Start codon

Definition

A start codon is a specific sequence of three nucleotides in messenger RNA (mRNA) that signals the beginning of translation, which is the process of synthesizing proteins from mRNA. The start codon establishes the reading frame for the ribosome, allowing it to correctly interpret the subsequent codons and synthesize a polypeptide chain. In most organisms, the start codon is typically AUG, which codes for the amino acid methionine, establishing it as the first amino acid in newly formed proteins.

5 Must Know Facts For Your Next Test

1. The start codon is crucial for ensuring that the ribosome reads the mRNA sequence correctly, as it sets the reading frame for translation.
2. In eukaryotes, AUG is not only a start codon but also codes for methionine, making methionine the first amino acid in nearly all eukaryotic proteins.
3. In prokaryotes, translation can begin before transcription is completed, allowing for a more efficient protein synthesis process.
4. Some mRNAs may have upstream elements or sequences known as Shine-Dalgarno sequences in prokaryotes, which help position the ribosome at the correct start codon.
5. Mutations in or around the start codon can lead to improper translation initiation and affect protein synthesis, potentially leading to various diseases.

Review Questions

• How does a start codon influence the accuracy of protein synthesis?
  • A start codon plays a critical role in determining where translation begins on an mRNA strand, thereby influencing the accuracy of protein synthesis. It sets the reading frame for the ribosome, ensuring that each subsequent codon is read correctly. If there were no defined start codon or if it were mutated, it could lead to frameshifts or misinterpretation of the mRNA sequence, resulting in incorrect protein synthesis.
• Compare and contrast the functions of start and stop codons in translation.
  • Start and stop codons both serve essential roles in translation but have opposite functions. The start codon initiates translation by signaling where protein synthesis should begin, typically at AUG. In contrast, stop codons signal termination by indicating where translation should end. Together, these codons ensure that proteins are synthesized properly and that the ribosome knows when to start and when to stop translating the mRNA into a polypeptide chain.
• Evaluate how mutations in start codons can affect cellular functions and contribute to diseases.
  • Mutations in start codons can significantly disrupt cellular functions by leading to improper initiation of translation. Such mutations may prevent ribosomes from binding effectively, causing incomplete or nonfunctional proteins to be produced. This can result in a range of diseases, including genetic disorders and certain cancers, where critical proteins are either not made at all or are produced incorrectly, affecting cellular processes and overall organismal health.

Related terms

Translation: The process by which ribosomes synthesize proteins using the sequence of codons in mRNA.

Ribosome: A molecular machine made of ribosomal RNA and proteins that facilitates the translation of mRNA into a polypeptide chain.

Stop codon: A sequence of three nucleotides in mRNA that signals the termination of translation, indicating that the protein synthesis is complete.