The nuclear membrane is the outermost part of a cell's nucleus that separates it from the cytoplasm. Also called the nuclear envelope, this double-membrane structure acts as a boundary for the nucleus, allowing it to keep its shape. It also allows controlled exchanges through its pores.
The nucleus is by far the most important structure in any cell, plant or animal, since it functions as the control center directing all of the cell's activities. The nucleus contains the chemical instructions deoxyribonucleic acid (DNA) needed to make a cell work properly. The nucleus is usually the largest separate structure in a cell and it typically has a round or oval shape. It keeps this shape because it has a double-layer membrane that keeps it separate from the rest of the cell's cytoplasm. Cytoplasm is the jelly-like contents of a cell that contains all of its other structures.
Besides acting as a boundary that keeps the nucleus together, the nuclear membrane also controls what passes between the nucleus and the cytoplasm. It carries out this regulating function by using nuclear pores that dot its surface. These pores are like a sieve (a strainer with certain size holes) and they allow small molecules in and out of the nucleus. They also selectively permit large molecules to pass through their openings.
The nuclear membrane also plays a key role during mitosis (my-TOH-sis), which occurs when a cell makes a copy of itself. During the later stages of mitosis, the nuclear membrane begins to break down, allowing the already-duplicated chromosomes to split into two groups. After each complete set of chromosomes moves to opposite ends of the cell, a nuclear membrane reforms around each group. Soon each new cell has a separate nucleus surrounded by its own nuclear membrane. This reforming of the nuclear membrane begins the completion phase of mitosis.