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.
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 going to be a membrane itself. The border of the nucleus has a specific name that we refer to as the nuclear envelope. The nuclear envelope defines the double membrane barrier that surrounds the nucleus, and again acts as the barrier of the nucleus, basically separating the inside of the nucleus from the outside. Nuclear pores refer to tiny pores or holes that are found in the nuclear envelope. These tiny nuclear pores in the nuclear envelope allow the entry and exit into and out of the nucleus. They can control and monitor what leaves the nucleus and also what enters the nucleus.
The nucleolus refers to a small dense structure inside the nucleus, and this is specifically 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 below to get a better understanding of the eukaryotic nucleus. Over here on the left-hand side, we are showing you our representation of the eukaryotic cell, and notice right here, we are showing you the nucleus of the cell.
If we zoom in here to the nucleus of the cell, you'll notice it is made up of a membrane, a double membrane. This double membrane, which you see here, acts as the barrier of the nucleus. The grey structures that you see here are referring to the nuclear envelope. We can label this as the nuclear envelope, the double membrane that surrounds the nucleus and acts as its barrier, basically what you see surrounding the nucleus here. Notice that in the nuclear envelope, there are these pores that you see throughout, these pores, if you will, the nuclear pores. This hole that you see right here in the nuclear envelope is referring to the nuclear pore.
Notice that inside the nucleus, we have DNA. These structures represent DNA, and most of the eukaryotic cell's DNA is found inside of the nucleus, which is important to keep in mind. Also, within the nucleus right here at the core, there's another small dense structure inside the nucleus, where ribosomes are being assembled. This is specifically the nucleolus. The nucleolus, a structure within the inside of the nucleus itself, is involved with building or assembling ribosomes.
The reason once again that the protein secretion process starts in the nucleus is that the nucleus contains the DNA of the cell. The DNA of a cell, found in the nucleus, is transcribed into RNA, and then the RNA is translated into a protein. The protein, recall, is being built by the ribosome. There's going to be a lot more details on this process that we're mentioning right here, later in our course. We'll talk more about this idea of transcription or being transcribed as well as 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. I'll see you guys in our next video.