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Before the DNA is replicated in a human somatic cell, the cell has 46 chromosomes. Also, after the sister chromatids are separated during Anaphase, the chromosome number in the cell doubles to 72, so when the sister chromatids are joined, why isn't the chromosome number also 72? On the internet, some sources say that one chromosome in a cell that hasn't replicated its DNA is equal to one chromatid. Is this correct, or are chromosomes and chromatids structurally different? 

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When I was learning genetics for the first time I have also found naming the two chromatids joined at centromere as chromosome a little bit strange. The number of DNA molecules and their behaviour in cell cycle seemed to me more important for understanding of genetics.

What you need to understand.is the term chromosome predates knowledge of DNA structure. The name literally means colored (chromo) body (soma). It is an object visible under the microscope when using specific dye. In metaphase two chromatids joined at centromere look like a single object - a chromosome. Sometimes the two chromatids of one chromosome are so close together it is difficult to tell them apart. In anaphase the chromatids separate and one object (two-chromatid chromosome) visibly splits to two objects (single-chromatid chromosomes of daughter cells)

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