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The central dogma—DNA → RNA → Protein—describes information flow in cells. DNA is transcribed into mRNA (eukaryotes: processed via capping, splicing, polyadenylation); mRNA is translated into protein on ribosomes using tRNAs as adapters. Eukaryotic translation requires initiation factors and is often coupled with post-translational modifications (phosphorylation, glycosylation) and protein targeting to specific compartments.
Use pulse-chase labeling to track protein synthesis, localization, and degradation. Identify post-translational modifications with 2D gel electrophoresis. Explain rapid response mechanisms (iron-responsive elements).
Central dogma is absolute—reverse transcriptase and alternative splicing permit deviations. All genes are continuously expressed—expression is tightly regulated. Proteins are final products—post-translational modification is essential.
You already know how transcription copies a gene's DNA sequence into messenger RNA, and how translation reads that mRNA on a ribosome to assemble a polypeptide chain. The central dogma of molecular biology ties these two processes into a single information pipeline: DNA → RNA → Protein. Think of DNA as a master blueprint locked in a vault (the nucleus), mRNA as a disposable photocopy carried to the factory floor (the ribosome), and the finished protein as the functional machine the cell actually uses. The direction of information flow matters — under normal conditions, information moves from nucleic acid to protein, never backward from protein to nucleic acid.
In eukaryotic cells, the journey from gene to protein involves several processing steps between transcription and translation. The initial transcript, called pre-mRNA, is capped at its 5' end, polyadenylated at its 3' end, and spliced to remove introns. Splicing is not merely housekeeping — alternative splicing allows a single gene to produce multiple different mRNA variants, each encoding a distinct protein isoform. This is how roughly 20,000 human genes can generate over 100,000 different proteins. The processed, mature mRNA is then exported from the nucleus to the cytoplasm, where ribosomes and tRNAs collaborate to translate its codon sequence into an amino acid chain.
Translation itself is tightly orchestrated. Initiation factors help the ribosome find the start codon, elongation factors ensure accurate and rapid amino acid addition, and release factors recognize stop codons to terminate the chain. But the polypeptide emerging from the ribosome is rarely the final product. Post-translational modifications — phosphorylation, glycosylation, acetylation, ubiquitination, and others — act as molecular switches that alter a protein's activity, stability, localization, or interactions. A kinase adding a phosphate group can activate an enzyme; a ubiquitin tag can mark it for destruction. These modifications give the cell fine-grained control over protein function without needing to make new mRNA.
The central dogma is a powerful organizing principle, but it is not absolute. Retroviruses like HIV use reverse transcriptase to copy RNA back into DNA, violating the strict one-way flow. Prions propagate information through protein conformation changes alone. And most genes are not expressed all the time — cells regulate which genes are transcribed, how mRNAs are processed and stabilized, and how efficiently they are translated. A liver cell and a neuron carry identical DNA, yet they express radically different sets of proteins. Understanding gene expression as a regulated pipeline — not an automatic readout — is the key insight that connects this topic to cell differentiation and development downstream.