Richard Peet, J.D., Ph.D.
GENES IN A WHOLE HUMAN: HOW MANY?
The conventional answer is that each human has somewhere between 20,000 and 25,000 genes, but is this accurate? Absolutely not! A whole human may contain more than 2,000,000 different genes.
Let’s agree on some terminology before we discuss why the conventional answer is inaccurate. Genes act as the basic units of heredity, transmitting information from one generation to the next through sexual reproduction. Genes are composed of sequences of DNA that code for a molecule that has a specific biological function. “Genes” are the term for specific sequences of nucleotides. Nucleotides are the basic building blocks of nucleic acids such as DNA and RNA. Genes serve as templates to manufacture proteins or functional RNA molecules.
For example, genes code for functional proteins, such as proteases, that digest the proteins we consume. Genes also code for functional RNA molecules such as ribosomal RNA, transfer RNA and non-coding RNAs, molecules involved in the manufacture of proteins or the control of gene expression. So, while it is true that the 46 chromosomes that make-up the human genome carry between 20,000 and 25,000 genes, other genes that contribute to what it is to be human, are located throughout the human body.
Are the missing genes found in mitochondrial DNA? Mitochondria are the “powerhouses” of the cell generating most of its chemical energy. Technically, mitochondria are the cell’s producer of adenosine triphosphate (ATP), a molecule used as a source of cellular energy. Each human mitochondria contains a circular DNA molecule that carries 37 different genes. These genes are involved in producing molecules that are essential for the function of the mitochondria, A very small number of the missing genes are found in mitochondrial DNA, so let’s look elsewhere.
So where are all the missing genes? The missing genes are found in the human microbiome. A very large number of genes are found in the gut environment containing microorganisms, including bacteria, archaea, viruses, and fungi. That is right; most genes don’t reside on human chromosomes. This is a radical new view of what it means to be human. After millions and millions of years co-evolution, humans and their microbiota have become co-dependent. Humans are dependent on the trillions of microorganisms that live on and in our bodies for health and well-being. Gut bacteria, for example, digest substances we are incapable of breaking down. In the process, these bacteria produce substances that are anti-inflammatory, energy generating, improve mental health, and more. The microorganisms in the microbiota depend on humans for food and shelter. Humans and their microbiota need each other for survival. Together, humans and their microbiota comprise what I call the “whole human.”
And a whole human contains millions of different genes. Estimates vary, but the human gut microbiota contains somewhere between 300 and 1,000 different species of bacteria. Think of each species of bacteria as a different model of car. There are Toyota Camrys, Mercedes 300s, and more. Let’s assume 500 different species of bacteria make up the average human gut microbiota. The number of genes in a bacterium can vary from as small as 500 to as high as 9,000. For our purposes, let’s assume the average number of genes in a human gut bacterium is 4,000. This means that that human gut microbiome contributes on average about 500 x 4,000 or 2,000,000 more genes to the whole human. And this number does not include the genes contributed by the archaea, viruses and fungi that are found in the human microbiota. Thus, a whole human is made up of human cells and microbes living together in a symbiotic relationship and these microbes contain millions of genes that are “unaccounted for” when you analyze your genetics with Ancestry.com or 23andMe.
Why should we care about those 2,000,000 genes? What do they have to do with “me,” myheredity, health, or identity? Each of those 2,000,000 genes code for functional proteins or RNAs that expand the amount of chemistry that takes place in our bodies. Because of these
genes, the gut microbiome can digest food that humans cannot digest, thereby contributing energy to our daily lives. These microbes produce Vitamin K, important for blood clotting. They produce various B Vitamins, such as folate (B9), riboflavin (B2), and cobalamin (B12), which play important roles in metabolism and neurological function. The genes in these microorganisms produce the hormone peptide YY involved in signaling satiety. The list of important microbiota-produced metabolites continues to grow along with our understanding of
how these substances contribute to human health and wellbeing.
A quote attributed to Chief Seattle, a prominent leader of the Suquamish and Duwamish tribes in the 19 th C Pacific Northwest of the U.S. states: “Humankind has not woven the web of life. We are but one thread within it. Whatever we do to the web, we do to ourselves. All things are bound together. All things connect.” Chief Seattle was referring to the physical environment in which his people thrived in harmony with the natural world. Tragically, humans have made some bad choices, with adverse environmental consequences. While we can’t control all that occurs in the world, our body is our personal domain. Thus, we must nurture our microbiome to sustain the
health and wellbeing of the whole human. We need to keep all those microbes and their genes within the human microbiome healthy to sustain our own health!
I would like to thank my friend Michael B. Reiner, Ph,.D. for editing some of these blogs. Any errors in the blogs are solely my responsibility.