DNA: A LIBRARY OF INFORMATION

The words of any language are a means to store and transmit information. For example, all English words are made from combinations of the 26 letters in the alphabet. Many, many words can be made from these letters. An unabridged dictionary contains 450,000 words, all made from only 26 different letters. In some ways, the nucleotides in DNA are like the letters of the alphabet, only the DNA letters are “chemical” letters. Because there are only four letters—A, T, G, and C—in the DNA alphabet, scientists thought that DNA was too simple to contain the complex genetic information of life. But what is also significant in DNA is the sequence of the letters, not only the letters themselves. Using these four letters in long sequences, nature can create an almost unlimited variety of genetic messages. In fact, by creating messages only ten nucleotides long, it is possible to make more than one million different sequences or messages with the four nucleotides.

When you realize that human DNA consists of not ten but three billion pairs of nitrogenous bases, you can begin to imagine how much information can be stored in the DNA of our cells! All of the information for constructing our bodies, determining all of our characteristics or traits, and keeping our bodies running is stored in the linear sequences of nucleotides in our DNA. The same is true for every bacterial cell, insect, fish, bird, tree, and all other organisms on Earth. Once scientists realized the importance of the linear sequence of DNA nucleotides, they were anxious to determine the sequence of nucleotides in a particular DNA molecule. In Chapter 2, you learned about one important reason for doing this. The evolutionary relationship of two organisms can be learned by comparing their DNA. The more similar their nucleotide sequences, the more recently the two organisms evolved from a common ancestor. (See Figure -8.)

To make use of the genetic information stored in DNA, organisms must change the information into proteins. Proteins are made up of amino acids that are linked to each other. So, a protein is another linear sequence of subunits. In the next chapter, you will learn how the information stored in DNA gets expressed in the form of proteins.

Check Your Understanding Why is the sequence of nucleotides in each strand of a DNA molecule so important? (Give more than one reason.)