# Genetic code

Groups of three bases on mRNA (codons) specify an amino acid. 64 codons cover 20 amino acids, plus start (AUG) and stop signals. The code is redundant: several codons can code for the same amino acid.

Biology (university) · Part 04: DNA, transcription and translation · Card 7 of 9 · Last updated 2026-10-19

## Analogy

A dictionary where each three-letter word means one building block.

## Example

**Reading codons**

mRNA: AUG GCU UAA
Met - Ala - STOP

## In practice

AUG means methionine and start. UAA, UAG and UGA mean stop.

## Practice

### Flashcards

- **What is a codon?** A group of three mRNA bases that codes for one amino acid or a stop signal.

### Interview questions

**Compare silent, missense and nonsense mutations and say which is likely to be most harmful.**

- Silent: same amino acid
- Missense: different amino acid
- Nonsense: premature stop
- Nonsense usually most damaging

## Questions people ask

### Why is the genetic code called redundant?

With 64 codons but only 20 amino acids, most amino acids have more than one codon. For example, leucine has six. This is useful because many mutations, particularly in the third base of a codon, change nothing in the protein. It protects cells from some damage.

### Is the genetic code the same in all organisms?

Nearly. Bacteria, plants and humans read the same codons as the same amino acids, which is strong evidence of shared ancestry and what makes genetic engineering possible. A few small variants exist, for example in mitochondria and some single-celled organisms.

## Related concepts

- [Point mutation](https://studyvisually.com/learn/science/biology-university/biology-university-part-4/biology-university-part-4-concept-9)
- [Central dogma](https://studyvisually.com/learn/science/biology-university/biology-university-part-4/biology-university-part-4-concept-4)
- [Transcription](https://studyvisually.com/learn/science/biology-university/biology-university-part-4/biology-university-part-4-concept-5)
- [RNA splicing](https://studyvisually.com/learn/science/biology-university/biology-university-part-4/biology-university-part-4-concept-6)

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