 The 3 billion DNA letters that make up the human genome encode roughly 21,000 genes. These genes in turn provide instructions for making an estimated 250,000 to 1 million distinct proteins. In the body, proteins are the worker bees. They are responsible for the make-up and proper functioning of cells. They help metabolize and digest our food. They make it possible for the brain to think and much, much more. How is it that such a small number of genes can provide directions for making at least 12 times as many different proteins? The answer lies in a very clever mechanism that nature has evolved called RNA splicing. The process works like this. A sequence of DNA comprising a single gene is copied. This copy is made of RNA, a molecule related to DNA but more temporary. You can think of this RNA copy as the gene's message. It contains the instructions for making a protein. Let's say a gene's full sequence looks like this. There is a message in there, but it's hard to see because there are a lot of other gibberish letters in between the words. The gibberish must be removed to make the message readable. The letters that are removed are called introns, and the remaining letters, the segments of the gene's message that encode a protein, are called exons. In this case, the RNA message is made up of 11 exons. As the introns are cut out, the exons are pasted together to form what biologists call a mature messenger RNA. The message now contains only those letters necessary to make a protein, or in our analogy a meaningful sentence with 11 words or exons. Draw a big house on the corner of the wide street. The process of going from a full length and edited RNA instruction to a coherent message, draw a big house on the corner of the wide street, is called RNA splicing. Now that we understand how proteins are made from DNA and RNA, let's get back to our original question. How do so few genes produce so many different proteins? The answer is by alternative splicing of a gene's RNA message. There are often many different ways to assemble the exons, just as there are different ways to combine words to form a sentence. Draw a big house on the corner of the wide street. We used 11 exons to form this thought, but these same exons could be combined differently to produce different meanings, the equivalent of different proteins. For instance, draw a big house on the corner of the street, or draw a house on the corner of the wide street, or even draw a house on the corner of the street. Alternative splicing enables organisms to expand the repertoire of proteins, which allows them to evolve complex biological functions, as well as adapt to new environmental conditions. It is truly one of nature's greatest inventions.