Chromosome | Types, Structure & Function
A Chromosome consists of two identical halves, the chromatids which are held together at one point called centromere. The centromere appears as a narrow region called primary constriction, of the chromosome . On the side of centromere, disc shaped structures are present known as kinetochores. Ends of chromosome are called telomeres. They seal the ends of chromosomes & prevent their shortening or chromosome loss.
Diagram of Structure of Chromosome
Depending on the position of centromere, the structure of chromosomes can be classified into four types :
(i) Metacentric Chromosome (median centromere) : The centromere is present at the centre & thus during anaphase divides the chromosome into two equal arms (Isobrachial). They appear V – shaped.
(ii) Sub – metacentric chromosome : The centromere is present slightly away from the centre of a chromosome or nearer to one end of the chromosome. As a result, chromosome is divide into one shorter & one long arm (Heterobrachial). They appear L-shaped during anaphase.
(iii) Acrocentric chromosome (subterminal centromere) : The centromere is present very close to one end of the chromosome. Thus, it forms one extremely short & one very long arm. They appear J – shaped during anaphase.
(iv) Telocentric chromosome (terminal centromere) : The centromere is present at the terminal end of the chromosome & thus, chromosomes appear to have a single arm. They appear L – shaped during anaphase.
A few chromosome may have additional constrictions termed as nonstaining secondary constrictions or NOR (nucleolar organiser) near their ends. The part of the chromosome beyond the secondary constriction is called satellite. A chromosome having satellite is called SAT-chromosome and these are considered as marker chromosome. In humans, 5 pairs of SAT chromosome are present.