The Same Four Letters
a read-aloud about the four letters that everything alive is written with, and the one thing about them nobody checks — with something you can try, and every source kept on the page for the grown-up
You are written down
Inside you, in nearly every tiny piece of you, there is something written down.
It is not written in English. It is not written in any language that anybody talks in.
It is written with letters, though. Actual letters.
And there are only four of them.
Four. That is the whole alphabet. There is no fifth one.
The thing being written on is DNA — deoxyribonucleic acid. The “letters” are the four bases: adenine, thymine, guanine and cytosine.
This zine is about the alphabet, not the code. Worth keeping separate, because popular writing runs them together constantly. The alphabet is which letters exist — four. The genetic code is a different thing: how three-letter groups get read as instructions for building proteins. The code is not in this zine at all. Nothing here says what any stretch of DNA does.
“Nearly every tiny piece” is doing real work. Mature human red blood cells have ejected their nuclei and carry no nuclear DNA. “Every cell” would have been false, and a child who has heard about red blood cells is entitled to catch us.
A, T, G and C
Here they are. All four.
A T G C
Each one is short for a much longer word, and the longer words are hard to say: adenine, thymine, guanine, cytosine.
You do not have to remember those. Nobody remembers those.
Even the people who spend their whole lives on this just write A, T, G and C.
The single-letter abbreviations are the working notation, not a simplification made for children. Sequence data is published, stored and read as strings of A, T, G and C.
One swap worth knowing about. DNA uses thymine (T). Its close relative RNA uses uracil (U) in the same slot. So “four letters” is true of DNA, and RNA has its own four with U where the T goes. This zine stays on DNA throughout.
A base is not a nucleotide. Strictly the letters name the bases; the repeating unit of the strand is a nucleotide — base plus sugar plus phosphate. The distinction does not change anything in the read-aloud layer, but it is the kind of thing a curious eight-year-old will meet next and be confused by.
They hold hands
The writing is not one line. It is two lines, side by side, running all the way down.
And every letter on one line is holding hands with a letter on the other line.
They are very fussy about who.
A always holds T.
G always holds C.
Never anybody else. Not sometimes. Not once.
This is the 1953 paper, and the sentence is almost this blunt. J. D. Watson & F. H. C. Crick, “Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid,” Nature 171, 737–738 (1953): “These pairs are: adenine (purine) with thymine (pyrimidine), and guanine (purine) with cytosine (pyrimidine).”
Why they cannot swap is geometry, and the paper says that too: “One of the pair must be a purine and the other a pyrimidine for bonding to occur.” A purine is the bigger ring and a pyrimidine the smaller; two big ones will not fit across the gap and two small ones will not reach. A–G and T–C are ruled out by size before anything else.
It had already been measured, by somebody else. The paper notes that “the ratio of the amounts of adenine to thymine, and the ratio of guanine to cytosine, are always very close to unity” — Erwin Chargaff's result, and the number their structure had to explain.
The credit problem, stated rather than smoothed. The acknowledgement in that paper reads, in full: “We have also been stimulated by a knowledge of the general nature of the unpublished experimental results and ideas of Dr. M. H. F. Wilkins, Dr. R. E. Franklin and their co-workers at King's College, London.” Rosalind Franklin's X-ray work was central to the structure and that sentence is how it was acknowledged. We have quoted it exactly rather than characterising it, because how thin it is, is the point. The child layer of this zine names nobody; this is the whole of what it says about who did the looking.
But nobody checks the order
So the pairs are fixed. A with T. G with C. No arguing.
Now here is the other question, and it is the good one.
What about the order?
Going down the line — A, then G, then G, then T, then C — is there a right order? One that is correct, that you are supposed to have?
No. There is not.
The letters can come in any order at all. Any order there is.
And that is not a thing nobody has checked yet. It is the actual rule. The same two people who worked out the holding-hands part wrote the other part down in the very same year, in the very same paper.
Verbatim, from the same 1953 paper: “The sequence of bases on a single chain does not appear to be restricted in any way.”
This is the hinge of the zine, and it is a real finding rather than a nice reading of one. It sits in the paper directly after the pairing rule and directly before its consequence: “if the sequence of bases on one chain is given, then the sequence on the other chain is automatically determined.” The constraint is entirely across the two strands. Along one strand there is none.
Why an unrestricted order is the useful half. A sequence that is free to be anything is a sequence that can carry information; one constrained to a single permitted arrangement could carry none. The variety is not a tolerance the system extends. It is the property that makes the system work at all.
What this does not mean. Unrestricted at the level of chemistry is not the same as “any sequence works equally well in an organism” — selection is a separate matter entirely, and nothing on this spread speaks to it. The claim here is narrow, chemical and exactly what the sentence says.
You, and the oak, and the whale
Now. Who else is written with these four letters?
