Start codon

Definition

A start codon is a specific sequence of nucleotides in mRNA that signals the beginning of translation, where protein synthesis occurs. It is essential for initiating the process of translating mRNA into a polypeptide chain. The most common start codon is AUG, which also codes for the amino acid methionine, establishing the first amino acid in newly formed proteins.

5 Must Know Facts For Your Next Test

1. The start codon serves as a critical signal for ribosomes to assemble and begin protein synthesis.
2. In addition to AUG, other start codons exist in certain organisms, but AUG is the most universally recognized among eukaryotes and prokaryotes.
3. The presence of the start codon ensures that translation begins at the correct location on the mRNA strand.
4. Mutations that affect the start codon can lead to significant changes in protein function or may prevent translation altogether.
5. In some cases, alternative start codons can be utilized, allowing for different proteins to be produced from the same mRNA sequence through a process known as leaky scanning.

Review Questions

• How does the start codon influence the initiation of translation and what role does it play in determining the reading frame?
  • The start codon is crucial for initiating translation as it marks where ribosomes begin synthesizing proteins. It establishes the reading frame, ensuring that codons are read correctly in groups of three nucleotides. If the start codon is incorrectly positioned or mutated, it can lead to a shift in the reading frame, resulting in entirely different and often nonfunctional proteins being produced.
• Discuss the importance of the AUG start codon and its implications if altered or mutated within a gene.
  • The AUG start codon is vital as it not only signals the beginning of translation but also codes for methionine, the first amino acid in most proteins. If this codon is altered or mutated, it may prevent proper initiation of translation, leading to truncated proteins or complete failure to produce a protein. Such changes can have serious consequences on cellular function and organismal development.
• Evaluate the potential consequences of using alternative start codons during translation and how this affects protein diversity.
  • Using alternative start codons can allow cells to generate multiple protein variants from a single mRNA transcript, enhancing protein diversity without requiring additional genetic information. This phenomenon can be advantageous for an organism's adaptability, as different isoforms might have unique functions or regulatory mechanisms. However, if misregulated, it may also lead to dysfunctional proteins that disrupt normal cellular processes and contribute to disease.

Related terms

codon: A codon is a sequence of three nucleotides in mRNA that corresponds to a specific amino acid or a stop signal during protein synthesis.

anticodon: An anticodon is a set of three nucleotides in tRNA that pairs complementary to the codon in mRNA, ensuring the correct amino acid is added during translation.

translation: Translation is the process in which ribosomes synthesize proteins by decoding the information carried by mRNA and linking the appropriate amino acids together.