My textbook says that during prophase, the chromosome longitudinally splits (except at the centromere) into two sister chromatids. But what does this mean? The DNA is already duplicated in the S phase. So does this mean the uncoiling of the plectonemic coil formed by the sister chromatids? Perhaps the degradation of cohesin(I think unlikely)? I am a grade 10 student unable to find the answer(although this is not a homework question) so please try to keep things simple.
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The duplicated set of DNA in each chromosome(formed at replication in s phase ) gets separated and repacked as a new chromatids that we call as sisters, which are held together at centromere by COHESISNS & CATANANES(mechanically interlocked DNA) But the arms are seperated from each other by further condensation of the DNA, DECATANATION by TOPOISOMERASE & SELECTIVE BREAKUP OF COHESISNS at NON CENTROMERIC REGIONS (ARMS) from late S PHASE to PROPHASE & PROMETAPHASE through a CLEAVAGE INDEPENDENT PATHWAY (which is REVERSIBLE). But the complete seperation of sister chromatids at the centromere is due to the additional action of SEPARASE through CLEAVAGE DEPENDENT PATHWAY (QUICK, IRREVERSIBLE) which occurs from LATE METAPHASE TO ANAPHASE.
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