How do codons of three bases on mRNA correspond to specific amino acids in a polypeptide?

Sourav PanNovember 8, 2024

Answered

Codons, which are sequences of three nucleotides in messenger RNA (mRNA), play a crucial role in determining the amino acid sequence of polypeptides during protein synthesis. This relationship is defined by the genetic code, which specifies how each codon corresponds to a specific amino acid or signals the termination of protein synthesis.

The Structure of Codons

1. Triplet Nature: Each codon consists of three nucleotides, making it a triplet code. Given that there are four types of nucleotides (adenine [A], uracil [U], cytosine [C], and guanine [G]), the combination of three nucleotides allows for a total of 43=6443=64 possible codons. This is sufficient to encode all 20 standard amino acids used in protein synthesis, as well as three stop codons that signal the end of translation .
2. Redundancy: The genetic code is redundant, meaning that multiple codons can specify the same amino acid. For example, both GAA and GAG code for glutamic acid. This redundancy helps to minimize the effects of mutations, as changes in the DNA sequence may not always lead to changes in the resulting protein .

Codon-Amino Acid Correspondence

1. Mapping Codons to Amino Acids: Each codon corresponds to a specific amino acid or a stop signal. For instance:
   • The codon AUG not only codes for methionine but also serves as the start signal for translation.
   • The codon UAA is one of the stop codons that terminate protein synthesis .
2. Transfer RNA (tRNA): tRNA molecules are essential for translating codons into amino acids. Each tRNA has an anticodon region that is complementary to a specific mRNA codon. When a tRNA with the appropriate anticodon binds to its corresponding codon on the mRNA, it brings along the specific amino acid that matches that codon .
3. Aminoacyl-tRNA Synthetases: These enzymes are responsible for attaching the correct amino acid to its corresponding tRNA, ensuring that each tRNA carries the appropriate amino acid specified by its anticodon .

The Translation Process

1. Initiation: The ribosome assembles around the mRNA and begins translation at the start codon (AUG). The initiator tRNA carrying methionine binds to this start codon .
2. Elongation: As the ribosome moves along the mRNA, each subsequent codon is read in sequence from 5′ to 3′. Corresponding tRNAs enter the ribosome, pairing their anticodons with mRNA codons, and bringing their attached amino acids into position for peptide bond formation .
3. Termination: When a stop codon (e.g., UAA, UAG, or UGA) is reached, no corresponding tRNA exists, and release factors promote the release of the completed polypeptide chain from the ribosome .