How are chromosomes arranged and made up?

In a dividing cell, the hereditary material is compactly packed. Due to DNA helixing during cell division, the hereditary material becomes visible under a light microscope as rod-shaped bodies – chromosomes.
Chromosomes have shoulders and the primary constriction is the centromere. Before DNA doubling, the chromosome is formed by one DNA molecule, and after reduplication it includes two DNA molecules – chromatids bound in the centromere region. The shape of the chromosomes depends on the primary constriction, to the area of ​​which, during division of the cell, the threads of the division spindle are attached. The centromere divides the chromosome into 2 shoulders. There are 3 main types of chromosomes:
1) equal shoulders – with shoulders of equal or almost equal length;
2) unequal shoulders – with shoulders of different lengths;
3) rod-shaped – with one long, I have another very short shoulder.
Some chromosomes have a secondary constriction — a region of lesser spiralization of the chromosome material corresponding to the position of the nucleolar organism in it.

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