When considered in the simplest of terms, the subject of DNA is quite understandable​—and fascinating. So let us take another trip to the inside of a cell. This time, though, we will visit a human cell. Imagine that you are going to a museum designed to teach you about how such a cell works. The whole museum is a model of a typical human cell​—but magnified some 13,000,000 times. It is the size of a giant sports arena, the kind that can seat an audience of about 70,000 people.

You enter the museum and stare awestruck at this wondrous place full of strange forms and structures. Near the center of the cell stands the nucleus, a sphere about 20 stories tall. You make your way there.

You go through a door in the nucleus’ outer skin, or membrane, and look around you. Dominating this chamber are 46 chromosomes. Arranged in identical pairs, they vary in height, but the pair nearest you is about 12 stories tall (1). Each chromosome has a pinched place near the middle, so it looks a bit like a link sausage but is as thick as a massive tree trunk. You see a variety of bands running across the model chromosomes. As you draw closer, you see that each horizontal band is divided by vertical lines. Between those are shorter horizontal lines (2). Are they stacks of books? No; they are the outer edges of loops, packed tightly in columns. You pull at one of them, and it comes free. You are amazed to see that the loop is composed of smaller coils (3), also neatly arranged. Within those coils is the main feature of all of this​—something resembling a long, long rope. What is it?

THE STRUCTURE OF AN AMAZING MOLECULE

Let us simply call this part of the model chromosome a rope. It is about an inch (2.6 cm) thick. It is looped tightly around spools (4), which help to form the coils within coils. These coils are attached to a kind of scaffold that holds them in place. A sign on the display explains that the rope is packed very efficiently. If you were to pull the rope from each of these model chromosomes and lay it all out, from end to end it would stretch about halfway around the earth!a

One science book calls this efficient packaging system “an extraordinary feat of engineering.”¹⁸ Does the suggestion that there was no engineer behind this feat sound credible to you? If this museum had a huge store with millions of items for sale and they were all so tidily arranged that you could easily find any item you needed, would you assume that no one had organized the place? Of course not! But such order would be a simple feat by comparison.

In the museum display, a sign invites you to take a length of this rope in your hands for a closer look (5). As you run it between your fingers, you see that this is no ordinary rope. It is composed of two strands twisted around each other. The strands are connected by tiny bars, evenly spaced. The rope looks like a ladder that has been twisted until it resembles a spiral staircase (6). Then it hits you: You are holding a model of the DNA molecule​—one of the great mysteries of life!

A single DNA molecule, tidily packaged with its spools and scaffold, makes up a chromosome. The rungs of the ladder are known as base pairs (7). What do they do? What is all of this for? A display sign offers a simplified explanation.

THE ULTIMATE INFORMATION STORAGE SYSTEM

The key to the DNA, the sign says, lies in those rungs, the bars connecting the two sides of the ladder. Imagine the ladder split apart. Each side has partial rungs sticking out. They come in only four types. Scientists dub them A, T, G, and C. Scientists were amazed to discover that the order of those letters conveys information in a sort of code.

You may know that Morse code was invented in the 19th century so that people could communicate by telegraph. That code had only two “letters”​—a dot and a dash. Yet, it could be used to spell out countless words or sentences. Well, DNA has a four-letter code. The order in which those letters​—A, T, G, and C—​appear forms “words” called codons. Codons are arranged in “stories” called genes. Each gene contains, on average, 27,000 letters. These genes and the long stretches between them are compiled into chapters of a sort​—the individual chromosomes. It takes 23 chromosomes to form the complete “book”​—the genome, or total of genetic information about an organism.b

The genome would be a huge book. How much information would it hold? All told, the human genome is made up of about three billion base pairs, or rungs, on the DNA ladder.¹⁹ Imagine a set of encyclopedias in which each volume is over a thousand pages long. The genome would fill 428 of such volumes. Adding the second copy that is found in each cell would make that 856 volumes. If you were to type out the genome by yourself, it would be a full-time job​—with no vacations—​lasting some 80 years!

Of course, what you would end up with after all that typing would be useless to your body. How would you fit hundreds of bulky volumes into each of your 100 trillion microscopic cells? To compress so much information so greatly is far beyond us.

