During prophase, the structures 'start to disappear'. but at the start of metaphase, th structures completely disappear

Explanation:
During the cell cycle, nuclear structures undergo various changes and rearrangements. The disappearance of nuclear structures occurs during the prophase stage.

Prophase:
- Prophase is the first stage of mitosis or meiosis, characterized by the condensation of chromatin into visible chromosomes.
- During prophase, the nuclear envelope disassembles, and the nucleolus, a substructure within the nucleus, also disappears.
- The chromatin fibers condense and become tightly coiled, forming distinct chromosomes.
- The replicated DNA strands are visible as sister chromatids, held together at a region called the centromere.
- The mitotic spindle, composed of microtubules, begins to form and extends from the centrosomes located at opposite ends of the cell.
- The disappearance of the nuclear envelope and nucleolus allows the spindle fibers to interact with the chromosomes.

Functions of the Nuclear Envelope and Nucleolus:
- The nuclear envelope is a double membrane structure that surrounds the nucleus. It separates the genetic material (DNA) from the cytoplasm and regulates the passage of molecules into and out of the nucleus.
- The nucleolus is involved in the production and assembly of ribosomes, which are responsible for protein synthesis.

Significance of Nuclear Structure Disappearance:
- The disappearance of the nuclear envelope and nucleolus during prophase is essential for proper chromosome segregation and cell division.
- It allows the spindle fibers to access and interact with the chromosomes, facilitating their alignment and separation during subsequent stages of mitosis or meiosis.
- The disappearance of the nucleolus ensures that ribosome production is temporarily halted during cell division, as the cell's energy and resources are focused on other processes.

Conclusion:
In summary, during the cell cycle, the nuclear structures disappear during prophase, which is the first stage of mitosis or meiosis. The nuclear envelope disassembles, and the nucleolus disappears, allowing the spindle fibers to interact with the condensed chromosomes. This disappearance is crucial for proper chromosome segregation and cell division.