7.4: Using the DNA Blueprints to Make Protein

Thus, a sequence of triplets in DNA is the blueprint for a chain of amino acids—for a protein. The DNA in our cells is much like an extremely long tape—3 billion bases long—on which about 20,000 blueprints for proteins are imprinted. Using these blueprints, the cell gathers the amino acids and arranges them into a wide array of proteins, much as children use a small variety of toy blocks to make a wide variety of structures.

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Each blueprint to make a protein is called a gene. Human DNA has about 20,000 genes. These genes are collectively called the human genome.

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Let’s watch a tiny section of a long thread of our cell’s DNA to see how this happens. We see its double strand “unzip” for a short length, letting the two strands separate:

We assume that the body has called for the production of a certain protein. And for clarity, we look at the end of the chain. But actually, the “unzipping” can happen anywhere along the chain, depending on which instructions are needed to make whatever kind of protein is in short supply. The length of the unzipping, of course, is governed by the length of the instructions needed to make that protein.

To keep matters as simple as we can, we will now follow just one side of the unzipped double chain. And we will follow just one triplet—the genetic code for a particular amino acid—in the instructions to make this protein.

There are three main stages involved. First, the blueprint for the protein has to be reproduced. Then the reproduction has to be sent out from the nucleus to the cytoplasm of the cell, where the protein is made. Finally, the needed raw materials (amino acids) have to be brought together and assembled in a chain.

The reproduction of the blueprint begins with certain submicroscopic bits which float freely in the nucleus. These bits are like the chemical units that make up DNA, but differ slightly. One difference is in the sugar which joins with the phosphate and base.

In DNA, that sugar is deoxyribose (for deoxyribonucleic acid). But in the loose bits floating in the nucleus, the sugar is ribose, and thus, the new material is called RNA (for ribonucleic acid).

The base of each of the new loose bits is much like the base of one of the four DNA bases, which we illustrated with the four playing card suits (like a single heart, diamond, and so on). And the moment that the DNA spiral unzips, the RNA bits rush in to seize the open places that each matches: a diamond snaps onto the heart of our triplet, a club onto the spade and a heart onto the diamond:

As soon as the RNA bits lock onto their opposite numbers in the DNA, they link to one another and begin to make their own chain. When this RNA copy of the protein’s blueprint—perhaps fifty triplets long, perhaps a thousand—is completed, the RNA “tape” breaks its bond with the DNA and pulls free.

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The first step in protein synthesis is to make a copy of the blueprint for the protein.

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The blueprint for a protein has been reproduced, but with a reverse image. If we look back at the drawing of our triplet as it begins to form from RNA bits, we can see that the RNA reproduction is a diamond-club-heart, through the complementary matching of the DNA heart-spade-diamond. One might compare it to the negative of a photograph.

At this point, the newly formed RNA “tape,” having freed itself, begins to move. This is the second phase of the process. It leaves the cell’s nucleus and moves out into the cytoplasm, where it seeks one of the many tiny protein factories—the ribosomes. Because of the function it performs, our wandering RNA is given a more specific name. It’s called messenger RNA. And it delivers its message through a channel in the ribosome.

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The copy of the blueprint to make the protein is called messenger RNA.

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The journey looks like this:

The voyage of messenger RNA. The RNA “tape” breaks free from the DNA spiral, slips through the wall of the nucleus and out into the cytoplasm, where it goes to a ribosome, the site of protein synthesis.

To understand what happens now, it will help if we focus in on our triplet again, as a part of the messenger RNA that feeds into the ribosome.

As we move in closer, we begin to see some odd-looking particles. These particles are called transfer RNA. It, too, gets its name from its function, which is to attach to a certain kind of amino acid and bring it to the ribosome. There are at least 20 kinds of transfer RNA—at least one each for the 20 kinds of amino acids needed to make protein.

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Each of the 20 amino acids has its own special transfer RNA to carry it to the ribosome.

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Each transfer RNA is designed to select and attach itself to a specific amino acid, and also to match up with the triplet code carried by messenger RNA for that same amino acid. At one end, each of these new particles has its own triplet of hearts, clubs, spades, or diamonds in the special combination identifying the particular amino acid. At the other end, the new particles have a curious shape, which looks something like a piece from a jigsaw puzzle. It’s a highly specific shape—it’s made so that only the particular kind of amino acid coded for will just fit it.

