The genetic code

The correspondence between two biological languages

Previously in the blog, we saw the players and major steps of the central dogma of molecular biology. For the gene-to-protein journey to come to fruition, living organisms must convert a 4-letter-based language (the nucleotide sequence of DNA and RNA) into a 20-letter-based one (the amino acid sequence of a protein). Let’s see next how cells carry out the translation between both languages.

The genetic code is a set of instructions that direct the translation of DNA into these 20 amino acids. It is made up of codons, which are three-letter chains of nucleotides. The genetic code allows this translation, associating each amino acid to a sequence of 3 nucleotides (codons). One codon corresponds precisely to one amino acid while one amino acid can be encoded by more than one codon (e.g the codon GUG corresponds only to Valine (V) while the same amino acid can be encoded also by the codons GUA, GUU, and GUC).

The genetic code describes how an RNA codon is translated into one of the 20 different amino acids. From the inside out, the first three circles represent the first, second, and third nucleotides of a given codon. In the fourth, fifth, and sixth circles, the translated amino acid is defined in three different ways: by its full name, by its three-letter abbreviation, and by its one-letter abbreviation. Three of the 64 total RNA codons are stop codons, which interrupt translation and add an extra sign to the amino acid alphabet. Source: Open Clip Art.

Example of gene-to-protein journey

The reading frame

Each strain of DNA could be translated into 3 different amino acid chains, depending on the nucleotide from which the translation starts (each DNA strain has 3 possible reading frames).


A nucleotide sequence that has the potential to be translated into a protein is known as Open Reading Frame (ORF). It always starts with a start codon (the first codon that is translated into an amino acid) and finishes with a STOP codon. In eukaryotes, the start codon is always ATG (encoding for Methionine) while the STOP codon could be either TGA, TAA or TAG

Gene structure and mRNA maturation

Gene expression is a dynamic process: to accomplish their functions, different cells express different combinations of genes in well-defined moments.

The expression of a gene is regulated by a promoter: a sequence of DNA able to activate the transcription in a precise temporal window and into defined cell populations. 

A gene is composed by two types of nucleotide blocks: exons (whose sequence is present in the mature mRNA) and introns (non-coding sequences with regulatory functions).

During RNA transcription, intron and exons are incorporated into the precursor of the mRNA (pre-mRNA). 

The pre-mRNA also includes:

  • Two untranslated regions with regulatory functions located upstream of the first exon (5’UTR) and downstream of the last one (3’ UTR).
  • a 7-methylguanosine CAP at the 5’ of the molecule, protecting the nascent mRNA from degradation and assisting in ribosome binding during translation.
  • a poly(A) tail at the 3’ of the nucleotide chain. it protects the mRNA from degradation, aids in the export of the mature mRNA to the cytoplasm, and is involved in binding proteins involved in initiating translation.

Once the pre-mRNA is synthetized, introns are removed in the process of splicing giving rise to the mature mRNA that will be subsequently translated into a protein.

Source: Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings
Miriam-Martinez-ZeClinics By Miriam Martínez

Miriam is a Human Biologist expert in neuropharmacology. After a master’s degree in Pharmaceutical and Biotech Industry, she obtained her PhD in Biomedicine from Pompeu Fabra University (Barcelona). During her doctorate, she focused her research on the behavioral analysis of animal models for neurophenotypical characterization. Since then, she has been working in the healthcare marketing and publicity sector, where she has contributed to developing marketing campaigns for several pharmaceutical brands. In 2021, she joined ZeClinics with a branding and marketing strategy focus.

genetic bases