If you were to take all of the chromosomal DNA in a single human cell, unwind the strands and lay them end to end, you’d have about 6-8 feet of DNA. If you were to take all of the chromosomal DNA out of a human body and lay it out the same way, you’d have a strand long enough to reach to the sun and back again (http://hypertextbook.com/facts/1998/StevenChen.shtml).
Let me repeat that. You have enough chromosomal DNA in your body to reach all the way to the sun and back! That’s some serious distance all wrapped up inside of us. It’s that wrapping I want to talk about in this post. This will be a bit of a simplification, but you’ll get the point.
DNA exists as a double helix. A corkscrew is an example of a single helix—there’s only one strand that is twisted up to form the helical shape. A nautilus shell is another example of a helix. In the case of DNA, there are two strands twisted around each other, forming a double helix. Imagine a ladder, with the legs secured at one end. You’re holding the legs at other end, and you start twisting the ladder—that’s pretty much what DNA looks like in its simplest form.
The example I used in my lab classes involved shoelaces. Take two shoelaces and hold them closely together. Secure one end (in a desk drawer, tape on the bench, or under that thick bio book). Then, while pinching both ends together, slowly and gently twist the shoelaces around each other to form a double helix. Stop when you have a helix all the way down the shoelaces. This is your basic double helix.
Now, start gently twisting again, leaving a little slack. After a few twists, the shoelaces will start coiling around themselves, forming irregular loops. The more you twist, the more loops will be formed, and the more compact your shoelace helices will become. This is how meters of DNA fit into each cell—very tight winding, around and around and around. As you can imagine, the irregular coils-upon-coils could cause problems for DNA replication, since they may end up in knots.
Instead of forming irregular structures, DNA in higher animals is wound around proteins called histones (follow the link for a good illustration). Wrapping DNA around histones helps keep in in regular structures, which are then wrapped around themselves, and so on. When time to replicate, the DNA is easily uncoiled from its regular structures. Much like your shoelaces, DNA does not coil and uncoil on its own accord—there is a system of proteins that perform this task, and the deeper science is truly intriguing. It’s a very elegant and amazing system, and it has to be to pack millions of miles of DNA inside my 6’ 4” frame. Elegant and amazing is the take-home message for this post.
On a similar note, if you want to see some really cool images of cells and DNA, check out the Invitrogen-Molecular Probes gallery at http://probes.invitrogen.com/servlets/gallery?id=18&company=probes. These are examples of cells stained with fluorescent dyes for various proteins, organelles and nucleic acids. These photos are as cool as the Hubble images, IMHO, and very similar in some cases.