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.

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.