How does the mRNA sequence determine the amino acid sequence of polypeptides according to the genetic code?

SouravNovember 8, 2024

Answer

The mRNA sequence determines the amino acid sequence of polypeptides through the genetic code, which is a set of rules that defines how sequences of nucleotides in mRNA correspond to specific amino acids in proteins. This process involves several key components and mechanisms.

The Genetic Code

1. Triplet Code: The genetic code is based on codons, which are sequences of three nucleotides in mRNA. Each codon corresponds to a specific amino acid or a stop signal during translation. Since there are four nucleotides (adenine [A], uracil [U], cytosine [C], and guanine [G]) in RNA, using triplets allows for a total of 43=6443=64 possible combinations. This is more than sufficient to encode the 20 standard amino acids used in protein synthesis, leading to redundancy in the code where multiple codons can specify the same amino acid .
2. Codon-Amino Acid Mapping: The relationship between codons and amino acids is defined by the genetic code table. For example, the codon AUG not only serves as a start signal for translation but also encodes the amino acid methionine. Other amino acids can be encoded by one or more codons; for instance, phenylalanine can be specified by either UUU or UUC .

Translation Process

1. Initiation: Translation begins when the ribosome assembles around the mRNA strand. The small ribosomal subunit binds to the mRNA and scans for the start codon (AUG). An initiator tRNA carrying methionine binds to this start codon at the ribosome’s P-site .
2. Elongation:
   • tRNA Function: Transfer RNA (tRNA) molecules act as adaptors that bring specific amino acids to the ribosome based on the codons in the mRNA. Each tRNA has an anticodon region that is complementary to a specific mRNA codon, allowing it to pair accurately with the mRNA during translation .
   • Peptide Bond Formation: As each tRNA enters the ribosome’s A-site, its anticodon pairs with the corresponding mRNA codon. The ribosome catalyzes the formation of a peptide bond between the growing polypeptide chain (attached to the tRNA in the P-site) and the new amino acid brought by the tRNA in the A-site . This process continues as the ribosome moves along the mRNA, adding amino acids one by one according to the sequence of codons .
3. Termination: Translation concludes when a stop codon is encountered (UAA, UAG, or UGA). These codons do not correspond to any amino acid and signal for termination factors to release the completed polypeptide from the ribosome .