Individual chromosomes are no longer visible during three specific phases of the cell cycle: G1 phase, S phase, and G2 phase. These phases are part of interphase, which is the period between cell divisions.
During the G1 phase (Gap 1), the cell grows and synthesizes proteins necessary for DNA replication. Chromosomes are in a less condensed form, making them invisible under a light microscope.
In the S phase (Synthesis), DNA replication occurs. The chromosomes are still in a decondensed state, and individual chromosomes are not visible.
Finally, during the G2 phase (Gap 2), the cell continues to grow and prepares for mitosis. The chromosomes remain decondensed and are not individually visible until they start to condense in preparation for cell division during the prophase of mitosis.
In summary, individual chromosomes are not visible during the G1, S, and G2 phases of interphase because they are in a less condensed form, making them indistinguishable under a light microscope.