hanbubbles21
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im just revising the cell cycle including mitosis and I don't fully understand when the chromosomes are actually visible? I know that during prophase the chromosomes condense and get shorter and fatter until telophase when they uncoil and become long and thin but does this mean the chromosomes are visible when short and fat or long and thin? so when im working out the mitotic index do I count all the cells undergoing mitosis out of all the cells?
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OxFossil
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(Original post by hanbubbles21)
im just revising the cell cycle including mitosis and I don't fully understand when the chromosomes are actually visible? I know that during prophase the chromosomes condense and get shorter and fatter until telophase when they uncoil and become long and thin but does this mean the chromosomes are visible when short and fat or long and thin? so when im working out the mitotic index do I count all the cells undergoing mitosis out of all the cells?
The visibility of DNA using staining depends on how condensed it is. Whilst it is being actively transcribed or replicated, it is not condensed (so that the transcription/replication enzymes can get to it).

So the DNA of each chromosome replicates in the earliest part of prophase (some sources will say that prophase does not start until after this replication has taken place), and it is hard to see it using normal staining techniques. It just looks like a hazy mess.

Once replication is completed (and still during prophase), the DNA condenses and becomes visible in the form of the X shaped chromatids - identical lengths of DNA joined at the centromere.

It remains condensed and visible throughout mitosis until the end of telophase, when the nuclear envelope(s) form in the daughter cells and the DNA decondenses.

As far as I understand it, in effect, any cells you can see the chromosomes in are counted for the mitotic index. NB I don't know what your exam spec might say about this
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