosophilla showing the presence
Salivary gland chromosomes
Fig. 17
of salivary gland chromosomes
Fig. 18 Various components of polytene chromosomes: AmRNA, B-Chromosome puff,C-Chromonemata, D-Dark band,
E-Interband
3. B-Chromosomes: Many plant and animal species,
besides having autosomes and sex-chromosomes possess a
special category of chromosomes called B-chromosomes
without obvious genetic function. These B-chromosomes also
called
supernumerary
chromosomes,
accessory
chromosomes or accessory fragments etc. usually have a
normal structure, are somewhat smaller than the autosomes
and can be predominantly heterochromatic (many insects,
maize etc.) or predominantly euchromatic (rye).
In maize, their number per cell can vary from 0 to 30
and they adversely affect development and fertility only
when occur in large amount. In animals, the B-chromosomes
disappear from the non-reproductive tissue and are
maintained only in the cell-lines that lead to the reproductive
organs. B-chromosomes have negative consequences for the
organism, as they have deleterious effect because of
abnormal crossing over during the meiosis of animals and
abnormal nucleus divisions of the gametophyte plants. In
animals, B-chromosomes occur more frequently in females
and the basis is non-disjunction. The non-disjunction of Bchromosomes of rye plant is found to be caused due to the
presence of a heterochromatic knob at the end of long arm of
B-chromosome.
Heteropycnosis
When the prophase chromosomes are
stained with Feulgen, they take differential staining. This
phenomenon is called heteropycnosis. The darkly stained
regions were called heterochromatic and light regions were
called euchromatic (E. Heitz, 1928). Heterochromatin
comprises a portion of the chromatin which remains highly
compacted. It can be visualized under the microscope as
dense regions at the periphery of the nucleus, and probably
consists of closely packed regions of 30nm fiber, which
represents the configuration of transcriptionally inactive
chromatin. It is believed that much of the heterochromatin
may consist of the repeated satellite DNA close to the
centromere of the chromosomes, although in some cases
entire chromosomes can remain as heterochromatin, e.g.,
one of the two X chromosomes in female mammals. The
heterochromatin controls the metabolism of chromosome,
biosynthesis of the nucleic acids and the energy metabolism.
Two types of heterochromatin are
generally recognized- constitutive heterochromatin and
facultative heterochromatin. Constitutive heterochromatin is
one which is permanently condensed in all types of cells. It is
the most common type of heterochromatin and serves as
chromosome markers. Facultative heterochromatin is
condensed only in certain cell types or at special stages of
development.
In
facultative
heterochromatin,
one
chromosome of the pair becomes either totally or partially
heterochromatic, e.g. in female humans, one X-chromosome
is inactivated or becomes heterochromatic only facultatively.
Similarly in plants, accessory chromosomes in general, and
among diecious genera like Melandrium and Rumex, one or
both sex chromosomes may undergo partial or complete
heterochromatization.
Euchromatin is the more diffuse region of
the interphase chromosome, consisting of inactive regions in
the 30nm fiber. It is the region where all transcription takes
place. It represents most of he chromatin that disperse after
mitosis has completed. Euchromatin contains structural
genes which replicate and transcribe during G1 and S phase
of interphase. The euchromatin is considered genetically
active chromatin, since it has a role in the phenotype
expression of the genes.
Significance of Chromosomes
1. Chromosomes are hereditary vehicles, carrying genes
from parents to offsprings.
2. Chromosomes are the essential unit for cellular division
and must be replicated, divided, and passed successfully
to their daughter cells so as to ensure the genetic
diversity and survival of their progeny.
3. Chromosomal recombination plays a vital role in genetic
diversity.
4. These control metabolism and also all other activities of
cell by directing synthesis of required enzymatic
proteins.
5. These bring about continuity of life by replication.
6. Nucleolus formation is controlled by the nucleolar
organizer region of certain chromosomes.
7. Chromosomes play important role in sex determination.
8. Chromosomes undergo mutations and contribute to
evolution in all living organisms.