These are tiny molecular machines, and they are doing this inside your body – right now. To understand why, we have to zoom out. Every day, in an adult human body, 50 to 70 billion of your cells die. Either they’re stressed, or damaged, or just old. But this is normal – in fact, it’s called “programmed cell death”. But, to make up for all these lost cells, right now, billions of your cells are dividing, essentially creating new cells. And that process of cell division, also called mitosis — well, it requires an army of tiny molecular machines. So, let’s take a closer look. DNA is a good place to start – the double helix molecule we always talk about. This is a scientifically accurate depiction of DNA, created by Drew Berry at the Walter and Eliza Hall Institute of Medical Research. If you unwind the two strands, you can see that each has a sugar-phosphate backbone connected to the sequence of nucleic acid base pairs, known by the letters A, T, G and C. Now, the strands run in opposite directions, which is important when you go to copy DNA. Copying DNA is one of the first steps in cell division. Here, the two strands of DNA are being unwound and separated by the tiny blue molecular machine called “helicase”. Helicase literally spins as fast as a jet engine! The strand of DNA on the right has its complementary strand assembled continuously. But the other strand is more complicated, because it runs in the opposite direction. So it must be looped out with its complementary strand assembled in reverse, section by section. At the end of this process, you have two identical DNA molecules, each one a few centimeters long, but just a couple nanometers wide. So, to prevent the DNA from becoming a tangled mess, it is wrapped around proteins called “histones”, forming a nucleosome. These nucleosomes are bundled together into a fiber known as chromatin, which is further looped and coiled to form a chromosome, one of the largest molecular structures in your body. You can actually see chromosomes under a microscope in dividing cells. Only then do they take on their characteristic shape. Otherwise, the DNA is more strewn inside the nucleus. The process of dividing a cell takes around an hour in mammals, so this footage is from a time-lapse. You can see how the chromosomes line up on the equator of the cell. Now, when everything is right, they are pulled apart into the two new daughter cells, each one containing an identical copy of DNA. Now, simple as this looks, the process is incredibly complicated and requires even more fascinating molecular machines to accomplish it. So, let’s look at a single chromosome. One chromosome consists of two sausage shaped chromatids, containing the identical copies of DNA made earlier. Each chromatid is attached to microtubule fibers, which guide and help align them in the correct position. The microtubules are connected to the chromatid at the kinetochore, here colored red. The kinetochore consists of hundreds of different proteins working together to achieve multiple objectives. In fact, it’s one of the most sophisticated molecular mechanisms inside your body. The kinetochore is central to the successful separation of the chromatids. It creates a dynamic connection between the chromosome and the microtubules. For a reason no one’s yet been able to figure out, the microtubules are constantly being built at one end and deconstructed at the other. While the chromosome is still getting ready, the kinetochore sends out a chemical “stop” signal to the rest of the cell, shown here by the red molecules, basically saying, “this chromosome is not yet ready to divide.” The kinetochore also mechanically senses tension. When the tension is just right, and the position and attachment are correct, all the proteins get ready, shown here by turning green. At this point, the stop signal broadcasting system is not switched off! Instead, it is literally carried away from the kinetochore, down the microtubules, by a dynein motor – that’s the walking guy. This is really what it looks like: it has long “legs” so it can avoid obstacles and step over the kinesins, molecular motors that walk in the opposite direction. Personally, I’m astounded by these tiny molecular machines, how they’re able to routinely and faithfully execute their functions billions of times over inside your body at this exact instant. I’m also amazed by the scientists who were able to work out how this happens in such detail that we could create realistic depictions of them, like you saw in the animations in this video. But, perhaps, the most amazing thing is just how much is left to be discovered, like, figuring out how exactly the chromatids are pulled to opposite ends of the cell. There is still so much that we don’t quite know. You know, what I find exciting is, that in science fiction, for decades, we’ve been writing about tiny nanobots that will be injected into our blood streams that can heal us. But, what this suggests, the existence of these tiny molecular machines inside us, it suggests that there isn’t a physical limit that would prevent that. And so, I think it’s pretty likely that, in future, we will be able to develop our own tiny molecular machines that will be able to repair our bodies better than they can repair themselves.