What is the difference between chromatin, chromosomes, and sister chromatids?
Chromatin is a long chain of DNA.
Chromosomes is rolled up DNA when it is going through cell division.
Sister chromatids are the branches of the same chromosome.
Chromatin only begins to form into chromosomes in the beginning of mitosis or meiosis, so in metaphase and anaphase. A chromosome can be composed of two chromatids, as in most cases, or even one chromatid, as that is the case in Meiosis II.
Sister chromatids are the branches of one chromosome. If you had two chromosomes and you pointed out two of their branches, one from each chromosome, you would not be able to say they are sister chromatids, since the branches/chromatids belong to separate chromosomes, and not just one singular chromosome.