The DNA Coiling Demonstration

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 (

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  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.

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Formulating Hypotheses

In an effort not to write a Toltsoy-esqe blog post, I’m breaking some background information out into separate posts.  There’s a point, I promise, it’ll just take me a couple posts to get there.

The Scientific Method is a standard process which is used to properly frame an experiment and guide the experimenter (at least it should be, but there’s a lot of junk being done these days).  Various descriptions of the Scientific Method give it a variable number of steps–from 4 to 7.  Those with fewer steps combine a few things that others break out into separate steps–there’s no big divergence in process.  For the sake of discussion, we’ll use the description at

One step, in this case #3, is always stating your hypothesis.  Classically, one wouldn’t just make a single statement and test against it; one would make several statements, each predicting a different outcome of the experiment.  Usually there is a statement of no difference, called the Null Hypothesis, and abbreviated H0 (that’s H sub zero).  One would then offer Alternate Hypotheses predicting different effects, and abbrevizted Ha, Hb, etc.

Let’s look at a simple example: will fertilizer help us grow bigger tomato plants?  We’ve made our observation that Farmer Brown grows nice tomatoes, and he uses fertilizer.  Our hypotheses would be:

H0: Fertilizer will have no effect

Ha: Fertilizer will help our plants grow

Hb: Fertilizer will hinder our plants’ growth

Simple, and easily tested.

Not every hypothesis can be tested.  The human lifespan is too short, and the vastness of space and time and the complexity of our own bodies (let alone the ethical considerations) make some hypotheses virtually untestable.  In these circumstances, even with some solid evidence, the discussion is more academic and philosophical than scientific, especially if there are equally strong (or preposterous) interpretations of the same evidence.

Additionally, another important component of the Scientific Method is repeatability.  Performing an experiment numerous times, and getting the same (or wtihin statistical probablity) answer each time is essential.  In our tomato experiment, we can easily reproduce this hundreds or thousands of times under very controlled conditions.  Referring again to vastness and complexity, some experiments can’t feasibly be repeated.

Quod erat demonstratum (“that which is to be proven”) sometimes always remains as such.

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For anyone who’s new to the conversation, I hold a BS in Environmental Science and an MS in Biological Sciences.  I earned my MS studying the genetics of pollution-degrading bacteria and the external proteases of a pathogenic bacterium.  While in graduate school, I was both a teaching and research assistant; I was awarded two teaching awards, and was rated “excellent” by my students when I taught MCAT prep classes for The Princeton Review.  After graduation, I worked as a molecular biologist in medical research for many years before switching to computer programming.

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