What is Anticodon?

The anticodons are trinucleotide units in the transport RNAs (tRNAs), that are complementary to the codons in messenger RNAs (mRNAs). They allow the tRNAs to supply the correct amino acids during the protein production.

The tRNAs are the link between the nucleotide sequence of the mRNA and the amino acid sequence of the protein. Cells contain a certain number of tRNAs, each of which can only bind to a particular amino acid. Each tRNA identifies a codon in the mRNA, which allows it to place the amino acid to the correct position in the growing polypeptide chain as determined by the mRNA sequence.

In one tRNA there are complementary sections, forming the cloverleaf structure, specific for the tRNAs. The cloverleaf consists of several stem-loop structures known as arms. They are Acceptor arm, D-arm, Anticodon arm, Additional arm (only for some tRNAs) and TψC arm.

The Anticodon arm has an anticodon, complementary to the codon in mRNA. It is responsible for the recognition and binding with the codon in the mRNA.

When the correct amino acid is linked to the tRNA, it recognizes the codon for this amino acid on the mRNA, and this allows the amino acid to be placed in the correct position as determined by the mRNA sequence. This ensures that the amino acid sequence encoded by the mRNA is translated correctly. This process requires recognition of the codon from the anticoding loop of the mRNA, and in particular from three nucleotides therein, known as anticodon which binds to the codon based on their complementarity.

Binding between the codon and the anticodon may tolerate variations in the third base because the anticodon loop is not linear, and when the anticodon binds to the codon in mRNA, an ideal double-stranded tRNA (anticodon) – mRNA (codon) molecule is not formed. This allows the formation of several non-standard complementary pairs, called wobble base pairs. These are pairs between two nucleotides that do not follow the Watson-Crick rules for the pairing of bases. This allows the same tRNA to decode more than one codon, which greatly reduces the required number of tRNAs in the cell and significantly reduces the effect of the mutations. This does not mean that the rules of the genetic code are violated. A protein is always synthesized strictly in accordance with the nucleotide sequence of the mRNA.

What is Codon?

The gene sequence encoded in DNA and transcribed in the mRNA consists of trinucleotide units called codons, each of which encodes an amino acid. Each nucleotide consists of phosphate, saccharide deoxyribose and one of the four nitrogen bases, so there are a total of 64 (4³) possible codons.

Of all 64 codons, 61 are coding amino acid. The other three, UGA, UAG, and UAA don’t encode amino acid but serve as signals for stopping protein synthesis and are referred to as stop codons. The methionine codon, AUG, serves as a translational initiation signal and is called a start codon. This means that all proteins start with methionine, although sometimes this amino acid is removed.

As the number of codons is greater than the number of amino acids, many codons are “redundant”, i.e. the same amino acid can be encoded by two or more codons. All amino acids, except methionine and tryptophan, are encoded by more than one codon. Redundant codons usually differ in their third position. The redundancy is needed to ensure enough different codons encoding the 20 amino acids and stop and start codons, and makes the genetic code more resistant to point mutations.

A codon is entirely determined by the selected starting position. Each DNA sequence can be read in three “reading frames”, each of which would give a completely different sequence of amino acids depending on the starting position. In practice, in the synthesis of the protein, only one of these frames has meaningful information about protein synthesis; the other two frames usually result in stop codons which prevents their use for direct protein synthesis. The frame in which a protein sequence is actually translated is determined by the start codon, usually the first encountered AUG in the RNA sequence. Unlike stop codons, a start codon alone is not enough to initiate the process. Neighboring primers are also required to induce mRNA transcription and ribosome binding.

It was originally thought that the genetic code is universal and that all organisms interpreted a codon as the same amino acid. Although this is the case in general, some rare differences in the genetic code have been identified. For example, in mitochondria, UGA, which is normally a stop codon, encodes tryptophan, whereas AGA and AGG, which normally encode tryptophan, are stop codons. Other examples of unusual codons have been found in Protozoans.

Difference Between Anticodon and Codon

1.   Definition

Anticodon: Anticodons are trinucleotide units in the tRNAs, complementary to the codons in mRNAs. They allow the tRNAs to supply the correct amino acids during the protein production.

Codon: Codons are trinucleotide units in the DNA or mRNAs, coding for a specific amino acid in the protein synthesis.

2.   Function

Anticodon: The anticodons are the link between the nucleotide sequence of the mRNA and the amino acid sequence of the protein.

Codon: The codons transfer the genetic information from the nucleus where the DNA is located to the ribosomes where the protein synthesis is performed.

3.    Location

Anticodon: The anticodon is located in the Anticodon arm of the molecule of tRNA.

Codon: The codons are located in the molecule of DNA and mRNA.

4.   Complementarity

Anticodon: The anticodon is complementary to the respective codon.

Codon: The codon in mRNA is complementary to a nucleotide triplet from a certain gene in the DNA.

5.   Numbers

Anticodon: One tRNA contains one anticodon.

Codon: One mRNA contains a number of codons. 

Summary:

• Anticodons are trinucleotide units in the tRNAs, complementary to the codons in mRNAs. They allow the tRNAs to supply the correct amino acids during the protein production.
• Codons are trinucleotide units in the DNA or mRNAs, coding for a specific amino acid in the protein synthesis.
• The anticodons are the link between the nucleotide sequence of the mRNA and the amino acid sequence of the protein. The codons transfer the genetic information from the nucleus where the DNA is located to the ribosomes where the protein synthesis is performed.
• The anticodon is located in the Anticodon arm of the molecule of tRNA, while the codons are located in the molecule of DNA and mRNA.
• The anticodon is complementary to the respective codon, and the codon in the mRNA is complementary to a nucleotide triplet from a certain gene in the DNA.
• One tRNA contains one anticodon, while one DNA or mRNA contains a number of codons.

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