Termination Codon or Stop Codon

Introduction

The genetic code is the set of rules that determine the correspondence between nucleotide triplets (codons) and the amino acids they specify. In the language of genetics, the Stop codon or termination codon is a nucleotide triplet in mRNA that signals the end of the protein-coding sequence.

Stop codons are essential for the proper termination of protein synthesis and play a vital role in the regulation of gene expression.

What is Stop Codon?

• Stop codon, also known as termination codon, is a set of three nucleotide sequences in the messenger RNA (mRNA) that signals the end of protein synthesis during translation.

• The genetic code consists of 64 codons, of which 61 codons code for specific amino acids, and the remaining three codons function as Stop codons.

• The Stop codons do not code for any amino acids but instead signal the end of the protein-coding sequence.

Stop codons are essential for the proper termination of protein synthesis, and their recognition is a highly regulated process.

During translation, the ribosome reads the mRNA codons and assembles amino acids into a polypeptide chain based on the genetic code. When the ribosome reaches a Stop codon, the translation process ends, and the newly synthesized polypeptide chain is released.

The three Stop codons are UAA (Ochre), UAG (Amber), and UGA (Opal). The names of Stop codons are derived from early geneticists who used the colors of beads to denote different types of mutants.

Recognition Steps for Stop Codon

The recognition of Stop codons is a highly regulated process that involves the interplay of several factors. Here are the steps involved in the recognition of Stop codons ?

• Release Factors: Release factors are specialized proteins that recognize Stop codons and initiate the termination of protein synthesis. The release factor eRF1 recognizes all three Stop codons and promotes the release of the polypeptide chain from the ribosome.

• Ribosome Stall: Once the ribosome reaches a Stop codon, it stalls and waits for the release factor to bind to the codon.

• Peptide Release: The release factor binds to the Stop codon and catalyzes the hydrolysis of the peptide bond between the last amino acid and the tRNA, releasing the polypeptide chain from the ribosome.

Nomenclature of Stop Codon

Stop codons are named after the amino acid that would have been incorporated if translation had not been terminated. For example, UAA is named "ochre," UAG is named "amber," and UGA is named "opal." The names reflect the fact that early geneticists often used the colors of beads to denote different types of mutants.

How Does Stop Codon Work?

Stop codons are critical for the proper termination of protein synthesis. Here are a few examples of how Stop codons work ?

UAA

This is the most common Stop codon and signals the end of protein synthesis in most organisms. For example, the human insulin gene contains a UAA codon that marks the end of the protein-coding sequence.

UAG

This Stop codon is less common than UAA but is still used frequently. For example, in the human mitochondrial genome, the gene for the ATPase 6 protein ends with a UAG codon.

UGA

This is the least common Stop codon and is used in only a small number of genes. For example, the gene for the selenoprotein P in humans ends with a UGA codon, but in this case, the UGA codon does not signal the end of protein synthesis. Instead, it codes for the amino acid selenocysteine, which is incorporated by a specialized mechanism that bypasses the Stop codon.

Mutations in Stop Codon

Mutations in Stop codons can have severe consequences on gene expression and protein synthesis. Here are a few examples of different types of Stop codon mutations ?

Nonsense Mutation

This type of mutation creates a new Stop codon within the protein-coding sequence, resulting in premature termination of translation. For example, in cystic fibrosis, a mutation in the CFTR gene creates a premature Stop codon, leading to the synthesis of a truncated and non-functional protein.

Readthrough Mutation

This type of mutation suppresses the Stop codon and allows translation to continue beyond the normal endpoint. For example, in some cases of Duchenne muscular dystrophy, a mutation in the dystrophin gene can result in readthrough of the Stop codon, producing a slightly longer but still functional protein.

Conclusion

In conclusion, the Stop codon is a crucial element of the genetic code that signals the termination of protein synthesis. The recognition of Stop codons is a highly regulated process that involves the interplay of several factors.

Mutations in Stop codons can have severe consequences on gene expression and protein synthesis, and different types of mutations can result in premature termination or read through of translation.

Understanding the role and regulation of Stop codons is essential for a comprehensive understanding of the genetic code and its impact on biological processes.