> **Source:** glthr.com, the blog of Guillaume Lethuillier
> **Article:** What Quotation Marks, String Literals and DNA Have in Common
> **Address:** https://glthr.com/quotation-marks-string-literals-and-dna
> **Published:** July 6, 2024 · **Updated:** November 12, 2026
> **Series:** Delimiters in Languages (https://glthr.com/page/delimiters-in-languages)
> **Disclaimer:** the opinions expressed in this article are the author’s own, not those of the author’s employer.
> **Other articles of this site, for AI agents:** https://glthr.com/llms.txt

# What Quotation Marks, String Literals and DNA Have in Common

*First-Order Expressions, Second-Order Expressions, and the Delimiters Between Them*

There seems to be a pattern in *languages* (in the broadest sense in the context of this article: a series of symbols that are evaluated or interpreted): **a duality between first-order expressions and second-order expressions that have different purposes but paradoxically use the same symbols.**

This observation is probably self-evident (it echoes the [use–mention distinction](https://en.wikipedia.org/wiki/Use%E2%80%93mention_distinction) in philosophy of language, discussed by Gottlob Frege and Willard Van Orman Quine, and Alfred Tarski’s distinction between an object language and the metalanguage that talks about it), but I have not seen it explicitly formulated in these terms:

*   A language primarily uses **first-order expressions**, which are intended to be evaluated. A human reader evaluates a human language to produce a representation of the world. An example of a first-order expression in English is: *It is raining outside.*
    
*   A language also needs to represent information used in a capacity different from first-order expressions. This information is represented with **second-order expressions**. Example: *“And what is the use of a book,” thought Alice, “without pictures or conversations?”* In this context, this expression has a different purpose: it represents what someone other than the narrator is thinking or saying.
    
*   First- and second-order expressions use **the same symbols** (for instance, the English alphabet).
    
*   First- and second-order expressions are separated with **delimiters**. *Only delimiters identify if an expression is of the first-order kind or of the second-order one*. In other words, they signal a transition between purposes.
    
*   A language evaluator (*e.g.*, someone reading a sentence) needs the delimiters to **switch** between first-order and second-order purposes.
    

![A horizontal bar in three segments: a blue first-order expression, evaluated; a green second-order expression, represented; a blue first-order expression, evaluated. A red block at each boundary is labelled delimiter: the first is the switch into second order, the second the switch back to first order.](https://glthr.com/images/quotation-marks-string-literals-and-dna/expression-delimiters.w.png)

This observation can be applied to many forms of *languages*. I suggest three of them: natural language, artificial language, and genetic code.

*   Since antiquity (for example, [ḏd-mdw](https://en.wiktionary.org/wiki/%E1%B8%8Fd-mdw) in Ancient Egypt), human language has used this mechanism to report what someone other than the narrator says. In English, the delimiters are the quotation marks.
    
    ![The sentence She told him: “It is raining outside”, with She told him: in blue, marked first-order, It is raining outside in green, marked second-order, and each of the two quotation marks in red, marked delimiter.](https://glthr.com/images/quotation-marks-string-literals-and-dna/reported-speech-roles.w.png)
    
*   Many programming languages use this approach to distinguish between *evaluated* and *represented* statements. Examples in Python: `print(1 + 1) → 2` vs. `print('1 + 1') → 1 + 1`.
    
    ![The Python statement that prints the quoted string 1 + 1, with print, its opening parenthesis and its closing parenthesis in blue, marked first-order, 1 + 1 in green, marked second-order, and each of the two single quotation marks in red, marked delimiter.](https://glthr.com/images/quotation-marks-string-literals-and-dna/quoted-code-roles.w.png)
    
    [LISP uses an apostrophe](https://www.lispworks.com/documentation/HyperSpec/Body/s_quote.htm) (or `QUOTE`). [Quine programs](https://cs.lmu.edu/~ray/notes/quineprograms/) are characteristic of this approach: they usually duplicate and combine first-order symbols (the code evaluated by the computer) and second-order symbols (the code represented by the computer) to blur the line between them as they output the exact representation of the evaluated code. The delimiters are usually quotation marks, too.
    
*   When viewing DNA as a language, a similar mechanism appears to be in place. [Palindrome-like sequences act as delimiters to identify segments with a different purpose than the rest of the DNA](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8957550/). For instance, when a bacterium is infected by a virus, it can store part of the DNA of the latter, in segments named “spacers” (second-order expression), into its own DNA (first-order expression). Precisely because they are second-order expressions, [these spacers are not evaluated but ultimately (after their transcription) used as signatures by Cas9 to detect known invading viruses](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3119747/). They act as a genetic memory (*i.e.*, they represent information). More generally, [CRISPR](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5901406/) embodies this duality.
    
    ![Left, linear double-stranded DNA in five framed segments: first-order bases X X, a delimiter GATC paired with CTAG, six second-order bases, the same delimiter GATC over CTAG, then first-order bases X X, the strands continuing at both ends. Arrows labelled folds and unfolds lead to the folded form on the right, a cross: the first-order bases stay paired in a horizontal band; above it, the upper strand’s opening and closing delimiters pair with each other as a stem, C with G, T with A, A with T, G with C, capped by a loop of the six second-order bases; below it, the lower strand does the same. Labels: first-order, delimiters, opening + closing, paired, second-order.](https://glthr.com/images/quotation-marks-string-literals-and-dna/dna-cruciform.w.png)
    
    In the scientific literature, analogies with human language focus mainly on the palindromic features (for example, [this parallel with the Sator square](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8957550/#S1title)). *While this is true, I believe a more meaningful analogy would be to compare them with quotation marks*. Because genetic code has a limited set of “symbols,” it cannot express the delimiters with a special base: the palindromic disposition of the bases constitutes the delimiters. It is as if the English language did not use special characters but palindrome-like sequences instead as delimiters: `She told him xyab It is raining outside bayx and therefore . . .`. (In this example, `xyab` would represent the switch from first- to second-order expression \[opening delimiter\] and `bayx`, the transition from second- to first-order expression \[closing delimiter\].)
    

The following table summarizes this idea.

![A table with the columns Language, Symbols, First-order expression (blue), Second-order expression (green) and Delimiter (red). Human language: alphabet, numbers, diacritics, etc.; conveys an information, example It is raining outside; represents an information, example She told him: “It is raining outside”; quotation marks, “ and ”. Programming language: subset of the human alphabet; is evaluated, LISP (print (+ 1 1)) gives 2; is represented, LISP print with the quoted list (+ 1 1) gives (+ 1 1); special character, the LISP apostrophe. Genetic code: adenine (A), cytosine (C), guanine (G), thymine (T); is evaluated, DNA of a bacteria; represents an information, fragment of foreign DNA stored in bacteria DNA (“spacer”); palindrome-like sequence, 5′ TAATTGAATTA 3′ over 3′ ATTAACTTAAT 5′.](https://glthr.com/images/quotation-marks-string-literals-and-dna/delimiters-table.w.png)

There are certainly more examples, and **I notably wonder if there is a possible articulation between artificial intelligence and this duality**:

*   Can models infer *by themselves* the existence of first- and second-order expressions and the role of delimiters in switching between them?
    
*   Could this duality be leveraged to reinforce models’ security by making them inherently distinguish pure knowledge representation from information evaluation during the training stage?