DNA is big news nowdays. It's used as a plot device in a lot of science fiction movies, it pops up in the news, and the Human Genome Project even made it to the White House when the scientists involved announced that they had "decoded the genetic map of the human race."

All well and good. But what does it mean? What does it mean to say that a genetic "map" is created, that the "code of life" is "decoded?" Before we get into that, we need to explain just what DNA is, and what it does. No, no -- don't click that "Back" button! Just sit tight. It's not that hard to understand.

The Human Cookbook

Your body is composed of all sorts of materials. Tiny living components called cells make up all of your internal organs -- all of you, actually -- and they produce complicated molecules called proteins that they need to sustain themselves. But cells die, and proteins wear out. Both of these things must be renewed continually for as long as you are alive. This means that, in essence, if you look at your hand, wait two weeks, then look at it again, it's not the same hand! The cells that made it up two weeks ago have died in the meantime, and some of the proteins have worn out. New cells and new proteins have been created to replace these old ones. And in order for your body to carry on this continual renewal process, it needs a recipe book to consult to make new cells and proteins.

That recipe book is your DNA. It's been called the code of life, the parts list for a human being, the directions for building a human being. A far better way to think of it is to think of it as the recipe book for a human being -- the complete directions for making all of the gunk that goes into renewing you, each and every day of your life as the parts of your body wear out and need to be rebuilt and replaced, over and over.

Now, there are some oddities about this particular cookbook. Remember that it doesn't contain recipes for hearts and livers. It contains recipes for proteins, complex molecules that are used to build hearts and livers. It's very nitty-gritty, low-level stuff. Think of a cookbook that has not only recipes for cakes and breads, but directions for how to grow the wheat and make the flour as well. It's seriously cooking from scratch.

Now, in that cookbook, you might have a dozen recipes for different cakes and breads and pastas -- but they might all use the same recipe for flour. Similarly, heart valves and tendons are different items, but they both use connective tissue. And connective tissue is made up of a lot of ingredients, each of which has their little recipe on your DNA. And these recipes for the individual ingredients might be spread out all over the place -- one ingredient for connective tissue might be on Chromosome 3, another might be on Chromosome 15, another on Chromosome 20. And there might even be something on Chromosome 4 that isn't used to create connective tissue, but the protein that it makes interferes with something else on Chromosome 20. So there's not only the individual recipes ("genes") that you have to be aware of, but also how they might interfere and interact with one another.

Think again of that from-scratch cookbook I was talking about. It's as if you wanted to make a loaf of bread. You'd find a set of directions for how to culture the yeast on page 4, directions on how to make flour on page 29, directions on how to make an oven on page 186, and directions on how to cultivate chickens so you can get eggs on page 349. And then, the directions on how to put all these ingredients together is on page 412! And maybe there is a recipe for olive oil on page 46 that you aren't using, but that tells you to tear up the chicken coop to plant an olive tree, so even if you don't care about olive oil, if someone else is trying to make it, they might get in the way of your egg production!

This helps to illustrate what "genes" are -- they are recipes for each of the individual ingredients ("proteins") for all the bits and pieces that go into making up one of you. They are spread all over the place, and even the way they interact with one another can influence the end product.

Introns -- the hangers-on of the genetic code

But even that's not the whole story. Going back to that from-scratch cookbook with the directions for flour and olive trees -- print out a few thousand pages of gibberish, just random typing like "SECVER%^*ebtjy5&^#%^BUYbjfhjb5u6&%^#VWJHrhv6y2R", and interleave those pages in with the good ones with the recipes on them. Just stick them in randomly, thousands of pages of gibberish.

Your DNA also has lots of stuff in it that's similar -- just holdover DNA from the millions of years of evolution that has been going on ever since the first little globs of gunk started to make copies of themselves a zillion years ago. We don't use it now. It's useless to us. It doesn't code for anything whatsoever anymore. But it's there, taking up the vast majority of the space in our recipe book. These pages of gibberish are called "introns," just old, useless DNA code that doesn't do anything but take up space. And it makes up over 90% of our genetic code!

Why is it there? Well, why not? Unless it explicitly gets in the way, there isn't any real reason for it to evolve out. And evidently, it doesn't get in the way. It just . . . sits there. And as far as that useless gibberish goes, we are nothing but vessels by which it is replicated. It doesn't code for anything we use. It doesn't even know we exist. We are naught but machines that reproduce it, over and over.

There is, however, some sentiment that it does do something useful for us, though. Consider the process of mutation. When you photocopy that from-scratch cookbook, the copier might smudge something or make one letter look like something else. The copying process is inexact and makes mistakes. If your code was 100% useful materials, every single mutation would interfere with something you needed. And mutations happen a lot more than you think. Up to 30% of all cases of Marfan Syndrome are due to spontaneous mutation, when this copying process introduces a typo for no reason.

But thanks to that 90%+ of gibberish in your code, the vast majority of mutations don't affect anything you need. Those few genes that you actually use are surrounded by a nice padding of intron "bodyguards" who take the majority of the bullets of mutation to keep the useful code unharmed.

DNA and coding for more than one protein

The next odd wrinkle in the story of your DNA is strange enough that it took a long time for scientists to realize it. It's still weird, and no one is entirely sure how it happens.

