In the context of cell division, chromosomes can exist in two distinct forms: unduplicated and duplicated. Understanding when these forms appear during mitosis and meiosis is crucial for grasping cellular division processes.
During mitosis, chromosomes exist in their unduplicated form during the G1 phase of interphase. At this stage, each chromosome is comprised of a single chromatid. As the cell prepares to divide, it enters the S phase, where DNA replication occurs, resulting in duplicated chromosomes consisting of two sister chromatids. These duplicated chromosomes are visible during prophase and remain so through metaphase, anaphase, and telophase, until the cell completes mitosis and returns to the unduplicated state in G1 phase of the next interphase.
In meiosis, the process is a bit more complex due to the two rounds of division. During meiosis I, the chromosomes are unduplicated during the G1 phase before the S phase. After DNA replication in the S phase, the chromosomes are duplicated, consisting of two sister chromatids. The duplicated chromosomes become visible during prophase I and align at the metaphase plate during metaphase I. However, during anaphase I, homologous chromosomes are pulled apart, and they remain in this duplicated form until telophase I.
After meiosis I, the cells enter meiosis II, which resembles a mitotic division. Here, the chromosomes are still in their duplicated state during prophase II, metaphase II, and through anaphase II, where the sister chromatids are finally separated. At the end of meiosis II, cells become unduplicated again, resulting in haploid cells ready for fertilization.
To summarize, during both mitosis and meiosis, chromosomes transition between unduplicated and duplicated forms. In mitosis, the unduplicated state is primarily during G1, while the duplicated forms are visible in prophase through telophase. In meiosis, the phases are similar, but include the additional complexity of homologous chromosome separation and two rounds of division.