Corresponding to different positions of centromere, chromosomes would be called (i) acrocentric
having terminal centromere, (ii) submetacentric
having sub-terminal centromere and (iii) metacentric
having median centromere.
Besides centromere, which produces a primary constriction in chromosomes, secondary constrictions
can also be observed in some chromosomes. Such a secondary constriction if present in the distal region of an arm would pinch off a small fragment called trabant
The satellite remains attached to rest of the body by a thread of chromatin. Secondary constrictions may be found in other regions also and are constant in their position, so that these constrictions can be used as useful markers. Secondary constrictions can be distinguished from primary constriction or centromere, because chromosome bends or shows angular deviation only at the position of centromere. Chromosomes having a satellite
are marker chromosomes and are called SAT-chromosomes.
The chromosome extremities or terminal regions on either side are called telomeres.
If a chromosome breaks, the broken ends can fuse due to lack of telomeres. A chromosome, however, can not fuse at the telomeric ends, suggesting that a telomere has a polarity which prevents other segments from joining with it. Telomeres have been studied in great detail at the molecular level in recent years (see later in this section and in Chemistry of the Gene 2. Synthesis, Modification and Repair of DNA