Start Codon

Definition

The start codon is a specific sequence of three nucleotides in messenger RNA (mRNA) that marks the beginning of the protein-coding region and signals the translation machinery to start synthesizing a new polypeptide chain.

5 Must Know Facts For Your Next Test

1. The start codon is typically the AUG (adenine-uracil-guanine) triplet, which codes for the amino acid methionine.
2. The start codon is recognized by the small ribosomal subunit, which then scans the mRNA for the start codon to initiate translation.
3. Correct identification of the start codon is crucial for ensuring the proper reading frame and the synthesis of the correct protein sequence.
4. In eukaryotic cells, the start codon is typically located within a Kozak sequence, a specific nucleotide sequence that enhances the recognition and binding of the ribosome to the start codon.
5. Mutations in the start codon or the surrounding Kozak sequence can lead to improper translation initiation and the production of incorrect or non-functional proteins.

Review Questions

• Explain the role of the start codon in the initiation of protein synthesis.
  • The start codon plays a critical role in the initiation of protein synthesis by marking the beginning of the protein-coding region on the mRNA. The start codon, typically AUG, is recognized by the small ribosomal subunit, which then scans the mRNA to locate the correct reading frame. Once the start codon is identified, the translation machinery, including the large ribosomal subunit and various initiation factors, assembles at the start codon to begin the process of synthesizing a new polypeptide chain.
• Describe the importance of the Kozak sequence in the recognition of the start codon.
  • In eukaryotic cells, the start codon is typically located within a Kozak sequence, a specific nucleotide sequence that enhances the recognition and binding of the ribosome to the start codon. The Kozak sequence provides a favorable context for the start codon, increasing the efficiency of translation initiation. Mutations in the Kozak sequence can lead to improper recognition of the start codon, resulting in the production of incorrect or non-functional proteins. The Kozak sequence is, therefore, crucial for ensuring the proper identification of the start codon and the accurate translation of the mRNA into the desired protein.
• Analyze the potential consequences of mutations in the start codon or the surrounding Kozak sequence.
  • Mutations in the start codon or the surrounding Kozak sequence can have significant consequences for protein synthesis and the production of functional proteins. If the start codon is altered, the ribosome may fail to recognize the correct translation initiation site, leading to the synthesis of an incorrect protein sequence or the complete absence of the desired protein. Similarly, mutations in the Kozak sequence can disrupt the efficient recognition and binding of the ribosome to the start codon, resulting in improper translation initiation and the production of non-functional or truncated proteins. These types of mutations can have severe implications for cellular processes, potentially leading to genetic disorders, developmental abnormalities, or other disease states, depending on the specific protein affected and its role in the organism.

Related terms

Initiation: The first step of protein synthesis where the ribosome and other translation factors assemble at the start codon to begin the process of translating mRNA into a polypeptide chain.

Reading Frame: The sequence of three-nucleotide codons on the mRNA that are read by the ribosome during translation, with the start codon defining the correct reading frame.

AUG Codon: The most common start codon, which codes for the amino acid methionine and signals the translation machinery to begin protein synthesis.