The central dogma of molecular biology is the foundational concept that describes the flow of genetic information within a living organism. It explains how hereditary material is converted into the functional components of a cell. This principle was first proposed by Francis Crick in 1958, establishing a directional pathway for the transfer of biological sequence information. Crick stated that once this information has passed into a protein, it cannot flow back to the nucleic acids, defining the basic mechanism of heredity and cellular function. The central dogma remains a unifying concept in biology, providing the framework for understanding how the instructions for life are read and executed.
Transcription: DNA to Messenger RNA
The first step in expressing a gene’s information is transcription, the creation of a working copy. This initial stage takes place within the nucleus of eukaryotic cells. The goal of transcription is to copy the sequence of a single gene from the DNA molecule into a mobile, single-stranded molecule known as messenger RNA (mRNA).
The copying is performed by the enzyme RNA polymerase, which is recruited to the beginning of a gene sequence by specific regulatory proteins. RNA polymerase unzips a small section of the double-stranded DNA helix, exposing the individual nucleotide bases. It then moves along one of the exposed DNA strands, called the template strand, reading its sequence.
As it reads the template, RNA polymerase synthesizes a complementary strand of RNA, using the DNA as a guide. The mRNA molecule acts as an intermediary between the genetic archives in the nucleus and the protein-building machinery outside of it. Once the gene sequence is fully copied, the RNA polymerase encounters a termination signal, releasing the completed messenger RNA transcript.
Translation: Messenger RNA to Protein
The second major step is translation, where the genetic code carried by the mRNA is converted into a chain of amino acids that will fold into a functional protein. This assembly process occurs in the cytoplasm on complex molecular machines called ribosomes. Ribosomes are composed of ribosomal RNA and various proteins, providing the physical workbench for synthesis.
The mRNA molecule threads through the ribosome, where its sequence of nucleotides is read in three-base segments called codons. Each codon specifies a particular amino acid, forming the basis of the genetic code. Translation begins when the ribosome recognizes a start codon on the mRNA, signaling the arrival of the first specialized transfer RNA (tRNA) molecule.
Transfer RNA molecules act as molecular adaptors, each carrying a specific amino acid and possessing a three-base anticodon loop. The tRNA’s anticodon must match and bind to the complementary codon on the mRNA, ensuring the correct amino acid is brought into position. As the ribosome moves along the mRNA, it joins the amino acids delivered by successive tRNAs into a growing polypeptide chain. This process continues until the ribosome encounters a stop codon, releasing the completed amino acid chain. The newly synthesized polypeptide then folds into a precise three-dimensional structure, transforming it into an active, functional protein.
Exceptions to the Traditional Flow
While the directional flow from DNA to RNA to protein defines the core of the central dogma, subsequent discoveries revealed pathways that modify the original model. The most notable modification is reverse transcription, which involves the transfer of information from RNA back into DNA. This reversal of the typical flow is characteristic of certain viruses, particularly retroviruses like the Human Immunodeficiency Virus (HIV).
Retroviruses carry their genetic material as RNA and use the specialized enzyme reverse transcriptase to synthesize a DNA copy from the viral RNA template after infecting a host cell. This viral DNA can then integrate into the host cell’s genome, allowing the viral genes to be expressed. Another exception involves RNA replication, a process seen in some RNA viruses that use an RNA-dependent RNA polymerase enzyme to directly copy their RNA genome into new RNA molecules without a DNA intermediate. These non-traditional pathways demonstrate that the flow of genetic information is more flexible in the context of viral biology.