Chromosomes were first seen by Hofmeister (1848) in the pollen mother cells of Tradescantia in the form of darkly stained bodies. The term chromosomes (Gr: chrom = colour, soma = body) was used by Waldeyer (1888) to designate their great affinity to basic dyes. W. S. Sutton (1900) described their functional significance.
Chromosomes are the most significant components of the cell, particularly they are apparent during mitosis and meiosis. A chromosome can be considered as a nuclear component having special organisation, individuality and function.
The morphology of chromosome can be best studied at the metaphase or anaphase of mitosis when they are present as definite organelles, being most condensed or coiled.
Number: The number of chromosomes in a given species is usually constant containing diploid number (2n) of chromosomes in their somatic cells and haploid (gametic or reduced) number (n) of chromosomes in their sex cell. The number of chromosomes in variable from one to several hundreds among different species. For e.g., in Ascaris megalocephala it is 2, while in certain protozoans (Aulocantha), there are 1600 and in man there are 46.
Size: Chromosomes range, on an average from 0.5 to about 30 microns in length and from 0.2 to 3microns in diameter. The salivary gland chromosomes of Diptera are 2mm long. Usually all the chromosomes in a cell are of the same size. Plant cell normally posses larger chromosomes than animal cells. Among the animals, grasshoppers, crickets, mantids newts and salamanders have large chromosomes. Variation in size of the chromosomes can be induced by a number of environmental agents like temperature, rate of cell division, etc.
Shape: The shape of the chromosomes is changeable from phase to phase in the continuous process of the cell growth and cell division. In the resting phase or interphase stage of the cell, the chromosomes occur in the form of thin, coiled, elastic and contractile, thread like stainable structures, called the chromatin threads.
Structure: In the earlier light microscopic description of chromosome was thought to consist of a coiled thread called the chromonema lying in a matrix. The chromosome was supposed to be covered by a membrane called pellicle. However electron microscopic studies revealed that there is no pellicle. During metaphase a chromosome appears to possess two threads called chromatids.
Chromonema: The chromatids are really spirally coiled chromonemata (singular chromonema). They may be composed of 2, 4, or more fibrils depending on the species. The fibrils of the chromonema are coiled with each other. The coils are of two types:
1. Paranemic coils: When the chromonemal fibrils are easily separable from each other.
2. Plectonemic coils: When the chromonemal fibrils are closely interwined and they cannot be separated easily.
Chromomeres: The chromonema of thin chromosomes of mitotic and meiotic prophases have been found to contain alternating thick and thin regions and thus giving the appearance of a necklace in which several beads occur on a string. The thick or bead like structures of the chromonema are known as the chromomeres and the region in between the chromomeres is termed as the inter-chromomeres. Actually the chromomeres are regions of the super-Imposed coils.
Centromere: (Kinetochore or Primary Constriction) It is indispensable part of the chromosome and forms the primary constriction of metaphase. The position of the centromere is constant for a particular chromosome. The structure and function of the centromere is different form that of the rest of the chromosome. During division the centromere is functional, while the rest of the chromosome is genetically inactive. The position of the centr4omere varies in different chromosomes. Since the spindle fibers are attached to the centromere, the shape of the chromosome during anaphase will depend on the position of the centromere. Four different categories of chromosomes can be recognized, based on the position of the centromere. They are – Metacentric, Submetacentric, Acrocentric and Telocentric.
In Metacentric chromosomes the centromere is near the middle of the chromosome. The two arms of the chromosome are nearly equal and the chromosome appears ‘V’ shaped during anaphase.
In Submetacentric chromosome the centromere is situated some distance away from the middle; one arm of the chromosome is shorter than the other. Such a chromosome will appear L-shaped during anaphase.
In Acrocentric chromosome the centromere is situated near the end of the chromosomes. It appears rod-shaped.
In Telocentric chromosomes the centromere is truly terminal, i.e., situated at the tip of the chromosome.
