How is DNA Packaged? - Chromosome Condensation & Karyograms

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  • 0:06 Cell Division
  • 1:18 DNA
  • 4:27 Condensation
  • 6:24 Summary
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Lesson Transcript
Instructor: Greg Chin
Learn how DNA packaging makes it possible to fit approximately seven feet of DNA inside a single cell. You'll see how chromosome condensation helps cells get ready for cell division.

Cell Division

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
Chromatin And Histones Illustration

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?

DNA Packaging

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
DNA Pool Example

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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