A new nuclear membrane develops during telophase, the final stage of mitosis. This is when the separated sets of chromosomes have arrived at opposite poles of the dividing cell, and the cell rebuilds a nucleus around each set. The process also occurs during telophase I and telophase II of meiosis, meaning every type of nuclear division ends with new nuclear envelopes forming around the daughter chromosomes.
What Happens to the Nuclear Membrane During Mitosis
To understand why a new membrane needs to form at all, it helps to know what happens earlier. During prophase, the nuclear envelope breaks apart into small membrane fragments called vesicles. This breakdown releases the chromosomes into the cell’s cytoplasm so the spindle apparatus can grab them, line them up, and pull them apart. By the time a cell enters metaphase and anaphase, there is no intact nuclear membrane at all.
Once the chromosomes finish separating in anaphase and reach opposite ends of the cell, the cell enters telophase. This is the reversal of everything that happened in prophase: the chromosomes begin to uncoil, the nucleolus reappears, and most importantly, a new nuclear envelope assembles around each group of chromosomes.
How the New Membrane Actually Forms
The rebuilding process happens in a specific sequence. First, the membrane vesicles that were created when the old envelope broke down bind to the surface of the chromosomes. These vesicles then fuse together, initially wrapping individual chromosomes in a double membrane. Those smaller membrane envelopes then merge with each other until a single continuous nuclear envelope surrounds the entire set of chromosomes in each daughter cell.
The raw material for this new membrane comes largely from the endoplasmic reticulum (the cell’s membrane-manufacturing network), which is physically connected to the outer layer of the nuclear envelope. Phospholipids, the building blocks of all cell membranes, flow from the endoplasmic reticulum to supply the growing nuclear envelope.
After the double membrane is in place, nuclear pore complexes reassemble. These are the gateways that control what moves in and out of the nucleus. Only after the pores are functional does the structural scaffolding on the inner surface of the membrane fully rebuild, and the chromosomes finish decondensing back into their loosely organized form.
The Role of Structural Proteins
The inner surface of the nuclear envelope is reinforced by a mesh of proteins called lamins, which act like a skeleton giving the nucleus its shape and stability. During prophase, enzymes add phosphate groups to these lamins, causing the mesh to disassemble and the envelope to fall apart. During telophase, the reverse happens: phosphatases (enzymes that remove phosphate groups) strip those phosphates off, allowing the lamins to reassemble.
Not all lamins return at the same time. Lamin B1 is one of the first structural proteins to concentrate on the chromosome surface during the transition from anaphase to telophase, and it quickly surrounds the entire region of decondensing chromosomes. Lamin A, by contrast, doesn’t visibly associate with the new nucleus until late telophase or even mid-cytokinesis, well after the pore complexes and membrane are already in place. This staggered assembly means the nuclear envelope becomes functional in layers: membrane first, then pores, then the full structural scaffold.
Telophase in Meiosis
Meiosis, the type of cell division that produces eggs and sperm, includes two rounds of division, and the nuclear membrane reforms during the telophase of each round. In telophase I, a nuclear envelope reassembles around each set of chromosomes after the first division, producing two cells that each still have paired chromatids. In telophase II, the envelope reforms again after the chromatids separate, yielding four cells with half the original chromosome number. The mechanism is essentially the same as in mitotic telophase: vesicles bind to chromosomes, fuse into a double membrane, and the lamins and pore complexes rebuild.
Not Every Organism Breaks Down Its Nucleus
Everything described above applies to “open” mitosis, which is the form used by animals and plants. In open mitosis, the nuclear envelope completely disassembles and must be rebuilt from scratch each division. But this isn’t the only strategy in nature.
Most fungi, for example, undergo “closed” mitosis, where the nuclear envelope stays intact throughout division. The spindle forms inside the nucleus and pulls the chromosomes apart without the membrane ever breaking down. Some organisms use a middle ground called “semi-open” mitosis, where the envelope partially ruptures or develops holes but never fully disperses. Budding yeast takes advantage of its closed division to keep the nuclear contents compartmentalized even while chromosomes are separating.
Closed mitosis is thought to be the more ancient mechanism. Open mitosis appears to have evolved independently multiple times, in both animals and plants, likely because fully removing the nuclear barrier gives the spindle better access to the chromosomes in larger, more complex cells.