Are the sister chromatids exactly the same? I'm having doubts about whether the sister chromatids are **exactly** the same or not. I know that they are the arms that carry the information genes (the alleles), but I don't know if they are exactly the same and / or in what these arms differ, since in a partial exercise they ask me the following: Explain briefly why the sister chromatids are not exactly the same and the homologous chromosomes are different. To answer, consider the origin of both structures. Nor do I understand quite well when the statement clarifies "take into account the origin of both structures." Thank you very much for the help!

A new chromatid is synthesized by copying the other chromatid. As such two sister chromatids are (quasi) perfectly identical.

Why did I say quasi? Because there are sometimes "errors" when replicating DNA. These "errors" are called mutations. As far as I know, most mutations occur during meiosis though and only very few occur during mitosis. So, to a pretty good approximation, yes sister chromatids are identical.