Question

If there are 20 chromatids in a cell, how many centromeres are there?

Answer 1

Assuming that the chromosomes have replicated there should be ten centromeres.

Explanation:

Centromeres are the point at which two replicated sister chromatids are attached and only appear when DNA replication occurs. Meaning that the normal number of chromosomes in the cell should be 10 and there should normally be 5 chromosome pairs. Now that there are 20 chromatids they should be in identical pairs of 2 (sister chromatids) as they have undergone replication. Therefore there would be 10 centromeres.

Reference
Allott, Andrew, and David Mindorff. Biology: Oxford IB Diploma Programme. Oxford: Oxford UP, 2014. Print.