22 Biology -- Cell Division

It is written in my book that during Mitotic cell division replication of DNA takes place in S-phase of interphase resulting each chromosome with two sister chromatids. But formation of thick distinct chromosomes from chromatin fibre is formed in prophase of karyokinesis after interphas...

It is written in my book that during Mitotic cell division replication of DNA takes place in S-phase of interphase resulting each chromosome with two sister chromatids. But formation of thick distinct chromosomes from chromatin fibre is formed in prophase of karyokinesis after interphase. I am confused here. Does replication of DNA takes place within entangled chromatin fibre or condensed distinct chromosomes with chromatids during interphase? Is it possible to have chromosomes with distinct chromatids in interphase where DNA is replicated? 

During the S-phase of interphase, DNA replication takes place within the entangled chromatin fibers. As a result of DNA replication, each chromosome in the cell is composed of two identical sister chromatids held together at the centromere.

During prophase of mitosis, the chromatin fibers condense and become visible as thick, distinct chromosomes. This condensation is necessary to ensure that the replicated chromosomes can be separated correctly during cell division. At this stage, the sister chromatids are still connected at the centromere, but they become visible as individual structures within the chromosome.

So, to answer your question, replication of DNA takes place within the chromatin fiber during the S-phase of interphase. The result of DNA replication is the formation of chromosomes composed of two sister chromatids held together at the centromere. During prophase of mitosis, the chromatin fibers condense to form visible chromosomes with distinct sister chromatids, which are necessary for proper separation of the replicated chromosomes during cell division. It is not possible to have chromosomes with distinct chromatids in interphase because at that stage, the sister chromatids are still held together as a single replicated chromosome.


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