MCAT Content / Genetic Code / The Triplet Code

The triplet code

Topic: Genetic Code

Each three nucleotides (triplet) in the genetic code, known as a codon, encodes a specific amino acid or stop signal.

After DNA is transcribed into RNA, the RNA is translated into a polypeptide sequence. Every three nucleotides in the RNA sequence is “read” as a separate codon, which encodes a specific amino acid; this is why codons are known as the “triplet code”. In the example below, a short polypeptide of three amino acids is encoded by nine RNA nucleotides. There is also a stop codon present, which signals the end of translation and synthesis of the polypeptide chain. If there is a mutation which occurs and causes an early stop codon in an mRNA sequence this is called an amber codon. Mutations can also have wider impacts upon translation and transcription. If the

Take a look at the chart below showing which codons encode which amino acids. You’ll see that many amino acids are actually encoded by more than one codon. Thus, the genetic code is degenerate, meaning many amino acids that are encoded by a few different codons. There are 64 possible nucleotide triplets (4 possible nucleotides in groups of three = 43), which is far more than the number of amino acids. Three of these codons are stop codons: UAA, UAG, and UGA. The remaining 61 codons encode 20 amino acids. Out of these 61 codons, the codon AUG (which encodes the amino acid methionine) also serves as a the start signal (start codon) for translation initiation. These codons are similar in all organisms and thus are considered universal.


Practice Questions


Khan Academy


MCAT Official Prep (AAMC)

Sample Test B/B Section Passage 4 Question 21

Practice Exam 4 B/B Section Question 29

Practice Exam 4 B/B Section Passage 10 Question 54

Key Points

• Every three nucleotides in the genetic code corresponds to one codon, which can either encode an amino acid or a stop signal for translation.

• A total of 64 codons are possible.

• The genetic code is universal because it is the same among all organisms.

• Three codons, UAA, UAG, and UGA, are known as stop codons as they stop the process of translation. If this is a mutation as a stop codon and its finishes prematurely it is called an amber codon.

• AUG is also known as a start codon, as it initiates the process of translation.

Key Terms

Nucleotides: The building blocks of DNA and RNA, these molecules are made up of a nucleoside and a phosphate group.

Amber codon: A stop codon that is a mutation of the genetic code during translation causing premature stop of the reading of the mRNA

Codon: A sequence of three adjacent nucleotides that encode a specific amino acid or stop signal during protein synthesis (translation).

Amino acid: Organic molecules that make up proteins; they contain an amino group (−NH2), a carboxyl group (−COOH), and a side chain specific to each amino acid.

Transcription: The process of converting a specific sequence of DNA into a new RNA molecule.

Translation: The process where RNA is used to create a new polypeptide chain (protein).

Polypeptide: A chain of amino acids that are connected by peptide bonds.

Stop codon: One of three codons (UAA, UAG, and UGA) that signals the end of translation.

Degenerate: Describes the fact that multiple codons can encode the same amino acid.

Start codon: The codon AUG, which both signals the start of translation and encodes the amino acid methionine.

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