In what part of mitosis does the nuclear membrane disappear? My book says that in prometaphase, "the nuclear membrane fragments", but when I looked online to verify this, I found some sources that said that nuclear membrane disappears in prophase, and some sources that said that the nuclear membrane disappears in metaphase. Which is correct?
The process of mitosis involves several distinct phases, including prophase, prometaphase, metaphase, anaphase, and telophase. During each stage, specific changes occur in the cell. Now, regarding the disappearance of the nuclear membrane, it is understandable to find some conflicting information, as different sources may describe it differently or use alternative terminology. However, I'll explain the general sequence, which should help clarify the matter.
In prophase, the nuclear membrane starts to disintegrate or break down. This breakdown occurs as the chromatin condenses into distinct chromosomes, and the mitotic spindle (composed of microtubules) begins to form. However, the nuclear membrane is still visible and partially intact during this stage.
The next phase, prometaphase, is when the breakdown of the nuclear membrane progresses further. The nuclear envelope fragments into small vesicles, dispersing throughout the cell. These vesicles are derived from the disassembled nuclear membrane and are commonly referred to as the nuclear envelope fragments. This fragmentation allows the microtubules of the mitotic spindle to interact with the chromosomes.
Following prometaphase, metaphase is reached. At this stage, the chromosomes align along the equatorial plane of the cell, forming what is referred to as the metaphase plate or equatorial plate. The nuclear membrane has completed its fragmentation at this point, and the vesicles from the breakdown are no longer visible as distinct structures. However, some of the components of the nuclear membrane, such as certain proteins, may still be associated with the chromosomes during metaphase.
To summarize, in prophase, the nuclear membrane begins to break down, in prometaphase, it further fragments into small vesicles, and by the time metaphase is reached, the nuclear membrane has completed its dissolution. Therefore, both your book and the online sources are partially correct, but it is important to understand the overall sequence of events in mitosis to avoid confusion.