The following points highlight the six main parts of a chromosome. The parts are: 1. Pellicle and Matrix 2. Chromatids, Chromonema and Chromomeres 3. Centromeres 4. Secondary Constriction 5. Satellite 6. Telomere.
Part # 1. Pellicle and Matrix:
A membrane which surrounds each chromosome is said as pellicle. A jelly substance present inside the membrane is called as matrix. Presumably the matrix and sheath are considered as non-genetic material. But the existence of a matrix and pellicle has not been supported by electron microscopic observations and several workers including Darlington have questioned the reality of matrix.
Part # 2. Chromatids, Chromonema and Chromomeres:
The most important and constant constituent of the cell nucleus is chromatin. The chromosomes are made up of chromatin. The structure of the chromosome is best studied in late prophase, metaphase and anaphase. During early prophase a chromosome appears to consist spirally coiled thin or filamentous long continuous structure called as chromonema (plural—chromonemata).
The chromonema thread is marked at intervals in a linear order by a number of knot or granular or beaded like structure arranged vertically in a single row along its length called as chromomeres. These are clear as small dense like structure at fixed intervals along the total length of chromonema thread and appears similar to the beads on a string.
The number and position of chromomere is found relatively constant in a chromosome on basis of which a chromosome can be differentiated from the other chromosomes. These chromomeres are responsible to carry the genes during inheritance.
In late mitotic prophase, the chromosomes appear as thick, dark and of various shape and sizes. Under electron microscope (magnification of about 2 lacs), chromosomes at metaphase stage are visible clearly as a double structure consisting of two chromonemata which become inter-wined with each other, has its own set of chromomeres and is known as chromatid. Thus, there are 2 chromatids in a chromosome.
The two chromatids are joined together at a point along their length which is said as centromere or primary constriction or kinetochore. The centromere is clear, non-stainable part of the chromosome. The centromere has a significant role in nuclear division. It performs two role i.e., holding the two chromatids and binding the spindle fibres. It may be single at one stage, double or four stranded at other stage like Meiosis, Prophase I Pachytene.
Some geneticists believe that the chromomere simply represent groups of superimposed coils, therefore, they appear like beads whereas other geneticists believe that the chromomeres represent points of condensation of nucleoprotein material.
Belling (1928) suggested that the chromomeres correspond to genes. Callon and Lloyd (1960) on lampbrush chromosomes and Beerman (1967) on mutations in polytene chromosomes suggested that a chromomere might be linked with the activity of one gene.
Part # 3. Centromeres (= Primary constriction):
The constricted non-stained portion of chromosome is known as centromere. It is the specific part of the chromosome where spindle fibres are attached. The position of centromere is constant for a particular chromosome.
The structure and function of the centromere is different from that of the rest of chromosome. During division, the centromere is functional while the rest part of the chromosome is genetically inactive.
In mitosis, metaphasic chromosomes which consist of two chromatids, four granules may be seen within the centromere. These granules are called centromeric chromomeres, and are about 0.5 micron in size. They are arranged in square.
During anaphase, when the two chromatids separate, each chromatid (chromosome) show two granules. At the same time duplication of the centromere occurs during mitosis and meiosis. The centromere is considered to be an extended region of the chromonema.
Depending upon position of centromere and comparative length of two arms of the chromosome, chromosomes may be of following types:
I-shaped, rod like chromosomes having one arm of very small size. Thus, arms are unequal. Position of centromere is sub-terminal.
(ii) Sub-metacentric (= Heterobrachial):
‘L’ or J-shaped. Centromere is not present in the centre of the chromosome. Arms are unequal.
(iii) Metacentric (= Isobrachial):
V shaped. Centromere is present in the centre of chromosome. Arms are equal in length.
Darlington (1939) defined telocentric chromosome as chromosome having a terminal centromere. I-shaped, single armed.
(v) Dicentric or Polycentric:
Chromosomes having two or more centromeres.
When the centromere is absent in the chromosome.
Part # 4. Secondary Constriction:
Normally each diploid set of homologous chromosome, exhibit additional ‘constrictions’ (one or more) called secondary constriction. This is also said as ‘nucleolar organising region’. These are so called because they are necessary for the formation of the nucleolus.
Nuclear organizers are probably of universal occurrence in several plants and animals. Each nucleolar organizer may be one or two in number and may be attached to one nucleolus. The nucleolar organizer represents about 0.3% of the total amount of nuclear DNA.
Part # 5. Satellite:
This is another structure exhibited by some chromosomes. These are terminal portion of the chromosome beyond the secondary constriction. Satellites are elongated, round shaped, variable in size. The chromosome with a satellite is referred to as ‘SAT-chromosome’. There are at-least two SAT-chromosomes in each diploid nucleus. Many polyploid species, however, have only two SAT-chromosomes. For example, hexaploids wheat.
Part # 6. Telomere:
The tips or the terminal ends of chromosomes are called telomeres. The telomere differs in structure and composition from the rest of the chromosome. It has a unique property that it prevents the ends of the chromosomes from sticking to each or one another.
If the chromosomes ends are broken by X-ray radiation, the newly formed broken ends undergo fusion. A telomere will not, however, unite with a broken end, nor will two telomeres unite. Telomeres are specially modified portion of the chromosomes for attachment to the nuclear membrane.