So far we’ve been comparing DNA to a book. The nucleotides are like letters and their sequence contains information, just like letters do when writing. But of course there is more to a book than just grammar. There is also the structure of a book to consider.

To really appreciate how complex and large DNA is, we can think of it more as an encyclopedia than a simple book. Because the amount of information contained in DNA really is so large.

Encyclopedias are very large, and so they need to be properly organized. There are specific volumes, editions, chapters, indices, tables of contents and so on.

DNA, for its part, is organized into chromosomes, which are like the volumes of an encyclopaedia. Each chromosome has a middle, called the centrosome, and two ends, called the telomeres. Those parts don’t really contain information, but are important to keep DNA working properly. Think of them like the covers of the book.

DNA is also associated with lots of proteins. The double helix is how pure DNA looks like, but in the cell, DNA is coiled around very small proteins that keep it tightly wound up. These proteins are called histones and DNA is wound around them to make something that looks like a beaded necklace. This is further wound up into spiraling rings, which then form loops. All this complex winding make DNA vary compact, so that the millions of nucleotides can be kept inside the nucleus. The histones are like the glue and strings that keep a book together.

When we talk about DNA, we refer to the string of nucleotides, but it’s important to remember that there are different regions of a chromosome and that DNA is always found in combination with proteins. So just like a book contains information stored in a physical package, so too does DNA. In up-coming podcasts, we’ll discuss how modifying the packaging, that is the histones, can have a major affect on what DNA does.