Anticodon- Definition, Principle, Functions, Examples

An anticodon is a sequence of three nucleotides that is part of a transfer RNA (tRNA) molecule. It is complementary to a codon, which is a sequence of three nucleotides on a messenger RNA (mRNA) molecule. The anticodon and the codon pair together during the process of translation, which is the synthesis of proteins from mRNA.

The anticodon is located on one end of the tRNA molecule, called the anticodon loop or arm. The other end of the tRNA molecule has a site where a specific amino acid can attach. The amino acid is determined by the anticodon sequence, according to the genetic code. For example, the anticodon UAC corresponds to the amino acid methionine.

The function of the anticodon is to recognize and bind to the codon on the mRNA that matches its sequence. This ensures that the correct amino acid is brought to the ribosome, where the protein chain is assembled. The ribosome moves along the mRNA and reads each codon, then matches it with the appropriate tRNA anticodon. The amino acid carried by the tRNA is then added to the growing protein chain.

The pairing between the anticodon and the codon follows the rules of base complementarity: A pairs with U, and C pairs with G. However, there is some flexibility in the pairing of the third base of the anticodon, also known as the wobble position. This allows some anticodons to bind to more than one codon, which reduces the number of tRNAs needed to code for all 20 amino acids.

Some examples of anticodons and their corresponding amino acids are:

• UAC - methionine
• AAA - phenylalanine
• GAA - leucine
• CAA - valine
• AGA - serine
• UGA - threonine
• CGA - alanine
• AUA - tyrosine
• GUA - histidine
• UUU - lysine

In summary, an anticodon is a key component of tRNA that enables translation by pairing with codons on mRNA and delivering amino acids to the ribosome.

The anticodon principle states that the arrangement of mRNA and tRNA is antiparallel, meaning that the 5` end of the anticodon on tRNA binds with the 3` end of the codon on mRNA. The first two bases of the codon and the anticodon form normal Watson-Crick base pairs (A-U, U-A, G-C, C-G), while the third base pair may be less strict and allow some non-canonical base pairs. This phenomenon is known as the wobble hypothesis.

The wobble hypothesis was proposed by Francis Crick in 1966 to explain how one tRNA molecule can recognize more than one codon for the same amino acid. This reduces the number of tRNAs required to decode all 61 amino acid codons (three codons are stop signals). Crick suggested that some tRNAs have a modified base called inosine (I) at the first position of the anticodon, which can form hydrogen bonds with three different bases on the codon: U, C, or A. For example, the tRNA for methionine has the anticodon 5`-CAU-3`, which can pair with the codon 3`-AUG-5` (the start codon) or 3`-AUA-5`. Other possible wobble base pairs are G-U, U-G, and C-I.

The wobble hypothesis has several implications for the process of translation:

• It explains the degeneracy of the genetic code, meaning that more than one codon can code for the same amino acid.
• It allows for faster dissociation of tRNA from mRNA after transferring the amino acid to the growing polypeptide chain.
• It increases the efficiency and accuracy of translation by reducing the chances of misreading or frameshifting.
• It enables some tRNAs to recognize codons that differ only by a single nucleotide polymorphism (SNP), a common type of genetic variation.

The wobble hypothesis is supported by experimental evidence from various organisms, such as bacteria, yeast, and mammals. However, it is not a universal rule and there are some exceptions and variations in different species. For example, some tRNAs have other modified bases besides inosine at the wobble position, such as pseudouridine or queuosine. Some tRNAs have different wobble rules depending on their amino acid specificity. Some tRNAs have no wobble at all and only recognize one codon.

Therefore, the anticodon principle and the wobble hypothesis are important concepts to understand how tRNA molecules decode mRNA codons and facilitate protein synthesis.

Translation is the process of synthesizing proteins from the genetic information encoded in mRNA. Anticodon plays a crucial role in this process by recognizing and binding to the complementary codon on mRNA. The anticodon-codon interaction ensures that the correct amino acid is added to the growing polypeptide chain.

