Genes Are Like Sentences

I admit that I lose track sometimes of how the common genetic terms relate to each other. What’s the difference between a chromosome and a strand of DNA, for example? a gene and a genome? Are each of those three-letter sets in DNA a gene? I’m not a scientist, but I was an English teacher, so comparisons between genetic terms and units of written language—words, sentences, and so on—are helpful. Maybe they will be for you also.

Let’s start small. Diagrams of DNA include four letters: A, C, G, and T. These letters and the letter we write with are similar in some ways not in others. In both cases, they are the smallest units of their respective languages. But the four DNA letters stand for the four nucleotides—Adenine, Cytosine, Guanine, and Thymine—that are present in DNA, while the letters of our language stand mainly for the sounds we would pronounce if we were reading aloud.

In DNA, those four nucleotides, abbreviated as letters, make up the three-letter codons that are the DNA version of words. A difference is that the letters and words I am writing with don’t do the whole job. I also use punctuation marks, spaces, and capital letters to show where words and sentence begin and end.

In DNA, however, the three-letter codon-words are more efficient. They represent all the content—amino acids—and all the necessary divisions and instructions. No actual spaces separate the codons in a gene. Since all codons are three letters long, where they begin and end is automatic. (And in fact early writing in the ancient world lacked space between words also. As long as one could read slowly and figurethewordsoutspacesweren’tessential.)

chromosome (mayoclinic.org)

mayoclinic.org

Groups of these codons make up a gene, which can be compared to a written sentence. These gene-sentences say something like, “The hair will be red.” The sentence can also be read as a recipe: “Put this together with this and this to get red hair.” The codons for this and all genes include one that indicates where to switch on the gene and when, and another that says “Stop here; gene complete.”

Sometimes a spelling error occurs in one of the letters-nucleotides in a codon. Such a mutation may change the meaning of the gene to, “The hair will be white.”

To recap: the four nucleotides are the basic components much as the letters of our alphabets are; codons are DNA words; and a group of codons form a gene that is a sentence/recipe.

Now we come to chromosomes, genomes, and DNA itself.

The 23 pairs of chromosomes in each of our cells are like chapters in a strange book in which each chapter appears twice.  The number of genes in a chromosome runs from a couple of hundred to over a thousand, many of them about similar matters, like sentences in a chapter. In the 23rd chromosome pair, which determines sex, the two chromosomes are very different from each other about half the time: females have two X chromosomes, but males have an X and a much shorter Y chromosome.

Finally, your genome is like the book itself, the totality of all your genes on all your chromosomes. Your genome book is almost exactly like mine, since we are both humans, but about one tenth of one percent of our 20,000 or so genes are different. That’s similar to two copies of the same long book that differ only in a few sentences.

Finally, DNA itself. I think of DNA as similar to our writing system as a whole with all its symbols, spaces, and graphic conventions. DNA is not a unit as these other genetic terms are; it is the stuff that all these units are composed of. The same goes for our writing system.

And it’s interesting that writing itself, developed a few thousand years ago, is structured roughly like the genetic code that appeared with the first cells over three billion years ago. In communicating information over distance and time, whether it’s an email about a meeting next month or the genetic instructions for building a baby organism, it seems the key is a method to preserve the necessary groupings and sequencing of components.

My Genome and Me

Recently I sent a DNA sample on two Q-tips swabbed inside my mouth to the National Geographic genome project. The information I received back described the whereabouts of my ancestors over the last 50,000 years or so. I’d known bits and pieces about my parents’ parents, all from different parts of Europe. But the DNA analysis showed me when their own early ancestors came to Europe out of Africa and what groups they belonged to when they got there.

Long before their journey, about 2 million years ago, earlier human species began migrating from Africa. Then about 60,000 years ago Homo sapiens began crossing Egypt and the Arabian peninsula. When we got to Europe, our cousins the Neanderthals were settled there and we settled down with them. Really, with them. My genetic make-up is 1.1% Neanderthal, which is the average for modern day non-Africans.

(crystallines.com)

(crystallines.com)

The genome report goes on to describe the two particular migrations on my mother’s and father’s sides out of Africa. On my father’s side, Branch H5 spread into Central Asia to points east and west, around 10,000 years ago. Today, H5 genes are common around the Black Sea and less so throughout Europe. Viking King Sven Estridsen, born in England but king of Denmark from 1047 to 1074, belonged to this group. He may have been one of my ancestors, but more likely he collected tribute from them.

On my mother’s side, Branch L2, about which less is known, left their genetic marker most frequently in Algeria, in northern Africa, but also in Asia and Europe, routes indicated quite clearly on the inset map.

Finally, there is my more recent regional ancestry, the combined information about both parents connecting me to any of 18 population groups around the world six or more generations ago. My genome links to three such groups. Forty-one percent of my DNA is descended from the Jewish Diaspora, the dispersal of Jews from the Near East, in this case into Europe. Thirty-one percent comes from Southern Europeans, the original peoples of the northern Mediterranean Coast. Finaly, 28% comes from Scandinavians, the most recent of the groups, since that area was peopled only after its glaciers melted around 12,000 years ago.

I’ve shared these results with family and friends, with mixed responses. Some find them too general to mean much. But they’ve also set off alarms about the pitfalls of knowing such information—about oneself or about others. The percentages and group identities make it tempting to imagine that a person is the way he or she is because of his or her ancestry—that I’m studious, for example, because of Jewish ancestry. A TV commercial shows a man who enjoys Swiss dances and lederhosen and who then finds from his genome that he is Scots, so he quits the lederhosen and takes up kilts. As if his genes made him suited for one but not the other. Did he feel it would be inappropriate or hypocritical to continue in the lederhosen? That seems not just silly but a little frightening. His genes are not him, not his abilities, not his interests.

True, some physical features, some potential abilities, and predispositions to some diseases are inherited. But our specific and individual characteristics depend as much if not more on the web of family, community, and culture and the flux of time. Saying a person has a specific characteristic because of his ancestral group is a shade less inaccurate than saying he has it because of his astrological sign.

For me, the genome history helps fill in my reveries about ancestors leading their particular lives a very long time ago. I picture a family walking in Eastern Europe, another farming in Italy, or a group crossing the water from Denmark, none of them knowing that far in the future, the paths of their descendants will come together in me. I imagine myself greeting them from their future and watching their surprised smiles as they realize who I am and as I tell them how often I’ve been thinking of them.