Jonathan D. Grinstein, PhD

As if shrunk-down on the miniaturized ship in the Fantastic Voyage with a front-seat, super-magnified view of the inside of the human body, zooming through the blood-stream on your way to fix a blood clot in the brain, you may have started to notice the cellular wilderness around you.

Flat, fish scale-like cells line the cavities of blood vessels, packed together tight providing a smooth, low-friction surface for fluid to jet. Each looking like the love-child of a frisbee and a recently sat on half-deflated whoopee cushion, red blood cells fly by. Amoeba-like white blood cells patrol for and swallow-up harmful foreign particles. Sticky, blood-clot forming remnants of cells float by like debris in open water.

And that’s just a quick survey of the inside of a blood vessel on your cruise to the brain—a world-class zoo of cellular diversity!

There are somewhere between hundreds and thousands of different types of cells in the human body. Yet, inside the tens of trillions of cells that make up this human being lies the same biological code, the same sequence of DNA—the self-replicating genetic material that in different combinations encrypts nearly all living organisms that have existed on earth the past few billion years.

So, if each cell in a human’s body is packed with the same genetic cipher, how do many cell types arise from one set of inheritable materials unique to each human being?

How does the same encryption get differently decoded to spawn heart cells pumping turbulent fluid over frictionless cells lining blood vessels that carry a plethora of blood cells and their coagulating relics to the cellular rainforest that is the brain?

A hint lies in the same process by which a couple meters’ worth of DNA gets packed into a membraned structure, the nucleus, of each human cell—to help size that up, roughly ten thousand nuclei fit onto the tip of a needle.

Like a coiled cord twisted upon itself repeatedly until forming a thick cable, long stretches of the double helix wrap around little molecular balls. When cells replicate and divide, these intertwined beads of DNA strings begin to tangle and stack into chains called chromosomes large enough to be seen with a kid’s beginner microscope.

Although the sequence of genetic information may be static, the form DNA takes in each cell is constantly in flux. Like a movie reel spinning from one spool to another, DNA is wound up most of the time with only small stretches showing. Little living projectors read the exposed genetic-tape into organic streams called RNA—basically identical chemical creatures to DNA except that they’re made up of ever-so-slightly different sugars.

Cells in the human body are like trillions of identical movie libraries stocked with limited screens. And, at any given moment, each cell in the human body can only show certain genetic films. In each nuclear cinema, the snapshot of RNA on the cellular screen will be unique.

Certain cells may choose to show mostly romantic films, whereas others horror flicks or psychological thrillers; some cells will become heart cells, whereas others will become blood cells or of the astral brain. All cellular theaters, though, are always showing the classics–the universal foundation upon which each living unit thrives.

During development, a cascading chain of events, like simultaneous avalanches on all sides of a mountain range, lead naïve cells with the potential to become any cell type in the body down different fated paths. Under the showering influence of physical forces and signaling blasts, DNA begins to fold differently in each cell and, with it, the RNA dispatched.

Like the blueprint for a cell, RNA inscriptions get interpreted into animate structures and machines that pave the way for each cell’s shape and capabilities. Specific RNAs give instructions to make the components for heart cells to squeeze, vessel lining cells to tile, blood cells to float, and brain cells to—among many things—sense the world around us.

Check back soon for the next episode, Genomic Satellites And Dark Matter: A Journey Into The Lost Mines Of Junk DNA.