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 carries the recipe or the code for making the protein. That's exactly why protein secretion starts in the nucleus; 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 a membrane itself. The border of the nucleus has a specific name, referred to as the nuclear envelope. The nuclear envelope defines the double membrane barrier that surrounds the nucleus, separating the inside of the nucleus from the outside. Nuclear pores refer to tiny pores or holes found in the nuclear envelope. These nuclear pores allow the entry and exit into and out of the nucleus, controlling and monitoring what leaves and enters.

The nucleolus is a small, dense structure inside the nucleus, specifically where ribosomes are assembled. Recall from our previous lesson videos that ribosomes are important for making proteins, another reason the nucleus is the first step of protein secretion, as 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're showing our representation of the eukaryotic cell, and notice, the nucleus of the cell. Upon zooming into the nucleus, you'll notice it is made up of a double membrane that acts as the barrier of the nucleus. The gray structures you see here refer to the nuclear envelope, the double membrane surrounding the nucleus acting as its barrier. In the nuclear envelope, there are these pores, the nuclear pores.

Inside the nucleus, we have DNA; these structures represent DNA, and most of the eukaryotic cell's DNA is found inside the nucleus. Also, within the nucleus, at the core, there's another small dense structure where ribosomes are going to be assembled, known as the nucleolus.

The reason protein secretion starts in the nucleus is that the nucleus contains the DNA, which is transcribed into RNA, and then the RNA is translated into a protein. The protein, being built by the ribosome. There will be more details on this process, including transcription and translation, later in our course. For now, note that protein secretion starts in the nucleus because the DNA needs to be transcribed into RNA, which then 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 will 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.