Cell division and organism growth | High school biology | Khan Academy

In this video, we're going to talk about cell division and organism growth. Or another way to think about it is: how do we start with fertilization? We talk about this in other videos, but in sexually reproducing species, each individual starts off as a cell that is the result of the fusion between a sperm and an egg, between two gametes. Each of these gametes has half of the genetic material needed to make that organism. So once they fuse, once fertilization has happened, you are now diploid. You're haploid when you have half of the genetic material, as each of these individually has, each of these gametes. Then you become diploid once it's fused.

Well, once it's fused, that fertilized egg will then start to divide and replicate. A short period of time later, you might have eight cells that it has divided into. It'll first divide into two, then those two will become four, those four will become eight, the eight will become sixteen, and you keep dividing. Before you know it, you have trillions of cells, on the order of thirty to forty trillion, and you could create this very handsome organism right over here.

But an interesting question is: how does that division happen? Are the cells identical? Do they have the same identical genetic material? But if they are, how do they differentiate into all the different types of cells that make this handsome organism, the skin cells versus the heart cells versus the nerve cells? This process of cell division, in which a parent cell divides into two new daughter cells, each of which is genetically identical to the parent cell, is known as mitosis.

So let's dig into mitosis a little bit. Let's start with two daughter cells from a previous round of mitosis. Now, this would not be a human cell; what we're looking at is four chromosomes in two pairs. They've color-coded it: two of these are blue, and two of these are red. A human cell would have 23 pairs, or 46 chromosomes, but this is a eukaryotic cell. The genetic material is inside of the nucleus right over here.

Just to understand what these long strands are, these are chromosomes in their non-condensed form. A chromosome, you can really view as a long strand of DNA, and you're going to have segments of that DNA that code for specific proteins. Those segments are what we call genes. In a given chromosome pair, you will code for the same genes, but you might have different versions of the same gene. Maybe this gene right over here contributes to hair color in some way, and then this other one contributes in the same way, but it might lead to a different hair color. So each of these chromosomes in a pair, we call them homologous or homologous pairs; they're coding for the same genes. They have the same set of genes, but they might have different versions of those genes or different alleles. So that's what we're seeing here.

Now, to prepare for mitosis, what we need to see is each of these chromosomes need to replicate. Once they do, the cell might look like this. Now, it's hard to see it here, but the two copies that are now replicated and connected at the center, we call them sister chromatids. We can see that a little bit clearer in the early phases of mitosis.

First, we see that the chromosomes have condensed into what we classically imagine as chromosomes when we look at it in a microscope. This is really happening in preparation for mitosis, in preparation for cell division. You see those two homologous pairs, and you can see that each of those chromosomes actually now consists of two sister chromatids—one over here, one over there. As we go through mitosis, they're going to separate into individual chromosomes that are actually just copies of each other.

Now, the other thing that we're seeing in this early phase of mitosis is that the nuclear envelope starts to break down. We're going to see why that's important because we're going to have to separate the chromosomes. Now, the other thing that we see, and in other future biology classes, you'll go into much more detail, is the mitotic spindle is starting to form. We don't have to go into all the details of the different parts of the mitotic spindle, but imagine a bunch of structures made primarily by protein that are actually going to act to allow mitosis to happen, to grab the chromosomes, to pull them apart, to allow the cell to change its shape so that it'll eventually be able to divide.

But after that, we go into the middle phase of mitosis. As we enter into the middle phases of mitosis, we can see now that the chromosomes are aligned at the center, and they've actually now been attached to the mitotic spindle, which is going to be important because that's the machinery that's going to pull them apart and bring them to the different sides of the cell.

As we continue through the middle phases of mitosis, we see that right over there, those sister chromatids are now pulled apart, and they are copies of each other. They became copies of each other back here where the DNA originally replicated when it wasn't in its condensed form. But I think you see where this is going: as the mitotic spindle pulls on these chromosomes, we then enter into the late phases of mitosis. You can see here that the sister chromatids, which are now individual chromosomes, have been pulled to either side of that now elongated cell.

You can see that once they're there, a nuclear envelope begins to form again on either side, and then the mitotic spindle breaks down. The splitting of the cytoplasm finally into two daughter cells is known as cytokinesis. So it's pretty mind-boggling to realize that this is going on in your body continuously. This is what allowed all of us to start as a single-celled organism, a fertilized egg, and become an organism made up of tens of trillions of cells.

Now, I haven't gone into detail yet. Even though the great majority of our cells have the exact same genetics, they are actually able to differentiate and express different parts of the genetics based on the roles that they need to play. But we will talk about that in future videos. It's also important to realize that mitosis isn't just about growing an organism; it's also about maintaining an organism and doing tissue repair for an organism.

So I'll leave you there. Look at your hands, look at your arms, look in the mirror, and just realize that you are an organism of thirty to forty trillion cells that all start with one cell through many, many, many rounds of mitosis.