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 ad ova is known as the gametic, reduced or haploid sets 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 knows 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 suffix”-ploid” refers to chromosome “sets”. The prefix indicates the degree of the ploidy.
The number of chromosomes in each somatic cell is the same for all members of a given species. The organism with the lowest number of the chromosomes is the mamatode, Ascaris magalocephalus 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 gracilic to 2n=1200.
Lastly, while ‘n’ normally signifies the gametic or haploid chromosome number, ‘2n’ is the somatic or diploid chromosome number in an individual. In polyploidy individuals, however, it becomes necessary to establish an ancestral primitive number, which is represented as ‘x’ and is called the base number.
Autosomes and Sex Chromosomes
In a diploid cell, there are two of each kind of chromosome ( these are termed homologous chromosomes), except for the sex chromosomes. One sex has two of the same kind of sex chromosome and the other has one of each kind. For example, in human, there are 23 pairs of homologous chromosomes (i.e., 2n=46); a chromosome number which was established by Tijo and Levan in 1956). The human female has 44 non sex chromosomes, termed autosomes and one pair of homomorphic (morphologically similar) sex chromosomes given the designation XX. The human male has 44 autosomes and one pair of heteromorphic or morphologically dissimilar sex chromosomes, i.e., one X chromosome and one Y chromosome.
The size of chromosome is normally measured at mitotic metaphase and may be as short as 0.25 µm in fungi and birds, or as long as 30 µm in some plants such as Trillium. However, most metaphase chromosomes fall within a range of 3µm in fruit-fly (Drosophila), to 5µm in man and 8µm to 12µm in maize. The organisms with less number of chromosome contain comparatively large-sized chromosomes than the chromosomes of the organisms having many chromosomes. Further, the chromosomes in a cell are never alike in size, some may be exceptionally large and others may be too small. The largest chromosomes are lampbrush chromosomes of certain vertebrate oocytes and polytene chromosomes of certain dipteran insects.
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, the chromatin threads. In the metaphase and the anaphase, the chromosomes become thick and filamentous. Each chromosome contains a clear zone, known as centromere or kinetocore, along their length. The centromere divides the chromosomes into two parts, each part is called chromosome arm. The position of centromere vary from chromosome to chromosome and it provides different shapes to the later which are following:
1. Telocentric. The rod-like chromosomes which have the centromere on the proximal end are known as the telocentric chromosomes.
2. Acrocentric. The acrocentric chromosomes are also J-like in shape but these have the centromere at orte end and thus giving a very short arm and an exceptionally long arm. The locusts (Acrididae) have the acrocentric chromosomes.
3. Submetacentric. The submeta-centric chromosomes are L-shaped. In these, the centromere occurs near the centre or at medium portion of the chromosome and thus forming two unequal arms.
4. Metacentric. The metacentric chromosomes are V-shaped and in these chromosomes the centromere occurs in the centre and forming two equal arms. The amphibians have metacentric chromosomes.
Structure of the Chromosome
While describing the structure of the chromosomes during various phases of cell cycle, cell biologists have introduced many terms for their various components. Let us become familiar with the following terms to understand more clearly the structure of the chromosomes.
At mitotic metaphase each chromosome consists of two symmentrical 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.
2. Chromonema (ta)
During mitotic prophase the chromosomal material becomes visible as very thin filaments, called chromonemata. A chromonema represents a chromatid in the early stages of condensation. Therefore, “chromatid” and “chromonema” 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.
The chromomeres are bead-like accumulations of chromatin material that are sometimes visible along interphase chromosomes. The chromomere-bearing chromatin has an appearance of a necklace in which several beads occur on a string. Chromomeres become especially clear in the polytene chromosomes, where they become aligned side by side, constituting the chromosome beads. At metaphase the chromosomes are tightly coiled and the chromosomes are no longer visible.
Chromomeres are regions of tightly folded DNA and have great interest for the cell biologists. They are believed to correspond to the units of genetic function in the chromosomes.
4. Centromere and kinetochore
Originally it was considered that the centromere consists of small granules. The chromatin remains connected with the granules of the centromere. Currently it is held that centromere is the region of the chromosome to which are attached the fibres of mitotic spindles. The centromere lies within a tinner segment of chromosome, the primary constriction. Centromeres are found to contain specific DNA sequences with special proteins bound to them, forming a disc-shaped structure, called kinetochore. Under the EM, the kinetochore appears as a plate- or cup-like disc, 0.20 to 0.25 nm, in diameter situated upon the primary constriction or centromere. The main function of the kinetochore is to provide a centre of assembly for microtubules, i.e., it serves as a nucleation centre for the polymerization of tubulin protein into microtubules.
The chromosomes of most organisms contain only one centromere and are known as monocentric chromosomes. Some species have diffuse centromeres, with microtubules attached along the length of the chromosome, which are called holocentric chromosomes. In some chromosomal abnormality, chromosomes may break and fuse with other, producing chromosomes without centromere (acentric chromosomes) or with two centromeres (dicentric chromosome).
Each extremity of the chromosome has a polarity an therefore, it prevents other chromosomal segments to be fused with it. The chromosomal ends are known as the telomeres.
6. Secondary constriction
The chromosomes besides having the primary constriction or the centromere possess secondary constriction at any point of the chromosome.
7. Nucleolar organizers
Normally in each diploid set of chromosomes, two homologous chromosomes have additional ‘constrictions’ called nucleolar organisers. These are so-called because they are necessary for the formation of the nucleolus. The nucleolus is formed in the post-mitotic reconstruction phase. Under the light microscope the nucleolar organiser appears as a constriction near one end of chromosome.
Sometimes the chromosomes bear round elongated or knob-like appendages known as satellites. The satellite remains connected with the rest of the chromosome by a thin chromatin filament. The chromosomes with the satellite are designated as the sat chromosomes.