Triplet Codon & The Codon Dictionary

Codon is a sequence of three nucleotides in the DNA or RNA which carry the genetic information for a particular amino acid. Each of the codons represents an amino acid that joins the elongating polypeptide chain to form protein. As each of the codons consists of three nucleotides, it is known as a triplet codon. The codon consists of genomic information for initiating the synthesis of protein (start codon) and a signal for termination of protein synthesis (stop codon). So, what is the start codon? It is the first codon of the mRNA which acts as an initiation signal for translation by the ribosome and starts forming the polypeptide chain. The stop codon terminates the elongation of the polypeptide chain and hence terminates the protein synthesis. The topic gives detailed information about codon examples, charts, and their role in life processes. 

All about Triplet Codon and Codon Chart 

The triplet codon is a sequence of three nucleotides encoding for a specific amino acid. The nucleotides are the building blocks of genetic material. Guanine, Adenosine, and Cytosine are present in both DNA and RNA whereas the 4th nucleotide in DNA is thymine and in RNA is uracil. The codon example is GGC which stands for glycine amino acid or CUA which stands for Leucine. There are 20 amino acids and each is a combination of three nucleotides. The codon chart is a table where all the amino acids are arranged and correspond to which codon it is represented. It provides detailed information on various nucleotide combinations that create the 20 amino acids. There are triplet codons or three-nucleotide combinations that represent the same amino acid.  

What are start codons and their role in translation? 

So, an important question is what is the start codon and its role in the process of protein synthesis or translation.

• A start codon is a group of three-nucleotide sequences that provides an initiation signal for translation. The translation is a process of changing mRNA (messenger RNA) into protein.
• Protein is formed through the formation of the polypeptide chain. Transfer RNA or tRNA brings an anticodon that attaches to the codons present in the mRNA to form a polypeptide chain.
• Codon is present in the mRNA and anticodon attaches to complementary codon leading to the process of protein synthesis.
• The codon example for the start codon is AUG in prokaryotes which stands for N-formyl methionine. Methionine is the start codon for eukaryotes that stands for AUG.
• In some eukaryotes, CUG which represents leucine is also used as the start codon. The mitochondrial genome uses AUA and AUU at times as a start codon that codes for isoleucine.

What are stop codons and their role in protein synthesis?

• Translation involves a total of 64 different codons where 61 represent specific amino acids whereas 3 are stop codons.
• The stop codon is an important codon example that signals the termination of the translation. Once the polypeptide chain is terminated, it is released.
• When an early stop codon enters the mRNA sequence due to mutation, it is known as an amber codon.
• The three stop codons which cause termination of the polypeptide chain are UAA, UAG, and UGA.
• The codon chart gives thorough information on the amino acids and the codons representing them.

Properties of the Codons

• One of the basic properties of the genetic codon is it is a triplet. Each of the codes consists of three nucleotides forming a triplet. Each of the codons codes for a particular amino acid.
• There are a total of 64 codons out of which 61 codes for amino acids, whereas 3 codes for a stop codon.
• Codons are non-overlapping, which means they are adjacent to each other. This means a single base cannot participate in information of more than one codon while reading the base sequence from mRNA starting.
• The genetic codes or codons are universal. This means the same codons code for the same amino acids that exist in all forms of life.
• The codons are non-ambiguous because each of the codons codes for only one amino acid. However, it degenerates as several codons code for the same amino acid. The codon example in such cases is codons ACU, ACC, ACA, and ACG stands for threonine.
• The codons have polarity meaning it is read in a definite direction. It means a codon represents an amino acid only when it is read in one particular direction. It might represent another codon if read from the opposite direction. The polarity of the codon is from 5’ to 3’ end.

Conclusion 

Triplet codons are strings of three nucleotides that represent the 20 amino acids as well as the stop and start codons. Each codon consists of three nucleotides – guanine, adenosine, cytosine, and uracil (RNA)/cytosine (DNA). The start codon is the initial signal which initiates the process of translation and hence synthesis of the polypeptide chain and its elongation by attachment of the amino acid. However, the stop codon gives the signal for the termination of the polypeptide chain and prevents further elongation. The codon chart gives detailed information on which codon represents which amino acid, the start and stop codon.