Question

In meiosis II anaphase we have two cells, each containing 23 chromosomes. After they split, the chromosomes come apart into 23 chromatids in each cell. How do these chromatids replicate themselves into 23 chromosomes if they have crossed-over their genes?

Answer 1

These chromatids have already replicated before entering into meiosis cycle.

Explanation:

see the below image

DNA is replicated before the cell undergoes meiosis.

So, when DNA cell decides to undergo Meiosis, it goes through following steps:
1. DNA is replicated and sister chromatids are formed for each chromosome. (cell contain duplicated genome 2n).
2. During Meiosis I homologous chromosomes are paired, does crossing over and then separated into two different cells. Remember Chromosomes still retain sister chromatids with it, it is only the chromosome pair has separated. (2 cells containing n, as chromosome pair is separated).
3. As the cells move to Meiosis II the sister chromatids are separated and you get haploid genome. (4 cells containing n).