During which phase of cell division is the 700-nm fiber typically observed?

The 700-nm fiber, which corresponds to the chromatin that has condensed into visible chromosomes, is typically observed during metaphase of cell division. This phase is characterized by the alignment of chromosomes along the metaphase plate and their maximal condensation, making them clearly discernible under a microscope. At this stage, each chromosome consists of two sister chromatids that are tightly packed, forming the distinct 700-nm structure.

Understanding the significance of this fiber is crucial, as it reflects the process of DNA packaging that is essential for proper chromosomal segregation during cell division. In contrast, during interphase, DNA exists in a more loosely packed form, allowing for transcription and replication; during anaphase, the sister chromatids are being pulled apart and are not in their most condensed state; and in telophase, the chromosomes begin to de-condense back into chromatin as the nuclear envelope reforms. This clear differentiation of phases highlights the unique structural characteristics of chromosomes as they progress through division.