Proteins: From Genes to Degradation

Cells use transcription and translation to synthesize proteins. Transcription transfers genetic information from DNA to messenger RNA (mRNA) via RNA polymerase II. Translation then occurs in the ribosome, where the mRNA is decoded with help from transfer RNA (tRNA) molecules to synthesize a chain of amino acids that forms a polypeptide.

Pre-mRNA undergoes three processing steps in the nucleus: 5' capping, 3' polyadenylation, and splicing. These modifications create mature mRNA, which undergoes a quality check before transport through nuclear pore complexes to the cytoplasm. In the cytoplasm, the ribosome performs another quality check, and incorrectly processed mRNAs are degraded.

Transfer RNA (tRNA) molecules assist the ribosome in decoding mRNA during translation. They recognize triplet codons on the mRNA and deliver the corresponding amino acids. The ribosome catalyzes peptide bond formation between amino acids, building a polypeptide chain whose sequence depends on the mRNA's codon sequence.

After the polypeptide chain is synthesized, molecular chaperones and protein folding mechanisms fold it into a functional tertiary structure. This three-dimensional shape is essential for the protein to perform its biological functions. Proper folding ensures the protein becomes active and functional within the cell.

Errors during translation can result in incorrect polypeptide chains and misfolded proteins that are toxic to cells. Cells tag these abnormal proteins with ubiquitin molecules, marking them for degradation in proteasomes. This quality control mechanism prevents accumulation of defective proteins that could harm cellular function.

The nucleotide sequence in DNA defines the genetic code of mRNA through transcription. During translation, the ribosome reads triplet codons in the mRNA sequence, and each codon specifies a particular amino acid. This direct relationship between mRNA codons and amino acids ensures the polypeptide chain has the correct sequence for proper protein function.

Cells enforce quality checks at various stages—in the nucleus after pre-mRNA processing, in the cytoplasm when mRNA enters, and during translation—to ensure synthesis of correct proteins. Any defective RNA or protein is degraded through pre-defined mechanisms. These multiple checkpoints prevent errors from propagating and protect cells from toxic or nonfunctional proteins.