Why Don’t I Have an Arm for a Leg?
Authors: Can Atacan & Sam Marchetti
Humans are made up of billions of tiny cells, all of which have nearly identical instructions in the form of DNA. In fact, even between different people our DNA is over 99% identical. But if that’s true, why do we look different from each other? Why do we even have different body parts? Why are your eyes different from your skin? Why is your hair different from your hands? Or why is your brain different from your stomach?
If you asked the Teenage Mutant Ninja Turtles, they would suggest that it’s all because of mutations, also known as changes in our DNA. While there’s a lot of emphasis on mutations in popular media, it’s not about just having or not having genes that determine our physical traits! There are also special molecules which increase or decrease how much different genes get to be expressed, like a volume dial.1
You’ve probably seen a picture of DNA wound up (or “supercoiled”) in a spiral shape like fusilli pasta, known as a double helix. DNA isn’t always in this shape; it also unravels to become a straightened thread (called chromatin) when DNA needs to be duplicated. Chromatin is wrapped around cylinder-shaped molecules called histones. Think of it like a spool of thread–the histone is the spool, and the DNA is the thread wrapped around it. Once unravelled from the histone “spools,” it becomes easier for our cellular DNA copying “machinery” to access our genes.2
There are a few ways the histones (the spool) can change how genes work. The most well-known way is that other special molecules called methyl groups attach to parts of chromatin where genes need to be expressed less (like lowering the volume dial on those genes).3 The methyl group doesn’t change the gene itself, it just reduces its expression, aka how active that gene is. As an analogy, you might think of how an orchestra uses “mutes” for brass instruments. These devices lower the sound of specific players in the band, and change the effect they have on the overall sound of the orchestra. The methyl groups act in much the same way: being added to specific genes (or “players” in our genetic orchestra) to alter their expression and the overall sound.
Another way that histones control gene expression is by changing how chromatin is wrapped around them. The chromatin thread is either more tightly wound around the histone spool, making it harder to reach the genes and slowing or stopping them from being expressed, or more loosely bound to the histone spool, making it easier to reach the genes and increasing how much they’re expressed.4 This is often controlled by other special molecules called acetyl groups attaching to the histones, causing them to wind more tightly or loosely (adjusting the volume dial again).5 These mechanisms are part of what’s known as epigenetics–changes in how a gene works without any changes to the gene itself.6
Throughout our body, our cells mostly all have the same DNA. The big difference are the patterns of these special molecules on the DNA, and which genes are then expressed. The result is that you get different body parts and functions! Otherwise, we would probably just be a random blob of cells.7
Paro, Renato, Ueli Grossniklaus, Raffaella Santoro, and Anton Wutz. 2021. Introduction to Epigenetics. Learning Materials in Biosciences. Cham: Springer International Publishing. https://doi.org/10.1007/978-3-030-68670-3.
Paro, “Epigenetics”.
“What Is Epigenetics?” Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, August 15, 2022. https://www.cdc.gov/genomics/disease/epigenetics.htm#:~:text=Epigenetics%20is%20the%20study%20of,body%20reads%20a%20DNA%20sequence.
CDC, “What is Epigenetics”.
Paro, “Epigenetics”.
Paro, “Epigenetics”.
Paro, “Epigenetics”.