The oak tree on the road. The grass. A whale, the whole enormous thing. A ladybird. The mould on a piece of bread you forgot about.
Every single living thing you have ever seen in your life.
Not letters like yours. The same ones.
The very same four.
What is different between you and the oak tree is not the alphabet. It is the order.
The picture's letter strings are invented, and the caption says so. They are there to show same letters, different arrangement, and six letters is far too few to be anybody's actual sequence. Nothing here is a real organism's DNA.
The universality is real and is explained by common descent. Every cellular organism — every animal, plant, fungus, bacterium and archaeon — stores its heredity in DNA built from these four bases. The shared alphabet is evidence that everything alive has a common ancestor, rather than a coincidence between separate inventions.
Chemical decorations do not add letters. Cells tag bases — 5-methylcytosine is the familiar one — and those tags matter enormously for which genes are active. They are modifications of the four, not additional letters in the alphabet, and they are not inherited as sequence.
“Every living thing you have ever seen” is phrased that way deliberately. It is exactly true, and it leaves room for spread eight, which is about something nobody has ever seen and which does not use A at all.
Take one line away
You need paper and a pencil. That is all.
One. Write a row of eight letters. Only A, T, G and C — but in any order you like. Make it up. Nobody is checking.
Two. Underneath each letter, write the one that holds its hand. A gets a T. T gets an A. G gets a C. C gets a G.
Three. Now fold the paper over, or cover the top row with your hand, so you cannot see it any more.
Can you write the top row back, just from looking at the bottom one?
Try it. Then look.
You got every single letter right. You had to. There was only ever one answer.
That is the trick your body uses when it needs to make a new copy of you.
Your child has just performed the inference the 1953 paper is most famous for, and it is the paper's closing line: “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” Fixed pairing means either strand is sufficient to reconstruct the other. That is the whole idea, and it is genuinely available to a six-year-old with a pencil.
What the demonstration shows and what it does not. It shows the information property — that no data is lost when one strand is removed. It does not show the machinery. Real replication is enzymatic, runs in one direction along each strand, is semi-conservative (each new double helix keeps one old strand), and involves proof-reading and repair. The paper mechanism is the logic, not the biology.
If it goes wrong, that is also true to life. A child who writes a wrong partner has made the thing a cell makes: a mismatch. Cells have dedicated enzymes that find and fix exactly that, and the ones that slip through are mutations — which is where variation comes from in the first place.
Except for something in a pond
We said everything alive uses the same four letters.
We should be careful. Let us say nearly everything.
Because there is something that lives in pond water. It is so small that you could never see it, not with the best eyes anybody has.
And it does not use A at all.
Not once. Not anywhere in the whole of its writing. Where everything else puts an A, this one puts a different letter instead — a letter nothing else uses.
It still holds hands with T. It just is not an A.
The organism is cyanophage S-2L, a virus that infects cyanobacteria. Its DNA substitutes 2-aminoadenine — also called 2,6-diaminopurine, and written Z — for adenine throughout. Not partially. The genome is described as ZTGC-DNA.
The source is older than most people expect. M. D. Kirnos, I. Y. Khudyakov, N. I. Alexandrushkina & B. F. Vanyushin, “2-Aminoadenine is an adenine substituting for a base in S-2L cyanophage DNA,” Nature 270, 369–370 (1977). Not read in full — Nature's own page for a 1977 article answers with an authentication redirect. The finding, the substitution being total, and the ZTGC-DNA description are taken from the 2021 follow-up literature that cites it, which we read as full text at PubMed Central.
Z still pairs with T, and pairs with it harder. The extra amino group lets it make three hydrogen bonds to thymine where adenine makes two. So the pairing rule from spread four is not broken by this — the partner is unchanged. What changed is which molecule sits in the A slot.
It is not alone any more. Later work found the enzymes that build Z-genomes in other phages as well, so S-2L is no longer a single oddity — though it remains the only member of its own clade. Numbers vary between the 2021 papers and we have not reconciled them, so none is quoted here.
It was in the water the whole time
Here is the careful bit, and you are allowed to hold us to it.
The thing in the pond is a virus. Scientists argue about whether a virus counts as alive at all. Most of them say it does not, quite.
So when we said everything alive uses the same four letters — that was true. But only just. It got away with it.
And here is the part we like best.
People have known about this pond one since 1977. That is probably before the grown-up reading this to you was grown up.
It was not hiding. It was not far away. It was in the water, being itself, the whole time.
Somebody just had to go and look, and then say what they found.
Why the universality claim on spread six still stands. Viruses are not cellular organisms and are conventionally excluded from “living things” — they have no metabolism of their own and replicate only inside a host. The four-letter alphabet remains true of all cellular life. This is a genuine boundary case rather than a disproof, and the honest way to hold it is: the rule has an exception, and the exception sits exactly where the definition of life gets argued about.
