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2.6. Genetic code

The genetic code

The DNA contains the information of the proteins that have to be synthesized, but that information has to be transferred to the mRNA so that it reaches the cytoplasm.

In the cytoplasm, the mRNA will bind to ribosomes so that the translation of RNA into proteins takes place .

But RNA is written in a "nucleotide language" of only four nitrogenous bases (A, U, C, G), and proteins need a "20 amino acid language".

The passage from one language to another is carried out thanks to a code, the genetic code , in which the nucleotide sequence of the mRNA  and the amino acid sequence of the proteine are relatedThe genetic code determines which amino acid corresponds to three nucleotides of mRNA.

Every three successive nucleotides of the mRNA forms a triplet or codon that is equivalent to one amino acid of the new protein to be synthesized.

Some of the characteristics of the genetic code are:

  • The genetic code is degenerate. Combining the four nitrogenous bases of the mRNA in triplets or codons, there are up to 64 possible different codons. Instead, there are only 20 amino acids, so the same amino acid can be encoded by more than one codon. For example, the codons GAA and GAG specify glutamic acid (redundancy), but neither specify another amino acid (no ambiguity). All amino acids, except methionine and tryptophan , are encoded by more than one codon (synonymous codons).

This can be an advantage, since if a nucleotide is changed by mistake, it may not encode a different amino acid and another protein is generated.

  • Some codons, such as UAA, UAG, or UGA do not code for any amino acid, but indicate a signal for the termination of the translation process.
  • The AUG codon codes for the amino acid methionine and also as an initiation signal. Thus, the first AUG in an mRNA is the area that marks the place where protein translation begins.
  • The genetic code is universal, since it is shared by all known living beings (except in mitochondria, protozoa and mycoplasmas, which is slightly different). Thus, the same codon codes for the same amino acids in all known living beings. For example, the codon UUU encodes the amino acid phenylalanine in both prokaryotes and eukaryotes. This can prove that all known living things have the same origin. Although the term "universal" is used, it refers only to terrestrial life, since the existence of life on another planet has not been proven.
  • It does not present imperfection. There is no ambiguity, no codon can code for more than one amino acid, since if it did, it would be a big problem for the same gene to code for different proteins.
  • It is a code without overlaps. The triplets are arranged in a linear and continuous manner, without spaces between them and without sharing any nitrogen base. Its reading is done in one direction (5'→3'), from the initiation codon, which indicates the beginning of the protein, to the stop codon that indicates its end. However, the same mRNA can have several start codons, which means that several different polypeptides could be synthesized from the same mRNA .