In this video, we're going to talk about the first organelle of the endomembrane system, which is the nucleus. Protein secretion starts in the nucleus because the nucleus of a eukaryotic cell stores the DNA of the cell. Recall that the DNA of the cell carries the recipe or the code for making the protein. That's exactly why protein secretion starts in the nucleus, because you can't secrete the protein until after you've made the protein. The nucleus can be defined as a rounded structure that contains and protects most of a eukaryotic cell's DNA.
Now there are three structures in the nucleus that you all should be aware of. Remember that the nucleus is part of the endomembrane system, which means that its border is a membrane itself. The border of the nucleus has a specific name that we refer to as the nuclear envelope. The nuclear envelope is really just defining the double membrane barrier that surrounds the nucleus, and acts as the barrier of the nucleus, basically separating the inside of the nucleus from the outside. Nuclear pores refer to tiny little pores or holes found in the nuclear envelope, which allow the entry and exit into and out of the nucleus.
They basically can control and monitor what leaves the nucleus and also what enters. The nucleolus is a small dense structure on the inside of the nucleus, where ribosomes are assembled. Recall from our previous lesson videos that ribosomes are important for making proteins, which is another reason why the nucleus is the first step of protein secretion because it is involved with making ribosomes. Let's take a look at our image down below to get a better understanding of the eukaryotic nucleus. On the left hand side, we show our representation of the eukaryotic cell, displaying the nucleus of the cell.
If we zoom in to the nucleus of the cell, you'll notice it is made up of a double membrane, which acts as the barrier of the nucleus. The gray structures here refer to the nuclear envelope. Notice that in the nuclear envelope, there are holes, or nuclear pores, throughout.
This hole seen here in the nuclear envelope is a nuclear pore. Notice that inside the nucleus, we have DNA, and most of the eukaryotic cell's DNA is found inside the nucleus. Within the nucleus, at the core, there's another small dense structure, where ribosomes are going to be assembled. This is the nucleolus, a structure within the inside of the nucleus itself that is involved with assembling ribosomes. The reason, once again, that protein secretion process starts in the nucleus is because the nucleus contains the DNA of the cell. The DNA in the nucleus is transcribed into RNA, and then the RNA is translated into a protein. The protein, recall, is built by the ribosome.
There's gonna be a lot more details on this process that we're mentioning right here, later in our course. We'll talk more about the idea of transcription and translation, later in our course. For now, what you should note is that protein secretion starts in the nucleus because, in order for a protein to be made, the DNA in the nucleus needs to be transcribed into RNA, and then the RNA needs to be translated into the protein. Once the protein is made, it can then be secreted. This concludes our introduction to the nucleus, and we'll be able to talk more about the other organelles of the endomembrane system moving forward in our course.
So I'll see you guys in our next video.