Start codon

Definition

A start codon is a specific sequence of three nucleotides in messenger RNA (mRNA) that signals the beginning of protein synthesis. The most common start codon is AUG, which codes for the amino acid methionine. This codon plays a critical role in the initiation stage of translation, where ribosomes recognize the start codon to begin assembling amino acids into a polypeptide chain.

5 Must Know Facts For Your Next Test

1. The start codon not only marks where translation begins but also establishes the reading frame for the ribosome to ensure proper amino acid sequencing.
2. In eukaryotes, the small ribosomal subunit binds to the 5' cap of mRNA and scans for the start codon before translation initiation.
3. The presence of a start codon is essential; without it, ribosomes cannot properly initiate translation, leading to incomplete or nonfunctional proteins.
4. Mutations in or around the start codon can lead to significant consequences, including truncated proteins or changes in protein function.
5. In prokaryotes, the ribosome can directly bind to the mRNA at the Shine-Dalgarno sequence and find the start codon without scanning.

Review Questions

• How does a start codon influence the translation process?
  • A start codon is crucial for initiating translation as it indicates where the ribosome should begin assembling amino acids into a protein. When the ribosome encounters a start codon like AUG on the mRNA, it sets up the correct reading frame for translation. This ensures that subsequent codons are read accurately, allowing for proper protein synthesis and function.
• Compare and contrast the roles of start and stop codons in protein synthesis.
  • Start and stop codons serve opposite functions in protein synthesis. The start codon signals where translation begins and establishes the reading frame for assembling amino acids, while stop codons signal where translation ends. The ribosome recognizes these signals to ensure that proteins are synthesized correctly, with start codons facilitating initiation and stop codons ensuring termination.
• Evaluate how mutations in start codons can affect protein synthesis and cellular function.
  • Mutations in start codons can drastically impact protein synthesis by preventing the ribosome from recognizing where to begin translation. This could lead to truncated proteins or nonfunctional variants if the ribosome skips essential coding sequences. Such alterations can disrupt cellular functions and contribute to various diseases, highlighting the importance of accurate start codon recognition in maintaining healthy cellular processes.

Related terms

ribosome:

A molecular machine in cells that facilitates the translation of mRNA into proteins by linking amino acids together.

amino acid: The building blocks of proteins, which are linked together in a specific sequence as dictated by the mRNA during translation.

stop codon:

A nucleotide triplet within mRNA that signals the termination of protein synthesis, causing the ribosome to release the newly formed polypeptide.