Structure of the Chromosome

The structure of the chromosome will now be taken up in detail.

Chormonema. Under the light microscope the metaphasic chromosome appears to be made up of two subunits called chromatids. These are possibly twisted around each other. If the chromosomes are treated with trypsin to remove protein, each chromatic is seen to consist of two subchromatids.

Centromere. The chromosome has a constructed region called the centromere, kinetochore or primary constriction. The position of the centromere is constant for a particular chromosome. The structure and function of the centromere is different from that of the rest of the chromosome.

(1) Metacentric. If the centromere is near the middle of the chromosome, the two arms of the chromosome are nearly equal.

(2) Acrocentric. When the Centro mere is situated near the end of the chromosome (submedian), one arm of the chromosome will be shorter than the other. Such a chromosome will appear L-shaped during anaphasic movement and is called submetacentric.

(3) Acrocentric. When the cenetromere is situated near the end or the chromosome (subterminal) it appears rod-shaped and is called acrocentric.

(4) Telecentric. If the centromere is truly terminal, i.e., situated at the tip of the chromosome, the chromosome is said to be telocentric.

Polycentric Chromosomes. Generally the chromosome has only one Centro mere. However, in some animals and plants the Centro mere is not located in one position but lies in a diffused condition along the length of the chromosome. In such cases the centromere is called a diffuse centromere, and the chromosome is said to be polycentric.

Dicentric Chromosomes with two centromeres are sometimes produced. Rarely, a new centromere may appear on the chromosome resulting in an abnormality. Such a centromere is called a neocentromere.

Acentric Chromosomes. Sometimes a chromosome may undergo a break into two, so that only one part has a centromere while the order is without the centormere. The part lacking the centromere is called the acentric fragment.

Arm ratio. The ratio of length of the long arm to the short arm of the chromosome is called the arm ratio.

Centormeric chromomeric. In metaphasic chromosomes, which consist of two chromatids, four granules can be seen within the centromere. These granules are called centromeric chromomeres.

Karyotype. A set of chromosomes of an individual or species is called a karyotype. Homologous pairs of identified chromosomes can be arranged in a series of decreasing lengths. Such an arrangement is called an ideogram.