Anticodon

What is an anticodon?

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

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

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 being formed on 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 do not have any amino acids attached; these are the so-called stop codons.

Description

An anticodon is made up of 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, such that it works like a code.

Anticodons are always found in transfer RNA molecules and are always located 3 ‘-> 5’. The structure of these tRNAs is similar to a clover, in such a way that it is subdivided into four loops (or loops); in one of the loops is the anticodon.

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

Anticodon functions

The main function of anticodons is the specific recognition of triplets that make up codons in 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, the codons of the 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 a 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 messenger RNA to encode an amino acid sequence within a protein.

Differences between anticodon and codon

– Anticodons are trinucleotide units in tRNAs, complementary to codons in mRNAs. They allow tRNAs to supply the correct amino acids during protein production. Instead, codons are trinucleotide units in DNA or mRNA, encoding a specific amino acid in protein synthesis.

– Anticodons are the link between the nucleotide sequence of the mRNA and the amino acid sequence of the protein. Rather, codons transfer genetic information from the nucleus where DNA is found to ribosomes where protein synthesis takes place.

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

– The anticodon is complementary to the respective codon. Instead, the codon in the mRNA is complementary to a nucleotide triplet of a certain gene in the DNA.

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

The swing hypothesis

The swing 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 the joints to be less strict than normal. It is also known as wobble or wobble.

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

Crick proposed that since the base pairing (between base 59 of the anticodon in tRNA and base 39 of the codon in mRNA) is less stringent than normal, some “wobble” or reduced affinity is allowed at this site.

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

Normally, hydrogen bonds between the bases of tRNA anticodons and mRNA codons follow strict base pairing rules for only 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 was predicted for each amino acid with codons exhibiting complete degeneracy, which has been shown to be true.

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

  • TRNA for serine 1 (anticodon AGG) binds to codons UCU and UCC.
  • TRNA for serine 2 (AGU anticodon) binds to codons UCA and UCG.
  • TRNA for serine 3 (anticodon UCG) binds to the AGU and AGC codons.

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 purine hypoxanthine. Inosine is produced by a post-transcriptional modification of adenosine.

Crick’s wobble hypothesis predicted that, when inosine is present at the 5 ‘end of an anticodon (the wobble 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 GCU, GCC or GCA trinucleotide activated ribosomes.

The same result has been obtained with other tRNAs purified with inosine in the 5 ‘position of the anticodon. Thus, the Crick wobble hypothesis explains very well the relationships between tRNAs and codons given the genetic code, which is degenerate but orderly.

References

  1. Brooker, R. (2012).  Concepts of Genetics  (1st ed.). The McGraw-Hill Companies, Inc.
  2. Brown, T. (2006). Genomes 3 (3 rd ). Garland Science.
  3. Griffiths, A., Wessler, S., Carroll, S. & Doebley, J. (2015).  Introduction to Genetic Analysis  (11th ed.). WH Freeman
  4. Lewis, R. (2015).  Human Genetics: Concepts and Applications (11th ed.). McGraw-Hill Education.
  5. Snustad, D. & Simmons, M. (2011).  Principles of Genetics (6th ed.). John Wiley and Sons.

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