First Codon (Start Codon):
* Initiates translation: The first codon, typically AUG, signals the ribosome to start translating the mRNA sequence into a protein.
* Specifies methionine: The AUG codon also codes for the amino acid methionine, which will be the first amino acid in the polypeptide chain.
* Sets the reading frame: The start codon establishes the correct reading frame for the ribosome, ensuring that codons are read in the correct groups of three.
Last Codon (Stop Codon):
* Terminates translation: The last codon in the mRNA sequence is a stop codon (UAA, UAG, or UGA). It signals the ribosome to stop translating the mRNA and release the newly synthesized polypeptide chain.
* Does not code for an amino acid: Stop codons do not code for any amino acid. They are recognized by release factors, which promote the dissociation of the ribosome, mRNA, and the completed polypeptide chain.
In summary:
* The first codon (AUG) initiates protein synthesis, establishes the reading frame, and codes for methionine.
* The last codon (UAA, UAG, or UGA) signals the end of protein synthesis and does not code for any amino acid.
These codons are essential for accurate and efficient protein synthesis, ensuring that the correct protein is produced from the mRNA sequence.
