Chromatin consists of DNA complexed with histone proteins inside the nucleus.
At the time of cell division, chromatin starts condensing, and the most condensed chromatin stage is the metaphase chromosome. So we use this stage widely for representing ‘chromosome’.
A metaphase chromosome has the following features
- It has two chromatids held at centromere. These are called as sister chromatids.
- Centromere : it is the point of attachment for spindle microtubules that is responsible for the movement of chromosomes during cell division.
- Kinetochore: it is the protein complex on the centromere to which the spindle microtubules attach.
- A pair of telomeres: are the natural ends or tips of a linear chromosome. Its function is to stabilize the chromosome ends.
- Primary constriction: it is the region of centromere.
- Secondary constriction: any constriction on the chromosome other than primary constriction and often contains genes coding for ribosomal RNA.
- Satellites: round bodies separated from the rest of the chromosome by secondary constriction.
Actually human Karyogram or karyotype of 46 chromosomes is the chromosome pictures of the metaphase stage observed under microscope.
