Explanation:
Suppose that you have a very precious piece of information. Let’s imagine that this piece of information is a blueprint. In fact, it’s not just a blueprint for a house, or a car, or even a top-secret fighter jet. It’s a blueprint for an entire organism – you – and it not only specifies how to put you together, but also provides the information that enables every cell in your body to keep functioning from moment to moment.
Sounds important, right? You’d probably want to keep information this valuable in a secure spot, perhaps in a protected vault where you can keep an eye on it. In fact, that’s exactly what eukaryotic cells do with their genetic material, placing it in a membrane-enclosed repository called the nucleus.
Eukaryotic DNA never leaves the nucleus; instead, it’s transcribed (copied) into RNA molecules, which may then travel out of the nucleus. In the cytosol, some RNAs associate with structures called ribosomes, where they direct synthesis of proteins. (Other RNAs play functional roles in the cell, serving as structural components of the ribosome or regulating activity of genes.) Here, we’ll look in a little more detail at the structure of the nucleus and ribosomes.
[Do all cells have just one nucleus?]
The nucleus
The nucleus (plural, nuclei) houses the cell’s genetic material, or DNA, and is also the site of synthesis for ribosomes, the cellular machines that assemble proteins. Inside the nucleus, chromatin (DNA wrapped around proteins, described further below) is stored in a gel-like substance called nucleoplasm.
Enclosing the nucleoplasm is the nuclear envelope, which is made up of two layers of membrane: an outer membrane and an inner membrane. Each of these membranes contains two layers of phospholipids, arranged with their tails pointing inward (forming a phospholipid bilayer). There’s a thin space between the two layers of the nuclear envelope, and this space is directly connected to the interior of another membranous organelle, the endoplasmic reticulum.
Nuclear pores, small channels that span the nuclear envelope, let substances enter and exit the nucleus. Each pore is lined by a set of proteins, called the nuclear pore complex, that control what molecules can go in or out.
If you look at a microscope image of the nucleus, you may notice – depending on the type of stain used to visualize the cell – that there’s a dark spot inside it. This darkly staining region is called the nucleolus, and it’s the site in which new ribosomes are assembled.