The forty-four-year gap is the real content of this spread. The substitution was reported in 1977; the enzymes that make it possible were not worked out until 2021. A fact can be published, correct and simply sit there for decades before anyone builds on it. That is normal, and it is worth a child knowing that “discovered” is not the same as “understood” or “acted on”.
What we are not doing with this. The pond virus is an exception to a generalisation about chemistry. It is not an argument about people, and it is not being used as one. Spread eleven says why that matters.
The alphabet, and the sentence
Here is the whole thing, both halves together.
The letters are shared. Nearly everything alive uses the same four, and that is not a coincidence. It is because everything alive is related to everything else alive, a very long way back.
You and the oak tree are family. Not in a nice-thought way. In a letters way.
And the order is free. For everybody. Always.
Not one living thing has ever been told what order to be in. There is no list of correct ones. There never was.
The alphabet is shared.
The sentence is yours.
Both halves of that are the 1953 paper's, and neither is our gloss. The fixed pairing and the unrestricted sequence are stated in the same two paragraphs. The zine puts them side by side; it does not add anything to either.
One claim we deliberately did not make. The obvious ending here is nobody else has your order — you are unique. It is left out for two reasons. The first is that it is not quite true: identical twins develop from one fertilised egg and their sequences are as close to the same as sequences get, differing only by mutations acquired afterwards. The second reason is the better one. Worth that rests on being rare is worth that can be taken away by somebody turning up who is similar to you. We are not building that floor under a four-year-old.
What replaces it is a claim about permission rather than rarity: no order is the correct one, so no order is the incorrect one. That holds for identical twins, and for everybody else, and it does not depend on a head count.
What this one refuses
This page is for you rather than for the child. It is the shortest page and the one we thought hardest about.
A zine about DNA, for children, in a house that writes from inside the neurodivergent and Disabled community, has to say plainly what it is not doing.
The pairing is fixed and the order never was. That is not a gap in the rule that somebody will tidy up later. That is the rule.
When you covered the top row, how did you know what to put back?
Could you write a row that is wrong? What would make it wrong?
The pond one has been there since 1977. What else do you think is in a pond that nobody has looked at yet?
You and a tree use the same four letters. Does that make the tree family? What would you call it?
Where all of this comes from
Spreads four to seven, and the two sentences the zine turns on. J. D. Watson & F. H. C. Crick, “Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid,” Nature 171, 737–738 (1953). Read at full text. Source of the pairing rule, the purine-pyrimidine size constraint, the Chargaff ratios, the acknowledgement quoted on spread four, the unrestricted-sequence sentence on spread five and the copying-mechanism line on spread seven. Every phrase in quotation marks in this zine's grown-up boxes is verbatim from it.
Spreads eight and nine. M. D. Kirnos, I. Y. Khudyakov, N. I. Alexandrushkina & B. F. Vanyushin, “2-Aminoadenine is an adenine substituting for a base in S-2L cyanophage DNA,” Nature 270, 369–370 (1977), PMID 413053. Not read in full — nature.com answers a 1977 article with an authentication redirect. The substitution, its completeness and the ZTGC-DNA description come from the 2021 follow-up work that cites it, read as full text at PubMed Central. Marked on the spread itself as well as here.
How this one is built
Two layers on every spread. Large plain type for the child, first in the document, so a screen reader meets it first — then a bordered box for the adult, holding the exact wording, the citations and every place the evidence runs out. Pictures sit between the two, where a child following the large type will actually reach them. The refusals on spread eleven are addressed to you, not to the child.
The demonstration on spread seven needs paper and a pencil and nothing else, because a demonstration that needs a kit is a demonstration that will not happen. It is the only one in this collection where the child performs a historical inference rather than observing an effect — the conclusion they reach on that spread is the one the 1953 paper reaches in its last sentence.
What we left out, and what is still open
A whole ending, cut on purpose. This zine started from a September 2026 result in which an ordinary bacterial enzyme transcribed a synthetic eight-letter alphabet as readily as the natural four. It is a lovely finding and it is not in here. Our own weekly scan had already graded it and said why: the argument needs the biology to stay in the test tube, and a zine would want it out. A read-aloud that ended on scientists made new letters is one small mishearing away from scientists made new creatures. The pond virus does the same work and never leaves the water, which is why spreads eight and nine are wild rather than laboratory.
Open, and said on the page. Whether a virus counts as alive is genuinely unsettled, and spread nine leaves it unsettled rather than ruling for the convenient answer. The 2021 papers disagree with each other about how many Z-genome phages are now known, so this zine quotes no number at all.
For grown-ups who want the rest of it. You Were Never One Thing is the other end of the same claim — not the alphabet you share but the separate organisms you are assembled from. It is not written for a child, and this zine borrows nothing from it into the read-aloud layer.