Reading frame: three wrongs make it right
About this app
A sentence of three-letter words read in fixed frames without spaces: one insertion or deletion turns the rest into nonsense, three insertions make it readable again. The same on an mRNA with amino acids and a stop codon, plus the 1961 phage experiment by Crick and Brenner.
With explain mode: an AI tutor walks you through the app, points out what matters in the scene and answers your questions.
Read in groups of three
The genetic code is read from a fixed starting point in groups of three, with no separators between codons. If a base is inserted or removed, the reading frame shifts from that point: every following codon changes, and a stop codon usually appears soon. Three insertions, or one insertion and one deletion, restore the frame afterwards; only the piece in between stays changed. With this finding, Crick, Barnett, Brenner and Watts-Tobin showed in 1961, using phage T4, that the code consists of triplets.
What the app shows
A sentence of three-letter words stands without spaces in fixed boxes. Insertions and deletions are applied one after another, the letters shift, the boxes stay put, and a reading window reads frame by frame. Readable and shifted words are coloured differently. The codon view does the same on an mRNA with amino acids and a stop codon. There are also four phage plates with 0 to 3 insertions.
Things to try
Make one insertion and read where the sentence turns into nonsense. Then make two and three insertions and compare. Combine an insertion with a deletion and move the position of the first mutation. Switch to the codon view and see where the amino acid chain stops. With three insertions it runs to the end and has one extra amino acid.
Frequently asked questions
- What is a reading frame?
- The reading frame determines how a base sequence is divided into codons of three bases each. It starts at the start codon and continues without gaps. The same sequence read in a frame shifted by one base gives entirely different codons and therefore different amino acids.
- Why do three insertions restore the reading frame?
- Each insertion shifts the frame by one base. After three insertions it is shifted by exactly one codon, which puts it back in the original rhythm. Behind the third insertion the codons are read as in the original; only between the first and third insertion are they changed, and the protein has one extra amino acid.
- What did the 1961 experiment by Crick and Brenner show?
- Crick, Barnett, Brenner and Watts-Tobin used proflavine to create insertions and deletions of single bases in the rIIB gene of phage T4. The gene did not work with one or two insertions, but it did with three insertions or with one insertion plus one deletion. They concluded that the code is read in groups of three, without commas, from a fixed starting point.
- What can I set in the Reading Frame app?
- You choose no, one, two or three insertions, one deletion or an insertion plus a deletion, move the position of the first mutation and let the reading window run. You can switch between a sentence of words and an mRNA with amino acids.
Subject: Biology (Gymnasium)
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