If we start with one homologous chromosome with two sister chromatids, and then the chromatids get pulled apart so that each daughter cell gets only one, then why are those daughter cells considered diploid? It seems to me that if they have only one copy of each chromosome they are haploid until S phase when DNA is replicated. But my textbook and everything I look up says that mitosis produces diploid cells.
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Don’t confuse sister chromatids with mom and dad pairs. Sister chromatids are two identical dna strands. So when the DNA replicates the cell ends up with 2 sets of the maternal and 2 sets of the paternal chromosome. So for a brief moment they are tetraploid. The chromatids split giving one chromosome from each parent.
Let’s just focus on one chromosome for clarity. Directly before mitosis the cell has 2 different versions of this chromosome, each of which consists of 2 identical sister chromatids. This cell is diploid because it has 2 versions of the chromosome.
Like you said, during mitosis the sister chromatids get pulled apart. So the daughter cells each still have 2 versions of the chromosome however each of those copies now only consist of 1 chromatid. The cells are still diploid because they still contain 2 different versions of each chromosome. The missing sister chromatids can be replicated during the next S-Phase because they are exactly identical copies of the present chromatids. That’s in contrast to the 2 versions of each chromosome which hold slightly different information. The number of chromosomes doesn’t change during mitosis or the S-phase.
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