Understanding Chromosome Structure: Chromatids, Centromeres, and Kinetochores

Chromosome Structure: A Detailed Explanation

A chromosome consists of two parallel, elongated, rod-shaped subunits called chromatids. The chromatids are not equally thick throughout their length because they contain chromomeres, which are condensed fragments of chromatin.

Chromomere size varies, but the thickness of the chromatid is roughly constant, between 0.25 and 2 micrometers.

Primary Constriction (Centromere)

A structure called the primary constriction, or centromere, divides the chromatid into two arms, which may be of equal or different lengths. This distinction allows for chromosome classification:

• Metacentric chromosome: Central centromere and arms of equal length.
• Submetacentric chromosome: Centromere shifted upward, resulting in upper arms shorter than the lower arms.
• Acrocentric chromosome: The centromere is near the terminal end, resulting in very short arms. These are also called chromosomes with satellites.
• Telocentric chromosome: The centromere is at the terminal end of the chromosome.

The centromere position is variable among different chromosomes but fixed for the same chromosome.

Kinetochore Structure

At the level of the centromere is a cylindrical structure on both sides of the centromere that serves as a focal point for tubulin polymerization: the kinetochore. This cylindrical structure, approximately 0.3 μm in diameter and 0.1 μm thick, appears in pre-metaphase and is responsible for the polymerization of kinetochore microtubules. These microtubules are crucial for chromatid movement during anaphase.

One side of the kinetochore is in direct contact with the centromere, while the other face points toward one of the spindle poles. At this level, the kinetochores form microtubule connections, orienting the chromosome towards the spindle apparatus.

Secondary Constrictions and Nuclear Organizers

Some chromosomes exhibit other constrictions, known as secondary constrictions, near the lower arms. These constrictions have fixed positions and stain more weakly. An example is chromosome pair No. 9 in humans.

Certain secondary constrictions correspond to the region that carries the information for the synthesis of 18S and 28S rRNA; this region is called the nuclear organizer.

Satellites

Satellites can also be located in the sub-terminal portion of a chromosome arm, appearing as a rounded area. Chromosomes with nuclear organizers often also have satellites.