The term chromosome was coined by W. Waldeyer in 1888. The word chromosome means “coloured body”. Chromosomes were discovered by C. von Nageli in 1842. Each chromosome is composed of proteins and nucleic acid (DNA). The proteins in case of higher organisms are histones. Thus chromosomes are nucleoprotein in nature. The nucleoprotein material of the chromosome is referred to as chromatin. The chromatin component is present in two states: euchromatin and heterochromatin. Chromosomes are usually diagrammed in their most condensed state (during the metaphase of cell division). The metaphase chromosome contains two DNA molecules because the DNA of each chromosome replicates before the cell divides. The two DNA molecules are attached at a constricted site on the chromosome called the centromere. There are present protein bodies called kinetochores on the either side of the centromere. The kinetochores participate in separation of the replicated chromosome during mitosis.
The constricted centromere is often located near the middle of the chromosome. But the centromere may also be present at locations different form the middle of the chromosome. On the basis of centromere location, chromosomes can be metacentric (centromere in the middle), submetacentric (centromere located somewhat off centre), telocentric (centromere at the chromosome end), subtelocentric or acrocentric (centromere near the end). The term acrocentric is used when describing human and animal chromosomes; subtelocentric is used for describing plant chromosomes. There are no telocentric human chromosomes.
The centromere is also called the primary constriction. Occasionally, another constriction, called a secondary constriction is present near one end of the chromosome. Beyond the secondary constriction a small portion of the chromosome extends into a chromosome satellite. At these regions is found the nucleolus organizer region (NOR), where rRNA genes are clustered.
When the DNA in the centromere is duplicated, the two short and two long arms of the chromosome are attached to a single centromere. The term chromatid denotes the identical duplicated portions of the chromosome. The two chromatids of a chromosome are called sister chromatids, and each sister chromatid has one linear DNA molecule. The centromere divides the chromosome into segments called chromosome arms. It divides the chromosome into a long arm and a short arm except in the case where centromere lies in the middle. In metacentric chromosomes where centromere lies at the centre, the distinction between long and short arms diminishes. The short arm is called p arm (p=petit); the long arm is q arm.
The ends of eukaryotic chromosomes are specialized and are known as telomeres. Telomeres stabilize the ends of the chromosome, and prevents other chromosomes/chromosome fragments from fusing to it. Telomeres also protect the end of the DNA in the chromosome from enzyme degradation.