Back to the from-scratch cookbook -- suppose you had a page with directions on it for "how to grind flour." But further suppose that, if you read this recipe backwards, it tells you how to squeeze lemons for lemon juice!

Believe it or not, your genes do this! It was previously thought that each gene coded for one protein. But it's been discovered recently that -- and no one's sure how this happens -- genes can code for more than one protein. Each recipe, depending on how the body reads it, can tell the body how to make four or five different proteins! Weird, huh? Again, no one's entirely sure how your body does this, and scientists were pretty flabbergasted when they discovered it.

Aging and DNA

Then, there's the third wrinkle. Remember how I said earlier that your body was continually renewing itself? Your cells are continually making copies of themselves to replace old cells that die off, and they use your DNA recipe book to do it.

But there's a catch -- the cells also contain the recipe book! So the recipe book is also being replicated over and over and over again, and if you've ever tried serially photocopying a piece of paper, then copying the copy, then doing that again . . . you know that it doesn't take long before the page is unreadable.

Well, the same thing happens to your DNA. If it didn't, you'd just . . . keep going, continually renewing yourself without a single error of replication. Clearly, this doesn't happen. After a while, the genes that code for the ingredients in collagen have had enough mistakes introduced in them that they don't make as good collagen anymore, and you get wrinkles. The various genes that code for hair pigment have enough errors that your body doesn't have the right recipe for it anymore, and you start to go grey.

So if you look different at age 50 than you did at age 25, there's a reason for it. Your cookbook has changed, and you aren't renewing yourself as well as you used to.

So what IS the "Human Genome Project?"

What the Human Genome Project has done is essentially put a Table of Contents in front of the from-scratch cookbook. It's a starting point for learning what the DNA does. Having the Table of Contents in front of the recipe book won't necessarily turn you into Julia Child. It won't give you immediate insight on how the recipes interact. And if a recipe has a typo in it, it won't necessarily teach you how to fix the problem.

But, without it, you won't be able to learn anything. It's a starting point. At least you know what pages contain actual useful information, and you know which directions can be found where. You don't know how many different versions of each recipe there are. After all, we're not all identical, so somewhere in the recipes for body pigment there must be differences that allow one person's body to grow red hair and another's to grow black hair. And you don't necessarily know how many body systems will be influenced by a given recipe -- just as a typo in the flour directions in your from-scratch cookbook might screw up dozens of recipes for cake, bread, and pasta, a typo in one part of your DNA might adversely affect your eyes, your heart, your skeleton, and your ligaments, just as the typo on C15-FBN1 does in Marfan Syndrome. And it's a far cry from knowing what causes a disease to curing it. Sure we know what causes diseases like Marfan Syndrome, Cystic Fibrosis, Tay Sachs, and hemophilia, but what on Earth do you do to fix it? Repairing all of that faulty DNA is just beyond our means right now.

So the mapping of the human genome (putting a Table of Contents on the from-scratch cookbook) is a vital first step in learning how our recipe book results in one of us, but it's only the first step. We haven't so much learned how our genes work, but we've learned enough to start learning in earnest.

Human Genetic Diversity -- 99.9% the same!

One of the comments that people often make about the Human Genome Project is, well, whose genome was it that they studied? And how applicable is that to the rest of us? After all, we're all so different from one another -- different color hair, different sizes, different blood types. How can you learn about all humans by studying only one or two?

The answer to this usually surprises people. All humans are 99.9% the same. Identical. Absolutely genetically indistinguishable. And we're more similar than most species. What seem like huge differences to us are really nothing compared to the differences in most other species. Two chimpanzees are only about 80% the same. And dogs? You'd be hard-pressed to tell that a Yorkshire terrier and a Labrador retriever were even the same species! But humans? 99.9% identical. If you took the genetic material from an Asian, an African, and a Caucasian, and gave them to researchers to analyze, they would not be able to tell whose genetic material was whose.

The reasons for this are hazy. The most obvious explanation, and the one that most scientists believe, is that the human species bottlenecked a few tens of thousands of years ago. (There's a lovely article about this on the BBC's website.) We came within about a gnat's hair of total extinction, and died out to a population of only about 5,000 or so. All of the current (over)population of 6 billion human beings originated in that nearly-wiped-out population of 5,000, however many tens of thousands of years ago.

And when you think about it, what looks like amazing diversity among humans really isn't that diverse. Look at housecats. They're every color under the sun, with spots and stripes and all sorts of patterns. Look at dogs again. Their heads are shaped differently, their coats are widely varied in color and texture, their skeletons are different enough that they barely seem the same species. Their ears stick up, hang down, are all sizes. Humans, though? Same basic face, no extended muzzles or snouts. Ears the same shape, no pricked up versus hanging ears. No spots, no stripes. We're all pretty much the same size. We're even mostly just varying shades of the same color -- brown. No blue, no green. We're amazingly the same. Our differences seem bigger to us because well, they'll all there is to notice.

So while there are many different kinds of genes, and while people are all certainly different, our mutual genetic heritage is so similar that important information can be gleaned from studying any human being. As far as Earth species go, we're the most "the same" of any of them.

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