Building many different tools with a single blueprint – the power of introns

Many bacteria can and do live as single cells. Humans, on the other hand, cannot. We are composed of many trillions of cells with various functions, shapes, and sizes. Considering this discrepancy, you might think that humans have many more genes than bacteria, but this isn’t the case. Free-living, single-celled bacteria generally have a few thousand genes and 37 trillion-celled humans only have about 20,000 genes. This is confusing because these 20,000 genes are supposed to contain all of the blueprints for our 37 trillion cells. So where do the additional instructions come from???

Changing genetic instructions with introns

Part of the answer lies in the way human cells read genes. Think of your genes as sentences explaining how to create cellular parts. For example, one of your genes might read,

“Make a red and white square.”

Even though the DNA encoding this sentence would be the same in all of your cells, some sections of the sentence can be ignored in some cells.

For instance, your immune cells might read the above sentence as:

“Make a red and white square.”

While a muscle cell would read it as:

“Make a red square.”

And a brain cell would read it as:

“Make a white square.”

By ignoring or including particular sections of genes, individual cells create different cellular parts. Biologists call the ignored sections of genes “introns.”

This differential reading of genes makes cells more diverse. You can imagine that if you had a more complicated sentence like:

“Make a red, white, and blue square and place it next to any green triangles.”

Cells could read these instructions in MANY different ways. Thus, you might reach the cellular complexity and diversity found in the human body.

Introns aren’t the only route to cellular complexity

Before closing, I’d like to point out that, even though bacterial genes don’t have introns, that doesn’t mean they’re simple. Single bacterial cells often perform different functions under different conditions and both bacteria and humans have means other than introns to encode this complexity. Perhaps I’ll write about these additional means in another post so you’ll have to subscribe using the box to the right 😀