mRNA Translation: From Transcription to Protein Synthesis

Transcription and mRNA Processing

Transcription occurs within the nucleus. During this process, introns are removed from the pre-mRNA, creating a mature mRNA molecule. This mature mRNA then migrates from the nucleus to the cytoplasm.

Translation Initiation and Elongation

Once in the cytoplasm, the mRNA is recognized by the ribosome. This recognition occurs via specific sequences in bacteria and the 5' cap in eukaryotes. The ribosome then initiates the translation process.

tRNA molecules act as adapters between amino acids and mRNA. Each tRNA has a region that binds to a specific amino acid and another region, the anticodon, that recognizes a triplet of nucleotides (a codon) in the mRNA.

Translation begins when the ribosome identifies the start codon, AUG, which codes for methionine (or formyl-methionine in bacteria). As translation progresses, different tRNAs bind to their corresponding codons on the mRNA. The ribosome facilitates the formation of peptide bonds between the amino acids carried by the tRNAs. The tRNA then releases the growing peptide chain, and the ribosome moves to the next codon.

Translation Termination

Three codons, UAA, UAG, and UGA, do not code for any amino acid. These codons serve as termination signals. When the ribosome encounters one of these stop codons, the newly synthesized protein is released.

Reading Frames

The ribosome can read the mRNA in three different reading frames. A reading frame is a sequence of consecutive, non-overlapping codons. The reading frame is determined by the starting nucleotide. Different reading frames can result in different protein sequences. For example, a stop codon might appear in one reading frame but not in another, affecting the length and sequence of the resulting protein.