The structure labelled A in the figure is a centromere.
What is a centromere?
The centromere is a region of the chromosome that is constricted. The centromere plays an important role in the division of cells during both the processes of mitosis and meiosis.
The centromere is simply the region of the chromosome where the spindle fibres attach. Therefore, when the spindle fibres attach to the centromere, they pull the sister chromatics in the opposite directions of the cell during cell division.
Thus, at the end of the cell division, each of the daughter cells gets one-half of the chromosome containing identical DNA. The centromere is thought to be the middle region of the chromosome, even though it is often not in the middle. It is simply the region where the chromosomes are most constricted
The centromere separates the chromosome into the P arm and the Q arm. These are simply the names given to the short arm and the long arm of the human chromosome respectively.
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