Nuclear Envelope: Definition, Function & Structure
The nuclei of eukaryotic cells are separated from the cytosol by the nuclear envelope. In this lesson, we explore the structure of the nuclear envelope and the functions it performs in cells.
A major hallmark of eukaryotic cells is that they store their genetic material in the nucleus, a compartment that is separate from the cytosol. The nuclear envelope is the double membrane structure that surrounds the nucleus in eukaryotic cells and provides this compartmentalization.
Structure and Functions of the Nuclear Envelope
Below is a diagram of the nuclear envelope. In most cells, the nucleus is sphere-shaped, and this diagram shows a cross-section.
We'll go through each structural component of the nuclear envelope here, and learn about their functions at the same time.
Inner and Outer Nuclear Membranes
The nuclear envelope is made up of a double membrane structure that provides a barrier between the nuclear contents and the cytosol. The inner nuclear membrane and outer nuclear membrane are labeled in the diagram above. The two membranes are connected together, but their protein compositions are different.
Inner Nuclear Membrane
The inner nuclear membrane contains integral and peripheral membrane proteins that anchor the nuclear envelope to the lamina, which is a sturdy protein meshwork that gives the nucleus its structure and shape.
Outer Nuclear Membrane
As you can see in the diagram, the outer nuclear membrane is contiguous with the endoplasmic reticulum (ER), the intracellular compartment where lipids, as well as proteins that are going to be secreted or inserted into membranes, are made. The ER and outer nuclear membrane are both studded with ribosomes, which are the enzymes that translate mRNAs into proteins. The ribosomes are there so that ER proteins can be transported through the ER membrane as they are translated.
The space between the inner and outer nuclear membranes is called the perinuclear space. As shown in the diagram, it is contiguous with the inside of the ER, so the same processes occur in the ER as in the perinuclear space.
Nuclear Pore Complexes
Although the nucleus is a separate compartment from the cytosol, many molecules have to go in and out. These molecules include histones, DNA and RNA polymerases, transcription factors, and ribosomal proteins. They are transported into and out of the nucleus through the nuclear pores, which are large protein complexes that penetrate through both membranes of the nuclear envelope.
Like guards or doormen of the nucleus, the nuclear pores regulate which molecules can enter and exit the nucleus. Due to the size of the nuclear pores, small proteins can diffuse through them passively without any trouble; however, large proteins can't get through the pores unless they are tagged with a special amino acid sequence called a nuclear localization signal (NLS) and transported actively into the nucleus. The NLS is like a protein's ID card that allows it access into the nucleus. The details of active nuclear transport are still being worked out by scientists, but somehow the NLS allows the nuclear pore to open up wider so that larger molecules can go through.
In this lesson, we've seen that the nucleus is surrounded by a double membrane called the nuclear envelope. The nuclear envelope has several functions. First, it provides a physical barrier between the cytosol and the nuclear contents. Second, it is attached to the lamina, which gives the nucleus its sturdy structure and its shape. Third, its nuclear pores regulate which proteins can be transported into and out of the nucleus, based on size and nuclear localization signal. Finally, the outer nuclear membrane and perinuclear space are continuous with the endoplasmic reticulum and perform the same lipid and protein synthesis functions as this compartment.
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