Chromosomes are the condensed chromatin fibres formed during cell division. The number of chromosomes vary from organism to organism. However, their number remains constant in a particular species. Generally, chromosomes are monocentric (i.e. with one centromere). But di- or polycentric chromosomes are not uncommon. Four types of chromosomes have been recognised basing upon the position of the centromere.
They are metacentric, when the centromere is centrally located; submetacentric, when the centromere is one sided making two unequal arms of the chromosome; acrocentric, when the centromere is close to one end of the chromosome and telocentric, when one arm of the chromosome is completely absent and the centromere is terminal. The chromosomes may also be called acentric, dicentric and polycentric when they possess no centromere, two centromeres or many centromeres, respectively.
Classical cytogeneticists visualized a number of components of a typical chromosome with specific functions. The centromere or kinetochore is the constricted region present on the chromosome responsible for mitotic spindle attachment during cell division. Otherwise known asprimary constriction, this divides the chromosome into two arms providing definite shapes during anaphasic movement. Each metaphase chromosome constitutes two symmetrical strands called chromatids but anaphase chromosome has got only one. During prophase, each chromatid appears to be made of very thin and highly coiled subunits called chromonemata.
Cytologists observe the presence of certain bid-like structures formed due to accumulation of chromatin material which are visible along the entire length of the chromonema. These are called the chromomeres and are believed to be the regions representing genes. The tips of chromosomes are called telomeres, which provide stability to chromosomes. Besides primary constriction, some specific chromosomes possess an additional constriction called secondary constriction.
These constrictions are associated with ribosomal RNA synthesis that induces the formation of nucleoli and the chromosomes bearing these are called nucleolar organising chromosomes. Often, the chromosomes bear a small rounded body, separated from the body of the chromosome by secondary constriction called trabant. This region of the chromosome is without thymonucleic acid (Sine Acedo Thymonucleirico) and is called the SAT-Chromosome.
Differential staining divides chromosome into heterochromatic and euchromatic regions. The darkly stained regions associated with late replicating genes and containing genetically inert regions are called heterochromatin; but the lightly stained zones on chromosome, rich in DNA, is the euchromatin. It is generally the most active region of chromosome, as it harbours the functional genes.
Chemical analysis of eucaryotic chromosome shows that, in addition to DNA, the chromosomes contain three other kinds of macromolecule such as RNA, histories or basic proteins and nonhistone or acidic proteins. It also contains some metallic ions. The DNA histone complex is the basic unit of structure in the chromosome. Recent study, reveals that chromosome contains DNA which is packed in several successive orders and at regular intervals they form beads of nucleohistone called the nucleosomes.
The chain of nucleosomes are coiled; about six nucleosomes per turn and form a solenoid which is further coiled to form a supersolenoid that corresponds to the unit fibre or chromonema of light microscopy.
The primary function of chromosome is to carry the genetic information from one generation of cells to another. It also controls the different physiological and reproductive activities of living organisms through the genes they possess. They also play important role in mutation, evolution and variation of organisms.