Taxonomy of Finfish (1+2)

6.1.2 Structure of Chromosomes

Recent alwances like the use of colchicines, hypotonic treatment and air-drying cell suspensions, have allowed more detailed analyses of chromosome morphology. A karyotype is characterized by the different morphological types of chromosomes, i.e. metacentric, submetacentric, subtelocentric or telocentric (sometimes called acrocentric) according to the position of the centromere. Measurement of chromosomes which give an accurate location of the position of the centromere have contributed to an important gain of knowledge, entitled beta-level karyology as opposed to alpha-level karyology in which only the number of chromosomes is reported. These new techniques have permitted interspecific comparison of karyotypes and thus the identification of chromosomal rearrangement, which are an important or significant factor in evolution.

Chromosomes are best seen during the cell division. Chromosomes are not uniform in width throughout their length, but each of them has a constriction, called as centromere. It is known to be associated with the movement of chromosomes during cell division. The terminal ends of chromosomes are called telomeres, which confer stability of their ends. The centromeres and the telomere are the constitutive heterochromatic regions.

The portions of the chromosomes on their side of the centromere are called as the chromosome arms. The total number of chromosome arms in a karyotype is called as fundamental arm number (FN). From the arm length and position of centromere we can calculate three factors: the centromeric index, the arm ratio and the relative length of the chromosomes.

The length of the shorter arm of a chromosome

Centromeric index = The total length of that chromosome

This gives an idea of how much of percentage the short arm is of the whole chromosome. Based on the centromeric index, the chromosomes are classified as metacentric, sub-metacentric and telocentric.

Length of the long arm

Arm ratio = Length of the short arm

Arm ratio is always greater than or equal to one and is an alternative method to centromeric index for describing the position of the centromere and the relative lengths of two arms.

Length of a chromosome × 100

The relative length = Totla length of all the chromosomes in the haploid set

The relative length gives us an indication of the proportion of the whole genome of the organism that is represented by the particular chromosome. However; this is a linear measurement and not a measure of mass and volume.