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10.6.1. The genetic code

The genetic code

The genetic code is the set of rules that allows the translation of a sequence of nucleotides of the mRNA to a sequence of amino acids constituting a protein, in all living beings, which shows that has a single or universal origin, the least the context of our planet.

The code defines the relationship between sequences of three nucleotides , called codons, and amino acidsThus, each codon corresponds to a specific amino acid.

The sequence of genetic material is made up of four different nitrogenous bases , which have a function equivalent to letters in the genetic code : adenine (A), thymine (T), guanine (G) and cytosine (C) in DNA and adenine ( A), uracil (U), guanine (G), and cytosine (C) in RNA .

The translation of the genetic message into proteins can be done thanks to this "dictionary" called the genetic code . Although there are only 20 different amino acids to code for, only 4 different nitrogenous bases are available to do so. Therefore, it will take more than one nitrogen base to code for each amino acid. If they were two nitrogenous bases, they would encode 42 = 16 different pairs of bases that would encode 16 different amino acids. And if they were three different bases (a triplet), they would code, 43 = 64 different triplets, which could be used to code even more than the 20 amino acids that exist.

Because of this, the number of possible codons is 64:

  • 61 codons encode amino acids (one of them being the start codon, AUG, and encoding the amino acid methionine ).
  • 3 codons that do not code for any amino acids, but are stop signals (UAAUAG, and UGA).

Each triplet (group of three) nucleotides of mRNA is called a codon. The codon sequence determines the amino acid sequence in a specific protein, which will have a specific structure and function. The DNA triplets that have been transcribed to these codons are called coding.

Characteristics of the genetic code

The genetic code has the following characteristics :

  • It is universal. The genetic code is shared by all known organisms, including viruses and organelles. Thus, for example, the codon UUU encodes the amino acid phenylalanine in both prokaryotes and eukaryotes. This indicates that the genetic code has had a unique origin in all known living beings. The word "universal" refers only to life on Earth, as life on another planet has not been proven.

Although we say it is universal, it has some exceptions, such as in mitochondriaprotozoa and mycoplasmas (a type of bacteria), whose genetic code has some slight difference.

  • The genetic code is degenerate. It means that an amino acid is encoded by more than one codon. For example, although the codons GAA and GAG specify glutamic acid (redundancy), neither specifies another amino acid (there is 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.

  • 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 a single 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.

Basic ideas about the genetic code

The genetic code

The genetic code has these characteristics:

  • It is universalAll known beings share the same genetic code.
  • It is degenerate. The same amino acid is encoded by more than one codon.
  • It does not present imperfection. The genetic code is not ambiguous, a codon cannot code for more than one amino acid.
  • No overlaps. The triplets are arranged in a linear and continuous way and are read in the 5'→ 3' direction.