The nucleus

- [Voiceover] Here's a diagram of a eukaryotic cell. Let's look a little bit closer at the structure of one of the most important organelles in the eukaryotic cell, the nucleus. So here we have a depiction of the nucleus. And the most important function of the nucleus is to contain the genetic material of the cell. But what is the structure? Let's look at the outside of the nucleus first. The nucleus is surrounded by two separate membranes, an outer membrane and an inner membrane. Let's look at this structure up close. So, we have an outer membrane, and we have an inner membrane. These two membranes separate the nucleus from the cytoplasm, which is the liquid-filled space that makes up the majority of the cell, and all of the non-compartmentalized parts of the cell. Essentially, the parts of the cell that aren't enclosed within an organelle. On the inside of the nucleus is what's called the nucleoplasm. In the nucleoplasm is the fluid inside the nucleus. And so these two, the cytoplasm and nucleoplasm, aren't continuous. However, a lot of the times, the stuff needs to be transported between the nucleoplasm and the cytoplasm. So think the interior of the nucleus is where MRNA is produced, but MRNA needs to get into the cytoplasm where it can be translated into protein. And also, proteins in the cytoplasm need to be able to get into the nucleus, such as polymerases, which convert DNA into messenger RNA. So this transport of things like MRNA out of the nucleus, and protein into the nucleus, happens at a special complex called a nuclear pore. And so there's another nuclear pore over here in my blown-up diagram of the outer and inner membranes. What's interesting about the nuclear pore, is that it spans both membranes, the outer membrane and the inner membrane, so that compounds in the cytoplasm can be transported into the nucleus, and compounds in the nucleus can be transported out. But now, the cell has gone to a lot of trouble to create these two lipid membranes to isolate the nucleus from the rest of the cell. So the nuclear pore is actually very selective in what can be transported inside and out. And so the nuclear pore actually recognizes special signals on different proteins, and only with the presence of these signals, can proteins be transported into and out of the nucleus through the nuclear pore. So the nucleus' primary function is to contain the cell's DNA, or chromosomes. But when you look at the nucleus up close, you see there is a very densely compacted area. And this area is called the nucleolus. Just as the nucleus is kind of the center of the cell, the nucleolus is the center of the nucleus. And now, but why is the nucleolus so much more compact and dense than the rest of the nucleus? Well, that's because the nucleolus has a very important function. And that is, the site of ribosome assembly. So the nucleolus is densely packed with regions of DNA that produce not messenger RNA, but ribosomal RNA. Which is the RNA that makes up the majority of the ribosome. And it is at the site of the nucleolus where this ribosomal RNA is assembled with the proteins that also make up the ribosome, into a fully-formed ribosome. And the ribosomes then, can be trafficked through the nuclear pore, out into the cytoplasm. The ribosomes also have proteins in them, and these proteins are produced in the cytoplasm, and are trafficked back into the nucleus where they can be assembled, and then make the return trip back into the cytoplasm. So there's lots of stuff moving back and forth in the nuclear pores. But it's very important for the compartmentalization of the nucleus from the rest of the cell. To move into our next discussion, the nucleus is intimately associated with another organelle, the endoplasmic reticulum, which you can see in the faded green outline. An interesting topological feature of the nucleus, is that the outer membrane, remember which we've drawn up in the top left, is actually continuous with the membranes that make up the endoplasmic reticulum. So I'll just kinda draw that. This outer membrane just turns around and actually becomes the membranes of the endoplasmic reticulum. And I won't draw it all in. So the interior space of the endoplasmic reticulum, is continuous with the interior space of between the outer and inner nuclear membranes. And one final term used to describe the nucleus, is the nuclear envelope. And the nuclear envelope just refers to the combination of the inner and outer membranes along with the nuclear pores. So these three structures together comprise the nuclear envelope, which encloses the nucleus.