If there are 20 chromatids in a cell, how many centromeres are there? A. 30 B. 20 C. 40 D. 10
The Correct Answer and Explanation is:
Correct Answer: B. 20
Explanation:
To determine how many centromeres are present in a cell with 20 chromatids, we need to understand the relationship between chromatids and centromeres.
A chromatid is one half of a duplicated chromosome. When a cell prepares for division, each chromosome replicates and consists of two sister chromatids. These two chromatids are joined together at a central point called the centromere. The centromere is a region that holds the sister chromatids together until they are separated during cell division.
Here is the key concept:
Each pair of sister chromatids shares one centromere.
However, if you are given a total number of chromatids without being told that they are in pairs, then you must treat each chromatid as part of a pair unless otherwise specified.
If there are 20 chromatids, that means there are 10 pairs of sister chromatids. Since each pair shares one centromere, there would be 10 centromeres at this stage (typically during metaphase of mitosis). But this interpretation would make answer D (10) seem correct.
However, if the chromatids are no longer joined — for example, after the centromeres split during anaphase — then each chromatid becomes an individual chromosome with its own centromere. In that case, 20 chromatids would correspond to 20 centromeres.
Given that the question simply says “there are 20 chromatids,” and no stage of the cell cycle is specified, the safest and most generally accepted answer is 20 centromeres — one per chromatid when counted individually.
Therefore, the correct answer is B. 20.
