Genes Are Like Sentences, Genomes Are Like Books

I lose track sometimes of exactly what the common genetic terms mean and how the genetic pieces work together. What’s the difference between a chromosome and a strand of DNA? A gene and a genome? What are those three-letter sets in a DNA diagram called and what do they do? I’m not a scientist, but since I was an English teacher, connecting the names of genetic units to the units of written language—words, sentences, and so on—makes the picture a little clearer.  Maybe it will do the same for the reader.

Let’s start small.  The spiraling rungs on diagrams of a DNA (deoxyribonucleic acid) molecule are each marked with two of four specific letters: A, C, G, and T.  The four DNA letters stand for the four nucleotides—Adenine, Cytosine, Guanine, and Thymine—that make up DNA. Like the letters of the full alphabet, these letters–or rather the four molecules they indicate–are the smallest building blocks of their language.

codons

moodle.clsd.k12.pa.us

In DNA, combinations of the letters for the four nucleotides make up the three-letter codons that are DNA’s version of words. Each three-letter codon/word specifies one amino acid. And most codons are “synonyms” in that several different codons refer to the same amino acid because there are many more codons than there are amino acids. The codons are “read” by a ribosome, a cellular reader/assembly-machine that produces the required amino acid and attaches it to the chain of amino acids that will form a protein.

Groups of these codons make up a gene, much as words make up a sentence. The genes/sentences are long because most proteins are complex; human proteins consist of anywhere from several hundred to several thousand amino acid molecules.  The gene/sentence for red hair says something like “Put this together with that and that and that….”

Genes also include a codon at the start that says “Start the gene here” and another at the end that says “Stop here; gene complete.” Within the gene, however, no actual spaces separate the codons, but since all codons are triplets, it’s always clear where codons themselves begin and end.  (Somewhat similarly, writing in the ancient world often lacked spaces between words.  As long as one could read slowly and figurethewordsoutspacesweren’tessential.)

chromosome (mayoclinic.org)

mayoclinic.org

So, to recap.  The four nucleotides are basic components much like the letters of our alphabet. Groups of three nucleotides spell out codons that can be thought of as words, which in this case are actual amino acid molecules.  And a sequence of codons/amino acids forms a gene that resembles a sentence in a protein recipe for some aspect of the organism.

Finally there are chromosomes and genomes.

A molecule of DNA is very long, a continuous strand of anywhere from a couple of hundred to more than a thousand genes, many of them about related aspects of the organism. Each molecule is a chromosome which, because its genes concern similar aspects of the body, can be compared to a chapter in a book.  But it is a strange book in that each chapter appears twice, in anticipation of the day when the molecule/chapter reproduces itself. Each human cell contain 23 such paired chromosomes, duplicate copies of the assembly instructions for an entire human being. Only the chromosome pair that determines sex contains chromosomes that are different from each other about half the time: females have two identical female chromosomes while males carry one female and one male chromosome.

Finally, our genome is like the book itself, the totality of all our genes on all our chromosomes. The book might be called Me And Us. Your genome book is almost exactly like mine except for about one tenth of one percent of our 20,000 genes that are different. That’s similar to two copies of the same long book that differ only in a few sentences.

Simplified though the comparison is, it’s startling what genetics and written language have in common considering that the second is a recent human invention and the first represents the formation of life almost four billion years ago. Both are composed of the smallest building blocks, then the groupings created from the building blocks, then the meaningful statements/instructions/recipes coded in the groupings, and finally the conversion of the code into organic construction/action/speech.

We Are All Descended From an Actual “Eve.” So?

She lived between 100,000 and 200,000 years ago in southern Africa. These days she’s known as Mitochondrial Eve, but that’s a little misleading. Unlike the Biblical Eve, she wasn’t the first woman nor was she the only woman alive at the time—and there were plenty of men around as well. Still, Mitochondrial Eve was an actual person. We don’t know much about her except that she is the most recent woman to whom everyone alive today—male and female, all 7.6 billion of us—is connected through their mothers by a speck of DNA.

But as important as such a linkage may be to scientists, how significant is it for the rest of us? Frankly, I’m not sure. See what you think.

Every cell in any organism contains small particles that keep the cell alive. The  nucleus, with the genetic DNA masterplan of the body, is the cell’s control center. Smaller particles carry out other functions. Mitochondria produce energy for the cell. They contain their own, separate, bit of DNA because millions of years ago they were free-floating bacteria that were absorbed by cells, proved useful, and took a permanent place in the cell anatomy.

Mitochondria in a cell (Flickr)

Mitochondria in a typical cell. The long thread of genetic DNA in the nucleus is shown but not the bits of mitochondrial DNA, which are incidental and much smaller. (Flickr)

Over time and countless cell divisions, and separate from any mutations in the genetic DNA, the DNA in the mitochondria also changed in small ways. As a result, the early apes, then the pre-humans, then the earliest modern Homo sapiens all carried the slight variations in mitochondrial DNA that they inherited.

But they inherited them only through the females. Males couldn’t pass theirs along. Why? Because we inherit our cellular structure from mom’s egg. While men may deliver their genetic DNA by sperm to the egg, it’s mom’s egg cell itself that grows into the embryo and into all human cells. Complete with the mother’s mitochondria.

Over the course of five thousand generations or so, women around the world passed their variations of mitochondrial DNA to their daughters. Along the way, though, some mothers bore only sons and other women had no children at all. Gradually, all the variations of mitochondrial DNA fizzled out, except one. We all carry it, as did a woman a long time ago, Mitochondrial Eve.

What to make of all this? Compared to the Biblical Eve and her list of firsts—first woman, first human to be curious, first mother—we have little to show for our ancestry from the other Eve, Mitochondrial Eve. And the merging of genetic DNA from our mother and father has by far a greater influence on who we are and what we’re like. By comparison, Mitochondrial Eve is just a woman a very long time ago whom we all happen to be linked with inconsequentially on our mother’s side.

Still, as Siddhartha Mukherjee writes in The Gene, without elaborating, “I find the idea of such a founding mother endlessly mesmerizing.” For Mitochondrial Eve is one of our Most Recent Common Ancestors–an MRCA. The MRCA for any group of organisms, whether the same species or not, is the individual or type after which subsequent generations evolved in different directions. The MRCA of primates (humans as well as chimps, apes, monkeys, baboons) lived 65 million years ago. The MRCA of all animals, 600 million years ago. And the MRCA of all living things, 3.6 billion years ago. For many people, interesting to know but not so easy to imagine.

But it’s a little less difficult to imagine in the case of the most recent MRCA, the one who looked a lot like us. Maybe Mitochondrial Eve’s value lies here after all: by thinking about her, we may be learning to wrap our heads around the reality of many ancestors who seem impossibly ancient yet who made us what we are.