Your Body’s Molecular Machines

Every day in an adult human roughly 50-70 billion of your cells die. They may be damaged, stressed, or just plain old – this is normal, in fact it’s called programmed cell death.

To make up for that loss, right now, inside your body, billions of cells are dividing, creating new cells.

And cell division, also called mitosis, requires an army of tiny molecular machines.DNA is a good place to start – the double helix molecule that we always talk about.

This is a scientifically accurate depiction of DNA. 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.
It literally spins as fast as a jet engine! The strand of DNA on the right has its complimentary 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 compliment 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.

To prevent the DNA from becoming a tangled mess, it is wrapped around proteins called a 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.

The process of dividing the cell takes around an hour in mammals. This footage is from a time lapse. You can see how the chromosomes line up on the equator of the cell. When everything is right they are pulled apart into the two new daughter cells, each one containing an identical copy of DNA.
As simple as it looks, this process is incredibly complicated and requires even more fascinating molecular machines to accomplish it. 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 proteins working together to achieve multiple objectives – 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. This is really what it looks like. It has long ‘legs’ so it can avoid obstacles and step over the kinesins, molecular motors walking the other direction.

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