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

Verified step by step guidance

1. Step 1: Understand the context of the problem. The micrograph shows 19 chromosomes, each consisting of two sister chromatids. This indicates that the chromosomes have already undergone DNA replication during the S phase of the cell cycle, and the sister chromatids are still attached at the centromere.
2. Step 2: Recall the characteristics of the stages of mitosis and meiosis. In prophase of mitosis, chromosomes condense, and each chromosome consists of two sister chromatids. In prophase I of meiosis, homologous chromosomes pair up and form tetrads, but the number of chromosomes is still the diploid number. In prophase II of meiosis, the cell is haploid, and chromosomes consist of two sister chromatids. In telophase II of meiosis, the chromatids have already separated into individual chromosomes.
3. Step 3: Analyze the chromosome number. A mouse typically has a diploid number of 40 chromosomes. If the micrograph shows 19 chromosomes, this suggests the cell is haploid (half the diploid number). Haploid cells are formed during meiosis, not mitosis. This rules out prophase of mitosis (option a).
4. Step 4: Consider the stages of meiosis. In prophase I, homologous chromosomes pair up, and the cell is still diploid, so this does not match the micrograph (option c). In telophase II, sister chromatids have already separated, so chromosomes would not consist of two sister chromatids (option b). In prophase II, the cell is haploid, and chromosomes consist of two sister chromatids, which matches the description in the micrograph (option d).
5. Step 5: Conclude that the micrograph was taken during prophase II of meiosis, as this is the stage where haploid cells contain chromosomes consisting of two sister chromatids.

Key Concepts

Chromosome Structure

Chromosomes are structures within cells that contain DNA. Each chromosome consists of two identical halves called sister chromatids, which are joined at a region known as the centromere. During cell division, these chromatids are separated to ensure that each daughter cell receives an identical set of chromosomes.

Stages of Mitosis and Meiosis

Cell division occurs through two main processes: mitosis and meiosis. Mitosis results in two identical daughter cells, while meiosis produces four genetically diverse gametes. Each process has distinct stages, including prophase, metaphase, anaphase, and telophase, which dictate the arrangement and separation of chromosomes.

Chromosome Number in Cell Division

In diploid organisms, such as mice, somatic cells typically contain two sets of chromosomes (2n). During mitosis, the chromosome number remains the same, while meiosis reduces the chromosome number by half (n). A micrograph showing 19 chromosomes with sister chromatids indicates a diploid cell in a stage of division where chromosomes are duplicated but not yet separated.