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Quiz yourself on Protein Synthesis — Transcription and TranslationGet your flashcardsBuild my revision planTrack my progress📖 The Genetic Code
The genetic code is the set of rules by which the base sequence of mRNA codons is translated into the amino acid sequence of a protein. Three key properties make the genetic code remarkable:
1. Triplet code
Each codon consists of three mRNA bases. With four possible bases (A, U, G, C), there are 4 × 4 × 4 = 64 possible codons. Since there are only 20 amino acids used in proteins, most amino acids are coded for by more than one codon.
2. Degenerate (redundant) code
Multiple codons can code for the same amino acid. For example:
- UUU and UUC both code for phenylalanine
- GCU, GCC, GCA, and GCG all code for alanine
This degeneracy means that some mutations (particularly at the third position of a codon) do not change the amino acid — the mutation is silent and the protein is unaffected.
3. Universal code
The same codons specify the same amino acids in almost all living organisms — from bacteria to humans. This is powerful evidence for evolution from a common ancestor. It is also why genes can be transferred between species in genetic engineering.
Start and stop codons
- AUG — the start codon; also codes for methionine; signals where translation begins
- UAA, UAG, UGA — the three stop codons; no amino acid attaches; translation ends
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