Such a particle might be represented like this:

Each amino acid attaches to and is carried by the transfer RNA that carries its triplet code. The shapes of the amino acids are kept simple in the drawings, but they are actually ornate, three-dimensional structures that differ in size, shape, and chemistry.

As each triplet on the tape of messenger RNA enters the channel of the ribosome, it calls for a matching transfer RNA triplet. In the matching process, the “reverse image” sequence of the messenger RNA triplet is paired through a second reversing process with a transfer RNA triplet with the same sequence as the corresponding triplet on the original DNA. Going back to our example, its diamond club heart triplet attracts one bit of transfer RNA with the complementary heart-spade-diamond sequence.

If we think back to the start of all this, to the triplet we saw on the DNA spiral, it, too, was heart-spade-diamond—DNA’s code for a particular amino acid. That amino acid, and no other, is now attached to the other end of the transfer RNA.

As all the triplets on the messenger RNA tape pass through the ribosome, each triplet does the same job. It attracts the right transfer RNA, which in turn holds the right amino acid—until dozens, or hundreds, of amino acids are lined up in the exact sequence called for originally by the corresponding segment of DNA in the cell nucleus. The process looks like this in a very short section of the tape:

A simplified picture of the ribosome “assembly line” in action. The transfer RNAs arrive with amino acids in tow. These are lined up according to the instructions of the RNA messenger tape. The amino acids are held in proper order until they link with one another. gradually making the entire protein.

Now, as each amino acid falls in line, it joins with the others. And when—according to the blueprint—the chain is complete, the chain breaks free. That chain, of course, is a new protein.

Let us recap briefly. The DNA of a cell has about 20,000 blueprints (instructions) for making human proteins. When a particular protein is to be made, a matching copy of the blueprint to make that protein is made in the form of the tape-like messenger RNA.

The messenger RNA then travels to the ribosome where the protein is assembled. Here, the triplets in the messenger RNA are matched—one by one—by the incoming triplets of transfer RNA. Each matching transfer RNA brings with it the appropriate amino acid. The amino acid is linked into sequence to make one link of the chain we know as protein.

It’s as though the messenger RNA had the instructions for assembling a freight train. At the ribosome, which is like a marshaling yard, each type of car needed is summoned up and coupled in line. Once completed, the train is immediately released for its destination. Then new instructions are received, and another train—perhaps a completely different sort—begins to be assembled. A ribosome is capable of putting together any protein which the messenger RNA calls for.

It’s by this method that food becomes life. For, protein—as enzymes, and such—is what enables life as we know it.

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All of life uses the same genetic code in making proteins. This is why we can use bacteria to make human proteins: Using the techniques of biotechnology, we can snip out a gene in human DNA and insert it into the DNA of bacteria.

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Since the body assembles all these proteins from just 20 kinds of amino acids, we can see clearly why our need for dietary protein is so basic. We can also see that the basic need isn’t for the proteins themselves, but for the amino acids they contain.

What’s truly amazing is that all life forms make their proteins from the same 20 kinds of amino acids, using the same triplet codes for these amino acids, and using the same method of protein synthesis. Each with its own unique DNA as an orchestrator, a tomato seed grows into a tomato by using the same fundamental process and raw materials that a robin’s egg uses to become a bird.

Do We Need to Eat DNA?

A doctor recommending a “no aging diet” tells of the importance of DNA and RNA in our lives. He says that the quality of our DNA and RNA goes down as we age, and for good health, “Foods high in DNA and RNA—nucleic acids—are the key.”

According to this doctor’s thesis, we can counter these aging effects and recapture some of our youth by taking in new DNA from food. Is this a realistic possibility? Luckily not. The DNA and RNA in foods are broken down before being absorbed from the digestive tract. (Our cells make all the DNA and RNA we need—they aren’t required in the diet.) But if our cells were able, in fact, to absorb and use the DNA in food—or in certain “rejuvenating” skin lotions—we must consider the consequences.

In the “no aging diet,” foods such as sardines, salmon, and the organs of the cow are recommended as “DNA rich.” If we were able to use such DNA, we’d begin to make protein according to their blueprints. We’d form the structures and chemicals suitable to fins, horns, and tails. Our rejuvenated selves would be “newer” in the extreme.