The Central Dogma

The central dogma describes how genetic information flows from DNA to proteins. DNA is transcribed into messenger RNA (mRNA), which is then translated by ribosomes into chains of amino acids called polypeptides. These polypeptides fold into functional proteins that perform cellular functions. This process explains how genetic instructions stored in DNA ultimately direct protein synthesis.

During transcription, DNA serves as a template to synthesize mRNA in the nucleus. The mRNA sequence mirrors the DNA coding strand, except thymine nucleotides are replaced with uracil. In eukaryotes, the primary transcript is processed by removing non-coding regions, capping the 5' end, and adding a 3' poly-A tail to create mature mRNA, which is then exported to the cytoplasm for translation.

Codons are three-nucleotide sequences on mRNA that specify which amino acid should be added during translation. Transfer RNA (tRNA) molecules carry specific amino acids and bind to complementary anticodon sequences on mRNA. For example, the codon CCA binds to tRNA carrying proline, while AGC binds to tRNA carrying serine. This codon-anticodon pairing ensures amino acids are assembled in the correct order.

The genetic code is redundant because 64 possible codons encode only 20 amino acids in eukaryotes. Multiple codons can specify the same amino acid, often differing only in the third nucleotide position. For instance, GUU, GUC, GUA, and GUG all code for valine. This redundancy minimizes harmful effects of mutations, since changes at the third codon position may not alter the amino acid or protein function.

With few exceptions, most prokaryotic and eukaryotic organisms use the same genetic code for protein synthesis. This universality enables biotechnology applications like recombinant DNA technology, where human insulin genes are inserted into bacterial cells. The bacteria then perform transcription and translation to produce human insulin protein, which can treat diabetes. This demonstrates that genetic instructions are readable across different species.

Early scientists knew DNA stores genetic information and proteins perform cellular functions, but the connection remained unclear. Two key observations in eukaryotes prompted the search for an intermediary: protein synthesis occurs in the cytoplasm, not the nucleus, and DNA cannot leave the nucleus. RNA was identified as this missing link, synthesized in the nucleus from DNA and exported to the cytoplasm where it directs protein production.

The start codon AUG signals where protein synthesis begins on mRNA. Unlike most codons that can specify multiple amino acids, AUG uniquely codes for methionine and marks the initiation point for translation. This codon ensures ribosomes begin reading the genetic code at the correct position, establishing the proper reading frame for accurate amino acid assembly throughout the entire polypeptide chain.