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A cell nucleus is a membrane bound organelle which contains all the genetic information for a eukaryotic cell. This genetic information, or DNA, is stored in different forms depending upon the cell's stage in mitosis, but is often found tightly wound into chromosomes.
The nucleus is one of the key things that whenever you see it you know you're looking at eukaryotic cell and it is the brain or control center of the cell because it has the DNA. The DNA holds the genes which are the instructions of how to build your cell and how to operate your cell. Now there are three major things that you'll see indicated on a diagram of this nucleus that you need to know about that's the nuclear envelope, the chromatin and the nucleolus.
Let's take a look at the cell, here we see at the center of the cell the nucleus and in fact the word nucleus actually means center, so on the outside you'll see the nuclear envelope. That's the double membrane of the nucleus through it you'll see small openings those are called the nuclear pores and those are the structures that allow messenger RNA, MRNA for short, in and out of the cell in order for it to get the ribosomes so that the cell can make the proteins as per of the directions of the DNA.
Now within the nucleus, you'll find the DNA. Now DNA is in normal cells is in the form called chromatin and a word that literally means colored stuff with nitrogen in it, so the chromatin is the DNA which is wrapped up loosely on proteins and through it's throughout the cell. They called it colored stuff with nitrogen in it because when they were first looking through cells or looking at cells through microscopes, they couldn't see anything so they started adding dyes and they added the dye that stuck to nucleic acids like DNA and so what they saw was "hey! There's colored stuff" and we know has nitrogen in it so we call it colored stuff with nitrogen in it, chromatin. Now they saw sometimes that there will be other regions within the nucleus that got even darker. We know now that's because right there in the nucleolus which means little center of the center thing we have not just DNA but we also have another kind of nuclear gas it called ribosomal RNA and so because that ribosomal RNA is being made right here in the nucleolus, it collects more of the stain and looks darker, so to summarize, we have the nucleus with nuclear envelope on the outside with nuclear pores [IB] substances in and out, we have the DNA typically in the form called chromatin loosely spread out throughout the nucleus and deep inside we have the nucleolus where ribosome or RNA is being made.