A professor of molecular biology and computer science noted: “One gram of DNA, which when dry would occupy a volume of approximately one cubic centimeter, can store as much information as approximately one trillion CDs [compact discs].”²⁰ What does that mean? Remember, the DNA contains the genes, the instructions for building a unique human body. Each cell has a complete set of instructions. DNA is so dense with information that a single teaspoonful of it could carry the instructions for building about 350 times the number of humans alive today! The DNA required for the seven billion people living on earth now would barely make a film on the surface of that teaspoon.²¹

MACHINES IN MOTION

As you stand there in the quiet, you find yourself wondering if the nucleus of a cell is really as still as a museum. Then you notice another display. Above a glass case containing a length of model DNA is a sign that reads: “Push Button for Demonstration.” You push the button, and a narrator explains: “DNA has at least two very important jobs. The first is called replication. DNA has to be copied so that every new cell will have a complete copy of the same genetic information. Please watch this simulation.”

Through a door at one end of the display comes a complex-looking machine. It is actually a cluster of robots closely linked together. The machine goes to the DNA, attaches itself, and begins to move along the DNA as a train might follow a track. It moves a little too fast for you to see exactly what it is doing, but you can easily see that behind it, there are now two complete DNA ropes instead of one.

The narrator explains: “This is a greatly simplified version of what goes on when DNA is replicated. A group of molecular machines called enzymes travel along the DNA, first splitting it in two, then using each strand as a template to make a new, complementary strand. We cannot show you all the parts involved​—such as the tiny device that runs ahead of the replication machine and snips one side of the DNA so that it can twirl around freely instead of getting wound up too tight. Nor can we show you how the DNA is ‘proofread’ several times. Errors are detected and corrected to an amazing degree of accuracy.”​—See the diagram on pages 16 and 17.

The narrator continues: “What we can show you clearly is the speed. You noticed this robot moving at a pretty good clip, didn’t you? Well, the actual enzyme machinery moves along the DNA ‘track’ at a rate of about 100 rungs, or base pairs, every second.²³ If the ‘track’ were the size of a railroad track, this ‘engine’ would be barreling along at the rate of over 50 miles (80 km) per hour. In bacteria, these little replication machines can move ten times faster than that! In the human cell, armies of hundreds of these replication machines go to work at different spots along the DNA ‘track.’ They copy the entire genome in just eight hours.”²⁴ (See the box “A Molecule That Can Be Read and Copied,” on page 20.)

“READING” DNA

The DNA-replicating robots trundle off the scene. Another machine appears. It too moves along a stretch of DNA, but more slowly. You see the DNA rope entering one end of this machine and emerging from the other​—unchanged. But a single strand, a new one, is coming out of a separate opening in the machine, like a growing tail. What is going on?

Again the narrator provides an explanation: “DNA’s second job is called transcription. The DNA never leaves the safe shelter of the nucleus. So how can its genes​—the recipes for all the proteins your body is made of—​ever be read and used? Well, this enzyme machine finds a spot along the DNA where a gene has been switched on by chemical signals coming in from outside the cell nucleus. Then this machine uses a molecule called RNA (ribonucleic acid) to make a copy of that gene. RNA looks a lot like a single strand of DNA, but it is different. Its job is to pick up the information coded in the genes. The RNA gets that information while in the enzyme machine, then exits the nucleus and heads to one of the ribosomes, where the information will be used to build a protein.”

As you watch the demonstration, you are filled with wonder. You are deeply impressed by this museum and the ingenuity of those who designed and built its machines. But what if this entire place with all its exhibits could be set in motion, demonstrating all the thousands upon thousands of tasks that go on in the human cell at the same time? What an awe-inspiring spectacle that would be!

You realize, though, that all these processes carried out by tiny, complex machines are actually going on right now in your own 100 trillion cells! Your DNA is being read, providing directions to build the hundreds of thousands of different proteins that make up your body​—its enzymes, tissues, organs, and so on. Right now your DNA is being copied and proofread for errors so that a fresh set of directions is there to be read in each new cell.