My favorite image is one that I came across in the aforementioned Molecular Biology of the Cell textbook. It is a simple depiction, a series of images increasing in magnification from a human thumb to the atomic level, featuring “snapshots” of epithelial cells covering the skin of the thumb, one cell in close up with individual organelles (nucleus, ER, Golgi, etc.) revealed, a solitary mitochondrion, and a ribosome within the mitochondrion transcribing mRNA. This demonstrates the hierarchy of biological organization from basic atoms to a multicellular organism: atoms, organic molecules (including DNA and proteins), cellular organelles, cells, tissue, organism.
I’ve kept this image faithfully tacked above the bed of every room I’ve lived in since I began teaching myself molecular biology because it reminds me of the first night I began reading about the intracellular world. What was so mind-boggling and awe inspiring to me then (and still is now) is the fact that the things inside our cells — the mRNA, the cellular machinery, the basic building blocks — are the same in every living thing on the planet with no known exception. It is estimated that far more than10 million species currently exist on our planet. I specifically remember staring at my hands as it hit me that the basic components within them linked me to them all.
The basic components I refer to imply a number of features. On a larger scale in the hierarchy of biological organization, all things classified as living are made up of cells: small (but visible with a light microscope) balloons enclosed by a membrane and containing an aqueous solution encompassing the organelles and organic molecules briefly mentioned above. The vast majority of organisms are made up of only one cell; others, including ourselves, are an enormous conglomerate of over 1013 cells, all working together to generate pieces of the body as different as our eyes and blood. However, even the most complex multicellular organisms begin life as a single cell, which contains all of the information required to duplicate and differentiate into a human body capable of producing yet more humans in the future. Indeed, if one were to look at a light microscope image of the original fertilized egg cell that will produce a human, a sea urchin, or seaweed, they would appear nearly identical. Yet, each gives rise to an entirely different organism, demonstrating that the complete nature of these complex multicellular beings is contained within an initial fertilized egg.
It is this ability to faithfully pass on characteristics from one generation to another (defined as heredity), that is the defining job of every organism. I therefore often think of cells as small self-sustaining powerplants making up the body: they contain the blueprints and the equipment required to produce more nearly identical powerplants with the same blueprints and equipment. In the case of cells, the blueprints are our genetic material, and the equipment is the proteins and RNAs required to catalyze all of a cell’s life-sustaining functions.
What is truly extraordinary is that the current diversity of living things on earth has arrived over 3.5 billion years of evolving and diversifying — think of how different you and I are from the baker’s yeast in the bread you may have had for lunch. Given this diversity, there is no reason to suspect that all cells in these vastly different organisms would store this blueprint of hereditary information in the exact same form, or that the equipment required to read and replicate this hereditary information would be identical and interchangeable between organisms as different as ourselves and yeast. But, astonishingly, this is the case. Every known organism, without exception, stores its hereditary information in the form of double stranded molecules of DNA, which through the intermediate step of transcription into RNA, encodes the proteins required to carry out a cell’s life-sustaining functions. This is akin to 10 million intellectually isolated factories producing nearly identical products to fit a given need using the exact same assembly system. The fact that all living things contain DNA, which is transcribed to RNA and then translated to protein, is the central dogma of biology, and is the means through which living things faithfully pass on their hereditary information.
How does this work? The answer lies in the structure of DNA. DNA is made of two complementary long chains of the same four repeating subunits, called nucleotides, that are strung together in an order that encodes genetic information, synonymous to the way computer code is made up of specific sequences of 0s and 1s. These four nucleotides, given the nickname A, C, G, and T have a chemical composition such that A always pairs with T and C always pairs with G. Thus, the two complementary strands of DNA fit together much like a zipper into the classic “double helix” appearance that can be peeled apart to reveal the base pairs within. Each organism contains a complete genome made up of all its collective genetic material: roughly two meters’ worth of DNA tightly wound and folded into your 6 micrometer cell nucleus. Smaller sequences of this DNA (usually on the scale of a few thousand to tens of thousands of base pairs) are called genes and encode individual proteins.
The entire genome can be replicated and faithfully passed on to another cell during cell division thanks to this complementary nature of DNA. When DNA is replicated to be passed on to another cell during cell division, the two strands are peeled apart and a molecular machine called DNA polymerase reads the nucleotides on an individual strand and matches it with its compliment- A to T and C to G until two new doubles strand have been created from one original. In a very similar way, RNA polymerase can read the nucleotides on an individual strand for a specific gene and produce RNA, a nearly identical polymer to DNA but with a different sugar backbone and the base U instead of T. By patching U to A and C to G, RNA polymerase produces a long molecule of messenger RNA (mRNA) which is then threaded through a massive machine called a ribosome. The ribosome “reads” the mRNA in three base pair words called codons, and each codon is matched to one of 21 amino acids to produce a protein.
While the genome of each organism and species may vary, the central dogma of cellular biology holds true: the same four nucleotides present in 10 million different species are assembled into structurally identical DNA polymers to produce proteins using nearly identical molecular machines (polymerases, ribosomes, and many more). While it may seem inconceivable, it is even possible (and I do it on a near daily basis in my lab) to take the DNA sequence specifying a human protein, place it in the bacteria, E. coli, and watch it faithfully read out the template to produce the human protein. This is direct proof that although on the level of human vision life is incredibly diverse, at the molecular level, it shares shockingly identical characteristics that allow all life as we know it to be linked by a common molecular theme.