Cell division is an important part in the life of any multicellular organism. That seems pretty obvious, right? Otherwise, how could we get from being one cell to multiple cells, and even now, cell division is maintaining our bodies by preventing us from running out of things like red blood cells or skin cells.
One of the most important goals of cell division is to make sure that each daughter cell receives one copy of every chromosome. What makes DNA so special? Well, since DNA is the blueprint for building a cell, any error in the DNA could result in a defective cell. A single DNA error could potentially lead to disease or death, and since there are three billion bases of DNA that the cell has to move around and copy, there's a lot of chances for an error to occur. So, the cell has set aside a specific process just make sure that equal amounts of nuclear DNA end up in both of the daughter cells.
Illustration of DNA wrapped around histones to form chromatin
This process is called mitosis, but how does that cell fit all of the DNA inside the nucleus? I just said that there were roughly three billion bases in the DNA. If you were to put all the DNA molecules in a single human cell end to end, then you'd end up with a molecule that's two meters, or roughly seven feet in length. That's a lot of DNA to store in a pretty small place like the nucleus. The nucleus is only about six microns wide or .0002 inches. How could the cell fit that much DNA into so small of a space?
We've already learned that DNA is wrapped around proteins known as histones to form chromatin. This is just the first step in organizing and packaging the DNA so that it can fit inside of the nucleus. Packing the DNA into nucleosomes condenses the DNA approximately sevenfold. This is the first step in organizing and compacting the DNA. It can be further organized by winding the chromatin into more compact structures. Coiling the chromatin around itself decreases the space it occupies by another sixfold or so. Through a series of similar compacting strategies, the entire genome can fit inside the nucleus of a single cell.
So, we said that the goal of cell division was to produce two copies of a cell. Now, of course during that process, everything has to be copied, including the DNA. However, those chromosomes are essentially still just strings. Don't get me wrong - they're shorter strings, but they're still just strings. During cell division, the cell has to be able to move those chromosomes around to get them into those daughter cells. But the cell is basically just like a water balloon. The cell is going to have to move those strings around in a water environment.
Illustration of wading pool scenario
To understand how much of a problem that's going to be, let's consider the following scenario. Say we have two wading pools. We have 46 yard-long pieces of string in one pool and 46 pens in the other. Let's say we're going to have a race, and each of us is going to have to move either the strings or the pens into the middle of the wading pool without lifting the object from the pool. Do you think you could move the pens faster than I could move the strings? Assuming I'm not harboring any kind of comic book superhuman speed, I agree that you're probably going to beat me. The pens are pretty small and compact compared to the strings, so it's going to be easier for you to drag them through the water.
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The cell's going to have pretty much the same issues while it's trying to move around the chromosomes in the cytoplasm. If we weren't moving around strings of chromosomes - if instead, we were moving around nice, compact, little sticks of chromosomes, that would probably be a lot easier. To help with movement in that aqueous environment, the cell further compacts the DNA in a process known as chromosome condensation. And chromosome condensation is able to produce a mitotic chromosome structure that's approximately 10,000 times more compact than how the DNA started.
When scientists examine the number and appearance of chromosomes in an organism, they typically observe the chromosomes in the middle of cell division. Now, to give you an idea of how much easier that is than looking at chromatin in a non-dividing cell, let's take a look at chromatin versus highly-condensed chromosomes. Notice that it's very difficult to notice where one chromosome ends and the other begins in this mass of chromatin. On the other hand, here's a nice, organized picture of these condensed chromosomes in what's called a karyogram. Notice each chromosome is stained with a dye that gives each a slightly different pattern - this makes for a slightly different pattern and makes it easier to distinguish chromosome one from chromosome two. Additionally, most chromosomes can be distinguished by length. Notice that chromosome 1 is much larger than chromosome 21.
Portion of a karyogram highlighting the 23rd chromosome for a male individual
A karyogram can be used to study the karyotype of an organism, which is basically just the number and appearance of chromosomes in an organism. You can clearly see that this individual has 23 pairs of homologous chromosomes, and this karyogram belongs to a human, and humans have 23 pairs of homologous chromosomes, although it's important to note that the 23rd pair of chromosomes are not identical because this individual is a male.
Mitosis is the process of separating nuclear DNA into identical complements for the new daughter cells. Chromosome condensation packages DNA more compactly in preparation for mitosis.
A karyogram is an organized picture of all the mitotic chromosomes, which can be used by scientists to study the karyotype, which is the number and appearance of the chromosomes in an organism.
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