The human genome contains over 3 billion base pairs of genetic
information. If it were stretched out end to end, it would span approximately
6 feet. But each human cell that houses all this information is about 0.0002
inches across, approximately 360 thousand times smaller than the
length of all of our genetic information laid out side by side. So
how in the world are we able to fit that massive amount of information into
a tiny cell? As always, nature came up with a clever solution. Instead of all
that DNA being stuffed into cells as long, sinuous strands which wouldn't
leave room for much else and would be hard to untangle when the information
is needed, the information is coiled up around a series of proteins into a
highly organized condensed structure called chromatin.
As you'll see, it's similar to the process of turning wool into a knitted
sweater, where each piece provides structure and organization to the next.
Let's explore this concept a bit more. First, a small segment of the DNA
double helix is wrapped around a collection of proteins called histones,
which forms a structure called a nucleosome.
Nucleusomes then fold on top of each other into a single chromatin fiber.
From here, the chromatin fibers are further coiled and intertwined into a
chromosome, which organizes our genetic information in the form of genes, units
of DNA that provide instructions to make proteins. Each human cell has 23
pairs of chromosomes, or 46 total, which contain approximately 20,000 to
25,000 genes, all of which live inside the cell's nucleus. From here, all that
neatly packaged information is responsible for making sure each cell,
and in turn our whole body, functions properly. It turns out that this
packaging trick doesn't just save space inside a cell, but also makes sure DNA
is able to do its job properly. Think of it like a coiled up elastic band;
when it is compacted together, no individual strands are accessible, but
you can pull away particular pieces of the coil band as needed. Similarly,
when DNA is tightly wound around histones, also called heterochromatin,
it isn't accessible to the proteins required to read DNA, so those genes
remain turned off. In response to environmental changes, however,
histones are altered through a variety of molecular processes which triggers
the DNA to uncoil into a more relaxed state, known as euchromatin.
Euchromatin is accessible to the DNA reading proteins, thus turning the
necessary gene on. These dynamic changes in chromatin density, a process
called chromatin remodeling, are important for the process of cell
division— when cells copy all of their genetic material and then segregate and
repackage it into the newly formed daughter cell. In non-dividing cells,
chromatin remodeling allows for tighter control over when and where certain
genes are activated, which ensures complex organisms, like humans, run
like a well-oiled machine. Beyond ensuring the right DNA is accessible at
the right time, this packing structure also prevents DNA from getting tangled,
which reduces DNA damage. The structure of chromatin is one of biology's many
ingenious solutions to a mathematical challenge and simultaneously solves
numerous cellular complexities. Number one, it ensures vast amounts of genetic
information can fit into each cell. Number two, it enables complex
organisms to diversify their functionality. And number three, it
helps prevent DNA damage. More levels of biological intricacy, however, also
means more opportunities for mistakes to happen. Researchers are finding that
genetic mutations affecting chromatin structure are common in many types of
diseases, especially cancer. While scientists are still trying to uncover
the precise roles these mutations play in diseases, efforts to develop drugs
that target chromatin machinery have shown promise in early clinical studies,
revealing their potential to provide a new type of medicine for challenging
health problems.