DNA size to scale, the biggest instruction set ever written
Let’s say we can print your DNA in a book, how many pages should it have? Fortunately for us, The Human Code Foundation literally printed a human genome, which is the entire collection of DNA a living thing has, and this resulted in 175 volumes of 1,600 pages each! The enormous amount of information that defines a living organism is stored in these DNA molecules.
Not surprised? Well now consider this, that collection of books is the amount of information we can find in a single cell when it contains a unique copy of the code, also most of our cells include twice this amount and we are made of trillions of cells!
An even more interesting question might be: How cells manage to store so much in such a small space?
Instead of just giving you numbers like the length of DNA molecules inside a single cell, or the diameter of the nucleus, which is the inner compartment in which cells store DNA, allow me to picture the proportion between the size of both, DNA molecules and the nucleus, in terms of objects we are more familiar with.
First, consider that we are about to use as reference the equivalent of two copies of the entire Human Code books collection mentioned above because this is the amount of information present in most of our cells. Actually, if we could put all these 46 DNA molecules one next to each other as a stretched line it should reach 2 meters in length! On the other hand, the diameter of a nucleus is equivalent to 0.00001 meters, see where we are going?
Now let’s use our imagination:
If a mandarin orange represents the nucleus of a cell and the 46 DNA molecules inside it are represented as silk fibres, then the resulting length of putting the 46 fibres next to each other as a stretched line should reach 16 kilometres, which is longer than the depth of the Mariana Trench, the deepest spot on Earth’s oceans. Take a moment to picture that in your mind, you swimming at that point, and all the distance between you and the bottom of the ocean … inside a delicious mandarin orange!
To be fair the width of the fibres are also important and above we represented them as silk fibres following the same proportion considering the mandarin orange as a reference to a human cell nucleus. Also, the width of a silk fibre is slightly below the smallest object visible to the naked eye but still, I am sure you can better feel the dimension of the question we did before: How cells manage to store so much in such a small space?
As you might expect, a complex packaging system is part of the answer and the key idea behind it is that it shows several levels of folding. A group of auxiliary proteins called histones play a central role and the resulting DNA-protein structure is known as chromatin. Let’s take a closer look.
In the image only the first packaging level is visible: the nucleosome. These units, seen as black dots (black arrowheads), are made by histones and the DNA, seen as lines between the dots (white arrowheads), wraps around each unit almost two times. From here some sections of chromatin get more tightened as the fibre made of nucleosomes coils in the second level of packaging: the 30-nano meter fibre. Yes, most of the names in biology are as original as this one.
Additional levels of packaging arise when the cell enters division, but that is a story for another day. More research is needed, as new discoveries always come up with more questions.