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.

Life Before Fossils

king-kong-killing-pterodactyl-1024x766Seeing may not always mean believing, but when it comes to living things from millions of years ago, it helps. A skeptic these days would have difficulty doubting the reality of dinosaurs given all the bones in museums and the reconstructions come to life in countless films. When embedded in an adventurous and romantic story, oversized reptiles and even King-Kong-size versions of our primate ancestors put persuasive passion and flesh on the cool scholarship of paleontologists.

The trouble is that the stuff of the usual fossil history—old bones, insects trapped in amber, hardened imprints of early plants–date back no more than 600 million years. Such a number may seem very old, but from another perspective it is not nearly old enough. For life has been traced back three billion years before that, six times further into the past. It’s not surprising that life from that long ago is not the material for theme parks or movies about entrepreneurs like Carl Denham who searched out Kong’s island. For life was small for the first three billion years, with no animals or plants as such. There were only microbes, single cells that gradually acquired the complexities of modern cell life—a nucleus, the hunger for oxygen, sexual reproduction. But there are no two-billion-year-old bones from which to reconstruct cellular giants, no fossils to serve as relics to fire the romantic imagination.

Or almost none.

Stromatolites in Australia, probably looking much as they did 3.5 billion years ago. (www2.estrellamountain.edu)

Microfossils from 3.5 billion years ago (www2.estrellamountain.edu)

To find them, you have to search for the oldest rocks. Try Australia, Greenland, or South Africa for those that date back almost four billion years. Slice them thin, put them under a microscope, look for microfossils measuring a fraction of a millimeter, their cell walls mineralized into tough material.

 

 

stromatolites layers pinterest

petrified stromatolite (pintrest)

 

 

And look for petrified stromatolites, the layered habitats of colonies of bacteria that filtered sea water for nutrients as far back as 3.5 billion years.

 

 

 

But could the tiny remains and traces of chemicals from billions of years ago become the attractions of crowded museums and movie fantasies? Could they find their place in popular culture as both entertainment and subtle education, as dinosaurs and apes have?

I believe they could.  It’s not difficult to imagine oversized reproductions of ancient microbes which kids could walk through while trying to avoid getting snagged on strands of DNA or thrown off-balance by the cell’s motion from its flagellum, its tail. And climate change sets the stage for a movie thriller about bacteria mysteriously resurrected from three billion years ago that thrive on carbon dioxide and for whom oxygen is poison.

Then our wonder at the marvels of our pre-human ancestors would reach back through the full history of life.