Classification of Chromosomes based on Position of Centromere and Length of Chromosomal Arms

how chromosomes are classified

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.

what are telocentric chromosomesWhat are acrocentric chromosomes

(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.

what are sub-metacentric chromosomeswhat are metacentric chromosomes

(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.

Molecular Biology Short Notes

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