DNA, the Brain, and Human Behavior
If you're in the dark about chemistry as much as I have been, you might find Claudia Dreifus's article in the May 18, 2010 The New York Times interesting. In the article, titled "A Marine Chemist Studies How Life Began," Jeffrey L. Bada talks about the classic Stanley Miller-Harold Urey experiment on the chemical origins of life. Bada says, "They'd taken gases present on the early Earth like methane, ammonia and hydrogen and applied a spark discharge to them, to mimic lightning. From that, they produced amino acids, the compounds that make up the proteins in all living organisms."
Is DNA a thing? In John Brockman's The Third Culture, "A Package of Information," George C. Williams writes: "The gene is a package of information, not an object. The pattern of base pairs in a DNA molecule specifies the gene. But the DNA molecule is the medium, it's not the message. Maintaining this distinction between the medium and the message is absolutely indispensable to clarity of thought about evolution."
From the bottom up, it goes like this: There are four compounds: adenosine, thymidine, cytidine, and guanine. These compounds are joined together in base pairs. Adenosine (A) pairs with thymidine (T), and cytidine (C) pairs with guanine (G). The molecular structure for the guanine-cytidine pair is illustrated at right (links to source). These base pairs connect together to form two matched helices, known as deoxyribonucleic acid (DNA). In his book, Evolving Brains (2000), John Allman explains how the code of base pairs is read. I have added bold emphasis to his explanation.
The code [of base pairs] is read from one direction in one strand. Three-letter sequences, triplets, specify each amino acid, and the sequence of triplets in turn specifies the chain of amino acids that makes up a protein." Allman explains that there are 64 possible triplet sequences. "Each of 61 triplets encodes for one of the 20 amino acids. Thus some amino acids are specified by more than one triplet, although no triplet specifies more than one amino acid. The other three triplets are stop codons that signal the end of a particular protein. The complete sequence of triplets that encodes a protein is a gene.
Genes are grouped together in large volumes as chromosomes which are large enough to be seen under a microscope. The image below is a light microscopic presentation of a normal, human male chromosome set (karyogram) from the German Mental Retardation Network (image links to source).
Other than germ cells, all humans' cells normally contain 46 chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes—either a pair of X chromosomes in females or an X chromosome and a Y chromosome in males. In each pair of autosomes, one chromosome is inherited from an individual's father and one from his or her mother. When contributions of sex chromosomes proceed normally, the mother contributes an X chromosome and the father contributes either an X or a Y chromosome. Neil Shubin, in Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body (2009), explains that "DNA is peeled into two halves every time a cell divides, and so every chromosome must be teased into two daughter chromosomes at the same time. This peeling also takes place when eggs and sperm are made, and this is how genes are passed to offspring."
The image below right illustrating the DNA double helix is from the National Library of Medicine and links to a webpage titled "What is DNA?" In his book, Your Inner Fish, Shubin eloquently explains how cells in a single individual—all containing the same DNA packaged into 23 pairs of chromosomes—wind up looking and functioning so differently—some as nerve, some as bone. I have put his words into a block quotation here. He writes:
Our body is made up of hundreds of different kinds of cells. This cellular diversity gives our tissues and organs their distinct shapes and functions. The cells that make our bones, nerves, guts, and so on look and behave entirely differently. Despite these differences, there is a deep similarity among every cell inside our bodies: all of them contain exactly the same DNA. If DNA contains the information to build our bodies, tissues, and organs, how is it that cells as different as those found in muscle, nerve, and bone contain the same DNA?
The answer lies in understanding what pieces of DNA (the genes) are actually turned on in every cell. A skin cell is different from a neuron because different genes are active in each cell. When a gene is turned on, it makes a protein that can affect what the cell looks like and how it behaves. Therefore, to understand what makes a cell in the eye different from a cell in the bones of the hand, we need to know about the genetic switches that control the activity of genes in each cell and tissue.
Here's the important fact: these genetic switches help to assemble us. At conception, we start as a single cell that contains all the DNA needed to build our body. The plan for that entire body unfolds via the instructions contained in this single microscopic cell. To go from this generalized egg cell to a complete human, with trillions of specialized cells organized in just the right way, whole batteries of genes need to be turned on and off at just the right stages of development. Like a concerto composed of individual notes played by many instruments, our bodies are a composition of individual genes turning on and off inside each cell during our development.
So we now have a very basic understanding of how genes build our bodies. But what about behavior? In "A Gene for Nothing," published in Discover, October, 1997, Robert Sapolsky clarifies this confusing issue:
A gene, a stretch of DNA, does not produce a behavior. A gene does not produce an emotion, or even a fleeting thought. It produces a protein. Each gene is a specific DNA sequence that codes for a specific protein. Some of these proteins certainly have lots to do with behavior and feelings and thoughts; proteins include some hormones (which carry messages between cells) and neurotransmitters (which carry messages between nerve cells); they also include receptors that receive hormonal and neurotransmitter messages, the enzymes that synthesize and degrade those messengers, many of the intracellular messengers triggered by those hormones, and so on. All those proteins are vital for a brain to do its business. But only very rarely do things like hormones and neurotransmitters cause a behavior to happen. Instead, they produce tendencies to respond to the environment in certain ways.
The image to the right was originally obtained from the DOE Joint Genome Institute website. It links to a page titled "What is a Genome?" created by Northwestern University. In the illustration, you can see that a gene is literally a "stretch of DNA" as Sapolsky explains above. Sapolsky also discusses gene-environment interactions in Monkeyluv and Other Essays on Our Lives as Animals (2005). He points out that in mammals, it is estimated that more than 95 percent of DNA does not code for a specific gene, and thus is labeled "noncoding DNA." Sapolsky explains that some of the noncoding DNA is the instruction manual for how and when to activate genes. "These stretches [of noncoding DNA] have a variety of names—regulatory elements, promoters, repressors, responsive elements. Different biochemical messengers bind to these regulatory elements and thereby alter the activity of the gene immediately 'downstream'—immediately following in the string of DNA." What is the impetus for our body's release of such powerfully influential biochemical messengers, capable of altering the activity of a gene? "Often the environment," says Sapolsky.
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