So recall from our previous lesson video that before any cell can divide, it must first replicate, duplicate, or make an extra copy of its DNA. And so how the DNA is organized in the cell is very important. In this video, we're going to introduce the organization of DNA in the cell. What you'll notice is throughout this video, we're going to introduce various terms, and notice that the terms that link directly to the image down below are labeled with letters a, b, c, d, and e. Once again, these letters, labeling specific terms, are going to link directly to the letters that we have down below in our image, so keep that in mind as we go through this video.
The very first term that we're going to introduce here is a term called the genome, and the genome refers to the complete set of all of a cell's DNA, where "all" is the keyword. The genetic material refers to molecules that determine the inherited traits of an organism, and usually, when scientists use the term 'genetic material,' they are referring to the DNA of the organism. Within a cell, the DNA is going to associate with proteins called histones. These histone proteins are going to form units that are called nucleosomes. The nucleosomes themselves can be defined as units of 8 histone proteins at the core with DNA wrapped around it.
To start to understand these terms, let's take a look at our image down below. Starting with term a, the genetic material, DNA. Notice here we're showing you an image of DNA as we've discussed in our previous lesson videos, a double-stranded helix of nucleotides as we see. Within a cell, it turns out that the DNA is going to be organized into units called nucleosomes. The nucleosomes form when DNA is wrapped around these histone proteins. Notice these little purple circles that you see throughout are histones, these histone proteins.
The histone proteins will organize into these units of 8 histone proteins, and the 8 histone proteins form the core of the nucleosome, and DNA will wrap itself around the histone protein, almost like taking the core of a pencil or a pen if that was the histone core, and wrapping a yarn around the histone protein. That's basically what the nucleosome consists of: histone proteins at the core with DNA wrapped around it. It turns out that within the cell, these nucleosomes can actually take different forms depending on if the cell is in a non-dividing state or if the cell is in a dividing state. When the cell is in a non-dividing state, when the cell is not dividing, it turns out that the DNA, the nucleosomes, are going to be organized into what's known as chromatin.
Chromatin represents loosely packed or coiled nucleosomes in non-dividing cells. But when the cell is about to divide, in a dividing cell, the nucleosomes take a different form and they are no longer loosely packed or coiled. Instead, they are going to become tightly packed into what are known as chromosomes. So chromatin refers to loosely packed or coiled nucleosomes in a non-dividing cell, and chromosomes refer to tightly packed or highly condensed nucleosomes in a dividing cell. Let's take a look at our image down below for a better idea of this.
Notice on the left-hand side over here, we're showing you a non-dividing cell. Within the nucleus of this non-dividing cell, when you zoom in here, the DNA is organized in this format where the genetic material, the DNA, is wrapped around histone proteins to form these nucleosomes, and these nucleosomal units, when the cell is in a non-dividing state, are going to be in a loosely coiled state called chromatin. Over here in the non-dividing state, the DNA is in a chromatin form. Because it's so loosely coiled, you can really think of a ball of yarn that is really loosely coiled. Notice that we have this loosely coiled ball of yarn here or bunch of yarn.
But notice that over here on the right-hand side, what we have is the dividing cell. The DNA is going to condense, and we have condensing DNA here. The DNA condenses into highly condensed or tightly packed chromosomes. This term chromosomes, you can think of a ball of yarn that is really, really tightly coiled, nicely wrapped around unlike the ball of yarn that's over here. Both terms, chromatin and chromosomes, refer to DNA wrapped around protein, and the difference is really that chromatin is a looser form of the DNA, found in non-dividing cells, whereas chromosomes are more tightly coiled DNA found in dividing cells.
This condensing of the DNA into chromosomes is very important for moving the DNA around within a cell that is dividing. We'll talk a lot more about chromosomes and the dividing cell as we move forward in our course. But for now, this concludes our introduction to the organization of the DNA in the cell, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.
