The size and shape of the chromosomes are variable in the different phases of cell cycle. Chromosomes in the interphase of cell appear as thin, coiled, elastic and thread-like structures. This thread-like stainable interphase chromosome is called chromatin. During the mitotic or meiotic cell division, the chromatin materials become thicker in their width and shorter in their length. Chromosomes in the metaphase stage of cell division show maximum condensation.
Each metaphase chromosome contains a centromere (primary constriction). The centromere divides the chromosome into two parts called chromosomal arms. The small arm of the chromosome is denoted as ‘p’ – arm, whereas the large arm is denoted as the ‘q’ – arm. 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. The position of centromere and the relative size of chromosomal arms are used as a criterion for a morphological classification of chromosomes. This morphological classification is an important karyotypic feature of an organism.
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Classification of chromosome
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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.
(2). Acrocentric
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.
(3). Sub-metacentric
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.
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(4). Metacentric
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.
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