DNAs fourletter alphabet

Although DNA can be a huge molecule, it's actually a simple one. DNA is made up of just four different building blocks that are called nucleotides, or bases. The four nucleotides are


^ Adenine, abbreviated as A ^ Cytosine, abbreviated as C ^ Thymine, abbreviated as T ^ Guanine, abbreviated as G

The function of DNA is information storage, which is what's so cool about it. All the instructions needed to make you can be written with just four letters!

The structure of DNA is a double helix consisting of two strands winding around each other (see Figure 3-1). Each strand can contain up to thousands of the four nucleotides, and the two strands are joined in a very specific way:

i A always pairs with T. i C always pairs with G. i T always pairs with A. i G always pairs with C.

One strand is an inverted version of the other, so if you know the sequence of one strand, you know the sequence of both strands. To give you an oversimplified example, if you have one strand of CATG, you know that the other strand is GTAC.

You may wonder how four nucleotides could possibly be the basis of all life in all its complexity. Well, you wouldn't be the first. What scientists discovered is that these four letters actually appear in groups of three, called codons. It turns out there are 64 codons. (You can read more about codons and the genetic code in the section "Protein-coding RNA and the genetic code" later in this chapter).

The double-helix structure of DNA has two properties that make it an excellent molecule for information storage:

^ It's an incredibly tough molecule. DNA is so tough, in fact, that scientists have been able to isolate intact DNA from extinct mammoths found buried in Siberian ice and even, in some cases, from fossilized bones.

^ The double-stranded arrangement provides a very easy way to make accurate copies. During DNA replication, the two strands of the double helix separate, and an enzyme (a protein that's involved in facilitating a chemical reaction) called DNA polymerase makes two new strands by using the original strands as guides. This produces two double helix molecules. Each is an exact copy of the original.

A DNA cocktail: Extracting DNA at home

DNA tends to strike people as being somewhat mysterious, but really, it's just a very long molecule. Although it's certainly amazing how organisms use DNA for information storage, it's not magic — just chemistry. If you want to see DNA rather than just read about it, the following recipe for a DNA cocktail shows you how to do so. The chemistry is simple.


8 strawberries

Mi ounce blue Curaçao liqueur % ounce gin

2 ounces fresh pineapple juice Instructions:

1. Freeze the strawberries.

A tall narrow glass works best. A test tube would work too, but make sure it's a clean one!

3. When the glass (or test tube) is cold, add the Curaçao.

4. Tilt the glass or tube with great care; then pour the chilled gin down the side to form a layer above the Curaçao.

5. Purée the frozen strawberries with the pineapple juice for 10 seconds.

Strawberries contain DNA. Blending them with pineapple juice allows the enzymatic activity in the juice to free the DNA from all the other bits of the strawberry that it hangs onto. The fresher the pineapple juice, the more enzymatic activity it will have, allowing the experiment to work even better.

6. Layer the strawberry-pineapple mixture on top of the gin.

When the now-dissolved DNA comes into contact with the cold gin, it precipitates out of solution (that is, turns into a solid due to the chemical reaction), and you see little white wisps floating in the gin layer. Those white wisps are the actual DNA molecules, and they contain all the information that makes the strawberry plant what it is. The Curaçao doesn't serve any high-tech chemical function; it's just there to make the final product a nice red, white, and blue — and to make it taste nice!

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