Nirenberg experiment: cracking the genetic code
About this app
Why a codon is three bases long: 4, 16 or 64 words from U, C, A, G next to 20 amino acids. Then the 1961 lab: cell-free extract, poly-U and 20 tubes, each with one radioactive amino acid; only the Phe tube counts. Poly-A, poly-C, poly-G and Khorana's (UC)ₙ fill the code sun.
With explain mode: an AI tutor walks you through the app, points out what matters in the scene and answers your questions.
How the genetic code was cracked
Four bases make 4 words of one letter, 16 of two and 64 of three. Sixteen is not enough for 20 amino acids, so a codon needs at least three bases. In 1961 Marshall Nirenberg and Heinrich Matthaei added an artificial mRNA made only of uracil (poly-U) to a cell-free extract from E. coli. They set up 20 tubes, each with a different radioactively labelled amino acid. Radioactive protein formed only in the tube with labelled phenylalanine: UUU codes for Phe. Poly-A gave lysine, poly-C proline. Khorana’s (UC)ₙ gave alternating serine and leucine.
What the app shows
The combinatorics view sets the number of possible words for one, two and three bases next to the 20 amino acids and shows which codons code for the same amino acid. The lab view walks through the experiment: add mRNA, incubate, precipitate with acid and filter, measure radioactivity. Each result goes into a code sun, which can be filled with all 64 codons. The measured values are relative.
Things to try
Raise the word length from one to three bases and compare the number of words with the line at 20. Choose poly-U and run the procedure, then poly-A, poly-C and poly-G. Finally try (UC)ₙ and think about why two amino acids alternate. A control without mRNA shows the background. The full code sun shows the state of 1966.
Frequently asked questions
- What did the Nirenberg experiment show?
- In 1961 Nirenberg and Matthaei showed that an artificial mRNA made of uracil (poly-U) is translated into a chain of phenylalanine in a cell-free system. This deciphered the first codon: UUU codes for phenylalanine. It began the deciphering of the genetic code, which was complete by 1966.
- Why does a codon consist of three bases?
- There are four bases and 20 amino acids. With two bases per codon there would be only 4² = 16 different words, too few. With three bases there are 4³ = 64; 61 of them code for amino acids and 3 are stop codons. Many amino acids therefore have several codons; the code is degenerate.
- Why did poly-G give no result?
- Strands of guanine fold into stable structures called G-quadruplexes. The ribosome cannot read RNA folded like this. So GGG could not be determined with poly-G; the codon for glycine was found later with other methods.
- What can I try in the Nirenberg Experiment app?
- You set the word length and compare it with the 20 amino acids, highlight the codons of one amino acid, choose an artificial mRNA (poly-U, A, C, G, (UC)ₙ or none), run the lab procedure and see the result in the code sun.
Subject: Biology (Gymnasium)
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