Anticodón: Description, Functions and Difference With Codon

A anticodon is a sequence of three nucleotides that is present in a transfer RNA molecule (tRNA), whose function is to recognize another sequence of three nucleotides that is present in a messenger RNA molecule (mRNA).

This recognition between codons and anticodons is antiparallel; that is, one is located in the 5 '-> 3' direction while the other one is in the 3 '-> 5' direction. This recognition between sequences of three nucleotides (triplets) is fundamental for the translation process; that is, in the synthesis of proteins in the ribosome .

2D structure (left) and 3D (right) of a transfer RNA

Thus, during translation the messenger RNA molecules are"read"through the recognition of their codons by the anticodons of the transfer RNAs. These molecules are so named because they transfer a specific amino acid to the protein molecule that is forming in the ribosome.

There are 20 amino acids, each encoded by a specific triplet. However, some amino acids are encoded by more than one triplet.

Additionally, some codons are recognized by anticodons in transfer RNA molecules that have no amino acids attached; These are the so-called stop codons.

Description

An anticodon is formed by a sequence of three nucleotides that can contain any of the following nitrogenous bases: adenine (A), guanine (G), uracil (U) or cytosine (C) in a combination of three nucleotides, in such a way that It works like a code.

The anticodons are always found in the transfer RNA molecules and always lie in the 3 '-> 5' direction. The structure of these tRNAs is similar to a trefoil, in such a way that it is subdivided into four loops (or loops); in one of the loops is the anticodon.

The anticodons are essential for the recognition of codons of messenger RNA and, consequently, for the process of protein synthesis in all living cells.

Functions

The main function of the anticodons is the specific recognition of the triplets that form the codons in the messenger RNA molecules. These codons are the instructions that have been copied from a DNA molecule to dictate the order of amino acids in a protein.

Since transcription (the synthesis of copies of messenger RNA) occurs in the 5 '-> 3' direction, codons in messenger RNA have this orientation. Therefore, the anticodons present in the transfer RNA molecules must have the opposite orientation, 3 '-> 5'.

This union is due to complementarity. For example, if one codon is 5'-AGG-3 ', the anticodon is 3'-UCC-5'. This type of specific interaction between codons and anticodons is an important step that allows the nucleotide sequence in the messenger RNA to encode an amino acid sequence within a protein.

Differences between anticodon and codon

- The anticodons are trinucleotide units in the tRNAs, complementary to the codons in mRNAs. They allow tRNAs to deliver the correct amino acids during protein production. In contrast, codons are units of trinucleotides in DNA or mRNA, which encode a specific amino acid in protein synthesis.

- The anticodons are the link between the nucleotide sequence of the mRNA and the amino acid sequence of the protein. On the contrary, the codons transfer the genetic information from the nucleus where the DNA is to the ribosomes where the synthesis of proteins takes place.

- The anticodon is found in the Anticodon arm of the tRNA molecule, unlike the codons, which are located in the DNA and mRNA molecule.

- The anticodon is complementary to the respective codon. In contrast, the codon in the mRNA is complementary to a triplet of nucleotides of a certain gene in the DNA.

- A tRNA contains an anticodon. In contrast, an mRNA contains a number of codons.

The rolling hypothesis

The balancing hypothesis proposes that the junctions between the third nucleotide of the codon of the messenger RNA and the first nucleotide of the anticodon of the transfer RNA are less specific than the junctions between the other two nucleotides of the triplet.

Crick described this phenomenon as a"rocking"in the third position of each codon. Something happens in that position that allows unions to be less strict than normal. It is also known as wobbling or tamboleo.

This Crick wobble hypothesis explains how the anticodon of a given tRNA can be paired with two or three different mRNA codons.

Crick proposed that, since the base pairing (between the base 59 of the anticodon in tRNA and the base 39 of the codon in mRNA) is less strict than normal, a certain"wobble"or reduced affinity is allowed in this site.

As a result, a single tRNA often recognizes two or three of the related codons that specify a given amino acid.

Normally, the hydrogen bonds between the bases of the tRNA anticodons and the mRNA codons follow strict rules of base pairing only for the first two bases of the codon. However, this effect does not occur in all third positions of all mRNA codons.

RNA and amino acids

Based on the wobble hypothesis, the existence of at least two transfer RNAs for each amino acid with codons exhibiting complete degeneration was predicted, which has proven to be true.

This hypothesis also predicted the appearance of three transfer RNAs for all six serine codons. Indeed, three tRNAs for serine have been characterized:

- tRNA for serine 1 (anticodon AGG) binds to codons UCU and UCC.

- tRNA for serine 2 (anticodon AGU) binds to codons UCA and UCG.

- tRNA for serine 3 (anticodon UCG) binds to codons AGU and AGC.

These specificities were verified by stimulated binding of purified aminoacyl-tRNA trinucleotides to ribosomes in vitro.

Finally, several transfer RNAs contain the base inosine, which is made from the hypoxanthine purine. Inosine is produced by a posttranscriptional modification of adenosine.

The Crick wobble hypothesis predicted that, when inosine is present at the 5 'end of an anticodon (the oscillation position), it would pair with uracil, cytosine or adenine at the codon.

In fact, purified alanyl-tRNA containing inosine (I) at the 5 'position of the anticodon binds to ribosomes activated with trinucleotides of GCU, GCC or GCA.

The same result has been obtained with other tRNAs purified with inosine at the 5 'position of the anticodon. Therefore, Crick's wobble hypothesis explains very well the relationships between tRNAs and codons given the genetic code, which is degenerate but ordered.

References

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5. Snustad, D. & Simmons, M. (2011). Principles of Genetics (6th ed.). John Wiley and Sons.