What it does
Before a cell divides, it copies its DNA. The copying is very accurate, but not perfect: now and then a wrong base goes in. The next copy is made from the changed string, so the change is passed on, and new errors add to it. Over many generations small changes build up, and this is one source of the variation that natural selection works on.
This page copies a short string of 60 bases (A, T, G, C) over up to 20 generations. Changed bases are coloured so you can see exactly where they are. It is a teaching model with a far higher error rate than real cells have. See also central dogma lab for what the DNA does and natural selection for what happens to the variation.
How to use it
- Set the error chance per base (0 to 5 %) and the number of generations.
- Read the rows from top (the original) to bottom (the latest copy).
- Press Copy again for a fresh random string and fresh errors.
- Press Run 500 times to average many experiments.
What each setting does
- Error chance per base: how likely each single base is to be copied wrongly. A wrong base is always one of the other three.
- Generations: how many times the string is copied in a row, each copy made from the one before.
- Class 11: shows the exact average, which allows for a base being hit twice and changing back.
Worked example
With 60 bases, a 1 % error chance and 5 generations, the rule of thumb gives 60 × 0.01 × 5 = 3 changes on average. A single run might show 1 or 5, but the 500-run average lands close to 3 (the exact formula gives about 2.9).
FAQs
If the rate is 0, is there any variation? Not from copying. Every copy is the same.
Are all changes bad? No. Most do nothing noticeable, some harm, and a few help.
Do real cells make this many errors? No. Real error rates are around one in a billion per base after repair, so this page exaggerates to make the idea visible.
Where the numbers come from
- Variation from small copying errors in DNA, and the rule of thumb expected changes = length x error rate x generations, are standard school biology and probability. The exact average for Class 11 is 3/4 x length x (1 - (1 - 4p/3)^g), checked in the tests.
- A teaching model: the string is short and random, and real error rates are far smaller and are mostly fixed by repair enzymes. Drawings are fibub's own. Nothing you enter is stored or sent anywhere.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-10.