Classification of Chromosomes Based on Position of Centromere and Length of Arms
Ø The size and shape of the chromosomes are variable in the different phases of cell cycle.
Ø Chromosomes in the interphase of cell appear as thin, coiled, elastic and thread-like structures.
Ø This thread-like stainable interphase chromosome is called chromatin.
Ø During the mitotic or meiotic cell division, the chromatin materials become thicker in their width and shorter in their length.
Ø Chromosomes in the metaphase stage of cell division show maximum condensation.
Ø Each metaphase chromosome contains a centromere (primary constriction).
Ø The centromere divides the chromosome into two parts called chromosomal arms.
Ø The small arm of the chromosome is denoted as ‘p’ – arm, whereas the large arm is denoted as the ‘q’ – arm.
Ø When chromosomes are represented as a karyotype or ideogram, each chromosome is arranged in such a way that the ‘p’ arm is positioned above the centromere and the q arm is represented below the centromere.
Ø The position of centromere and the relative size of chromosomal arms are used as a criterion for a morphological classification of chromosomes.
Ø This morphological classification is an important karyotypic feature of an organism.
Classification of chromosome
Ø Based on the position of centromere and length of chromosomal arms, the chromosomes are classified into 4 groups:
(1). Telocentric chromosomes
(2). Acrocentric chromosomes
(3). Sub-metacentric chromosomes
(4). Metacentric chromosomes
(1). Telocentric chromosome
Ø In telocentric chromosomes, the centromere is located at the proximal end (tip) of the chromosome.
Ø The chromosomal tips are called as telomeres.
Ø Thus, telocentric chromosomes are long rod-like chromosomes
Ø These chromosomes appear as ‘i’ shaped structure in the metaphase stage of cell cycle.
Ø This type of chromosome has only one chromosomal arm.
Ø Telocentric chromosomes are very rare in occurrence and they were reported only in very few species.
Ø The centromere is positioned at one end of the chromosome in such a way that it produces a very short arm (p) and an exceptionally long arm (q).
Ø Acrocentric chromosomes appear as ‘J’ shaped structures in the metaphase stage of the cell cycle.
Ø The group Acrididae (grasshoppers) shows this type of chromosomes.
Ø The name is derived from the Acrididae (family of grasshoppers).
Ø All acrocentric chromosomes will be sat-chromosomes.
Ø Sat-chromosome = a chromosome with a secondary constriction and a knob-like structure at one end.
Ø In human, the chromosome number 13, 15, 21 and 22 are sat-acrocentric chromosomes.
Ø The centromere is located near the centre of the chromosome (NOT in the exact centre).
Ø Thus, these chromosomes will have two unequal arms; a small ‘p’ – arm and a large ‘q’ – arm.
Ø Sub-metacentric chromosomes appear as ‘L’ shaped structures in the metaphase stage of cell division.
Ø Majority of the human chromosomes are sub-metacentric chromosomes.
Ø The centromere is located exactly at the centre of the chromosome.
Ø Thus, these chromosomes will have two equal sized arms.
Ø The metacentric chromosomes will appear as ‘V’ shaped structures in the metaphase stage of cell division.
Ø Metacentric chromosomes are considered as a primitive type of chromosome.
Ø Primitive organism shows a karyotype with a majority of the chromosomes in metacentric shapes.
Ø Such a karyotype is called as an symmetric karyotype (primitive type).
Ø Amphibians usually show metacentric chromosome.
Ø Chromosome number 1 and 3 of human are metacentric chromosomes.