3. The name of the structure labelled B is centromere. The
centromere essentially holds the replicated chromosomes (chromatids) togehter
so they eventually go to where they are supposed to during nuclear division.
A protein structure known as the kinetochore forms on this
region, and it is what the microtubules attach to help ghide the chromosomes
as they move about the cell during nuclear division.
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Long & Slichter