The functions of anticodon in translation can be summarized as follows:

• Anticodon determines the amino acid specificity. Each tRNA molecule has a specific anticodon sequence that matches with a specific codon on mRNA. The amino acid attached to the tRNA is determined by the anticodon sequence. For example, the anticodon UAC on tRNA corresponds to the codon AUG on mRNA, which codes for methionine. Therefore, the tRNA with UAC anticodon carries methionine as its amino acid.
• Anticodon initiates and terminates the translation process. The first anticodon that binds to the mRNA is UAC, which recognizes the start codon AUG. This initiates the translation process by bringing the first amino acid (methionine or N-formyl-methionine) to the ribosome. The last anticodon that binds to the mRNA is one of the three stop codons (UAA, UAG, or UGA), which do not code for any amino acid. This terminates the translation process by releasing the completed polypeptide chain from the ribosome.
• Anticodon facilitates the movement of tRNA along the mRNA. The anticodon on tRNA forms hydrogen bonds with the codon on mRNA, creating a temporary attachment. This attachment allows the tRNA to move along the mRNA in a 5` to 3` direction, following the genetic code. As each tRNA delivers its amino acid to the polypeptide chain, it moves from the A site (acceptor site) to the P site (peptidyl site) and then to the E site (exit site) of the ribosome, where it detaches from the mRNA and leaves.
• Anticodon allows for some flexibility in base pairing. According to the wobble hypothesis, some anticodons can pair with more than one codon, depending on the base at the third position of both sequences. This allows for some variation in base pairing, which reduces the number of tRNAs required to decode all 61 codons. For example, the anticodon GCI (where I stands for inosine) can pair with three different codons: CUA, CUC, and CUG, which all code for leucine. This means that one tRNA with GCI anticodon can carry leucine for three different codons.

These are some of the main functions of anticodon in translation. Anticodon is essential for ensuring that the genetic information in mRNA is accurately translated into proteins. Without anticodon, translation would not be possible or would be prone to errors.

To illustrate how anticodons work in translation, let us look at some examples of anticodons for different amino acids. Remember that the anticodon is the sequence of three nucleotides on the tRNA that is complementary to the codon on the mRNA. The codon is the sequence of three nucleotides on the mRNA that specifies an amino acid or a stop signal.

For example, suppose we have a codon on the mRNA that reads UUU. This codon codes for the amino acid phenylalanine. To translate this codon, we need a tRNA molecule that has an anticodon that can pair with UUU. The anticodon that is complementary to UUU is AAA. Therefore, the tRNA molecule that carries phenylalanine has an anticodon of AAA.

Another example is the codon AUG on the mRNA. This codon codes for the amino acid methionine and also serves as the start signal for translation. To translate this codon, we need a tRNA molecule that has an anticodon that can pair with AUG. The anticodon that is complementary to AUG is UAC. Therefore, the tRNA molecule that carries methionine and initiates translation has an anticodon of UAC.

However, not all codons have a single corresponding anticodon. Some codons can be recognized by more than one anticodon due to the wobble hypothesis. The wobble hypothesis states that the third base of the codon and the first base of the anticodon can have some flexibility in pairing, allowing for some mismatches or non-standard base pairs.

For example, suppose we have a codon on the mRNA that reads GGU. This codon codes for the amino acid glycine. To translate this codon, we need a tRNA molecule that has an anticodon that can pair with GGU. The anticodon that is complementary to GGU is CCA. However, there are other possible anticodons that can also pair with GGU due to wobble. These are CCG, CCU, and CCC. Therefore, there are four different tRNA molecules that can carry glycine and have different anticodons.

Another example is the codon UGA on the mRNA. This codon does not code for any amino acid but signals the end of translation. To translate this codon, we need a tRNA molecule that has an anticodon that can pair with UGA and release the polypeptide chain from the ribosome. This tRNA molecule is called a release factor and has an anticodon of ACU. However, there are other possible anticodons that can also pair with UGA due to wobble. These are ACG and ACA. Therefore, there are three different release factors that can terminate translation and have different anticodons.

The table below summarizes some examples of anticodons for different amino acids and stop signals.

These are just some examples of how anticodons work in translation. There are many more possible combinations of codons and anticodons that determine the sequence of amino acids in a protein.

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