A micrograph of a dividing cell from a mouse showed 19 chromosomes, each consisting of two sister chromatids. During which of the following stages of cell division could such a picture have been taken? (Explain your answer.) a. prophase of mitosis b. telophase II of meiosis c. prophase I of meiosis d. prophase II of meiosis

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1. Identify the stage of cell division where chromosomes are visible and consist of two sister chromatids. During cell division, chromosomes condense and become visible under a microscope, and each chromosome consists of two identical sister chromatids.
2. Understand that in mitosis and meiosis II, chromosomes with sister chromatids are visible during prophase. In prophase of mitosis and prophase II of meiosis, the nuclear envelope breaks down, and the chromosomes condense, each consisting of two sister chromatids.
3. Recognize that in meiosis I, specifically during prophase I, chromosomes undergo synapsis forming tetrads (pairs of homologous chromosomes), which is different from just having sister chromatids.
4. Consider that in telophase II of meiosis, the chromosomes are at opposite poles and the nuclear membranes are reforming, making it unlikely to observe chromosomes with sister chromatids as they begin to decondense.
5. Conclude that the most likely stage for observing 19 chromosomes each with two sister chromatids in a dividing mouse cell, as described, is during prophase II of meiosis, where the chromosomes would have already been reduced in number during meiosis I and each chromosome would still consist of two sister chromatids.