Secondary constriction: In addition to the primary constriction or centromere the arms of the chromosome may show one or more secondary constrictions called Secondary Constriction II. These differ from nucleolar organizers called Secondary Constriction I. The location of secondary constriction II is constant for a particular chromosome, and is, therefore, useful for in identification of chromosomes.
Nucleolar Organizer: Normally in each diploid set of chromosomes, two homologous chromosomes have additional ‘constrictions’ called nucleolar organizers. These are so called because they are essential for the formation of nucleolus. The nucleolar organizer appears as a ‘constriction’ near one end of chromosome. The part of the chromosome beyond the nucleolar constriction is very short and appears like a sphere or satellite. In man chromosomes 13, 14, 15, 21, 22, and Y have nucleolar organizers and satellites. Chromosomes bearing satellites are called SAT-Chromosomes. The prefix Sat stands for ‘Sine Acid Thymonucleionico’ (without thymonucleic acid or DNA), since the chromosomes on staining shows relative deficiency of DNA in the nucleolar region.
Euchromatin and Heterochromatin: Certain segment of the chromosomes or the entire chromosomes, are more condensed than the rest of the karyotype during various stages of the cell cycle. Such difference in thickening has been called heteropycnosis. Heteropycnosis may be positive, where there is overcondensation or negative, where there is undercondensation. Chromosomes, which remain condensed during interphase, are called heterochromosomes and the non-condensed chromosomes, which extend during interphase, are called euchromosomes.
Chromatin material is of two types- heterochromatin and euchromatin. Chromatin material showing heteropycnosis at any stage is called heterochromatin. Regions of the chromosomes, which never show heteropycnosis, consist of euchromatin. Heterochromatin stains deeply whereas euchromatin stains less deeply. Heterochromatin is found in the condensed region of the chromosome, and is associated with tight folding and coiling of the chromosome fibre. Euchromatin consists of the diffused or less tightly coiled regions. There are two types of heterochromatin, constitutive heterochromatin and facultative heterochromatin. Constitutive heterochromatin shows heteropycnosis in all cell types and throughout the cell cycle. Facultative heterochromatin on the other hand is hyperpycnotic only in some special cell types, or at some particular stages of the life cycle.
ULTRA STRUCTURE OF CHROMOSOME
Two views have been proposed for ultra structure of chromosome.
1. Multistranded view: This was proposed by Ris (1966). By electron microscope the smallest visible unit of the chromosome is the fibril which is 100Ao in thickness. This fibril contains two DNA double helix molecules. Next largest unit is the half chromatid. The half chromatid consists of four 100Ao fibrils so that it is 400Ao in thickness and contains eight double helixes of DNA. Two half chromatids form a complete chromatid consisting of 16 double DNA helix molecules. As the chromosome consists of two chromatids, thus total number of helixes will be 32 and diameter 1600Ao thick before duplication or synthesis. In summary, the chromosome is composed of numerous micro fibrils, the smallest of which is a single nucleoprotein molecule.
2. Folded - Fibril Model: Dupraw (1965) presented this model for the fine structure of chromosome. According to this model, a chromosome consists of single long chain of DNA and protein forming what is called a fibril. The fibril is folded many times and irregularly entwined to form the chromatid. This measures 250 – 300Ao in thickness. Dupraw’s model of DNA association is now considered unlikely by the discovery that DNA itself looped around histone beads to form nucleosomes.
3. Nucleosome: The chromatin is formed of repeating units called nucleosomes. The term was given by Oudet et al, (1975). The nucleosome is made up of DNA and histone proteins. The proteins form a core particle, which consists of two molecules of each of four histone proteins – H2a, H2b, H3 and H4. The surface of core particle is surrounded by 1.75 turns of DNA (200 base pairs). The DN linking the core particle is called linker DNA. An other histone protein H1, is bounded to the linker DNA.