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DNA doubles in S-phase, but chromosomes don't double in S-phase, in spite of the fact that chromosomes are formed by the condensation of DNA. Why is this so?

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  • $\begingroup$ My answer won't be precise, so I post it as a comment. It's a question of terminology, and it's normal to be confused: a chromosome can have one or two chromatides, but we can talk about a single chromosome in both states. For less confusion, we could talk about a replicated chromosome when one chromosome is composed of two chromatides. But I'm looking forward for a better answer than mine, since I'm still a student myself and don't exactly know why we call them like that. $\endgroup$ – justdoit Jan 30 '18 at 13:44
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First, chromosomes are composed of an original and a replicated chromatid, called sister chromatids. Chromatids are composed of tightly packed DNA.

DNA is replicated after the end of the S-phase, or in other words, the material composing the two sister chromatids is there. The catch is that chromosomes and chromatids aren't actually condensed yet, the DNA is still completely unpacked at this stage. So in fact, while you do not see actual chromosomes, the material composing them has been duplicated during the S-phase.

DNA condensation occurs two steps later during the prophase, where distinct sister chromatids/chromosomes are then formed from the packing of DNA that was duplicated during the S-phase.

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