The Genetic Code and Translation

Key Concepts in Translation

• 3' End Correspondence: The 3’ end of mRNA matches the carboxyl terminus of the encoded polypeptide.
• Polypeptide Alignment: The amino terminus of the polypeptide lines up closest to the promoter.

UTR Definitions

• 5' UTR: Region of mRNA between the 5' end and the start codon.
• 3' UTR: Region of mRNA following the stop codon to the 3' end.

tRNA Interactions

• Base Pairing with mRNA: The anticodon region of tRNA forms base pairs with mRNA.
• Amino Acid Attachment: An amino acid is attached to the 3' end of the tRNA.

Translation Process

• Initiation: The start codon is located in the P site of the ribosome; ribosomes locate the correct AUG differently in bacteria (Shine-Dalgarno sequence) versus eukaryotes (5' cap scanning).
• Amino Acid Addition: Amino acids are added to the carboxyl terminus of the growing polypeptide during translation elongation, with aminoacyl tRNA entering the A site.
• Open Reading Frame (ORF): In a provided mRNA example, the open reading frame starts from a specific base and determines the polypeptide sequence.

Genetic Code and Codons

• Degeneracy: The genetic code is degenerate, meaning multiple codons can encode the same amino acid (e.g., CCC and CCU both encode proline).
• Sense and Antisense Strands: Codons are found in the sense strand of DNA, while the corresponding tRNA anticodons pair during translation.

Differences in Translation**

• Eukaryotic vs Bacterial: Eukaryotic translation utilizes the Kozak sequence; bacteria use Shine-Dalgarno. Eukaryotes also have separate transcription (nucleus) and translation (cytoplasm/ER).

• Initiation Factors: In eukaryotes, the 5' cap and polyA tail aid in ribosome assembly and start codon selection.

Key Terms

• Translation: Process of synthesizing polypeptides from mRNA.
• Amino Acid: Building blocks of proteins.
• Ribosome Sites: A site (aminoacyl), P site (peptidyl), E site (exit).
• Councodon Relationship: Each amino acid is encoded by a triplet code (codon).