Suppose an organism (2n) has a haploid number of n=6. How many chromatids would be present at prophase of mitosis?

24

Explanation:

An organism with n=6 chromosomes would have the diploid chromosome number 2n = 12 in its somatic cells. The process of DNA replication during the S phase of interphase doubles the DNA content. This newly formed DNA is accommodated in the sister chromatids of chromosomes. Therefore, after the S phase, each chromosome has two sister chromatids.

The prophase cells have clearly visible chromosomes due to the compaction of chromatin. Each of the prophase chromosomes has two sister chromatids. Therefore, the total number of chromatids in prophase cell of the given organism would be 12 x 2 = 24.