The Central Dogma of Molecular Biology describes the flow of genetic information within a biological system. This principle explains how instructions encoded in DNA are converted into the functional components of a cell, primarily proteins. Francis Crick, who co-discovered the structure of DNA, first articulated the dogma in 1958. The core idea is that information passes from nucleic acid to nucleic acid, or from nucleic acid to protein. Crucially, once information is in a protein, it cannot flow back to a nucleic acid.
The Mechanism of DNA Replication
Before a cell can divide, the genetic material must be duplicated through DNA replication (DNA \(\rightarrow\) DNA). This mechanism is described as semi-conservative because each new DNA double helix consists of one original parental strand and one newly synthesized strand. Replication begins when specialized proteins unwind the double helix, separating the two strands at specific points called origins of replication.
The enzyme DNA helicase unwinds the DNA, creating a Y-shaped structure known as the replication fork. Each separated strand serves as a template for constructing a new partner strand. DNA polymerase is the main enzyme that builds the new DNA, moving along the template strand and adding complementary nucleotides.
DNA polymerase only adds new nucleotides in the 5′ to 3′ direction. This restriction means the leading strand is synthesized continuously toward the replication fork. The lagging strand must be synthesized in short fragments, called Okazaki fragments, which are later joined by DNA ligase.
Creating the Messenger: Transcription
Transcription is the process of synthesizing an RNA molecule from a DNA template (DNA \(\rightarrow\) RNA). Since DNA remains protected within the nucleus of eukaryotic cells, genetic instructions must be copied onto a portable messenger molecule.
The main enzyme catalyzing this process is RNA polymerase, which binds to a specific DNA region called the promoter. The polymerase unwinds a small section of the double helix and reads the nucleotide sequence on the template strand.
As the enzyme moves along, it incorporates complementary ribonucleotides to form a growing RNA chain. This process is divided into three stages: initiation, elongation, and termination. The primary product is messenger RNA (mRNA), which carries instructions from the DNA to the cell’s protein synthesis machinery.
Transcription also produces functional RNA types, including transfer RNA (tRNA) and ribosomal RNA (rRNA). These molecules play structural and regulatory roles in protein assembly. Once RNA polymerase reaches a termination signal, the newly formed RNA transcript is released.
Building the Protein: Translation
Translation is the process where the genetic code carried by mRNA is decoded to synthesize a specific chain of amino acids, which folds into a functional protein (RNA \(\rightarrow\) Protein). This intricate assembly process occurs on ribosomes, complex structures made of protein and ribosomal RNA that function as the cell’s protein synthesis machinery. The mRNA sequence is read in sequential groups of three nucleotides, each group called a codon.
Each codon specifies a particular amino acid, forming the genetic code that dictates the protein’s structure. Transfer RNAs (tRNAs) are the adaptor molecules, having a specific amino acid attached to one end and a complementary anticodon sequence at the other. The ribosome moves along the mRNA, reading the codons sequentially.
As the ribosome reads a codon, the matching tRNA enters the active site, carrying its amino acid. The ribosome catalyzes the formation of a peptide bond between the incoming amino acid and the growing polypeptide chain. This cycle of binding and movement, known as elongation, continues for hundreds or thousands of steps.
Synthesis stops when the ribosome encounters one of three specific stop codons on the mRNA. Release factors bind to the ribosome, causing the newly synthesized polypeptide chain to detach. The finished protein then folds into its three-dimensional structure, determining its specific function.
How the Central Dogma is Modified
While the classic flow of DNA to RNA to protein represents the primary pathway of information in most organisms, discoveries have shown that the flow is not strictly unidirectional. The most recognized modification involves reverse transcription, a process where genetic information moves from RNA back to DNA (RNA \(\rightarrow\) DNA).
This occurs in retroviruses, such as the human immunodeficiency virus (HIV), which use the enzyme reverse transcriptase. This enzyme converts their RNA genome into a DNA copy that integrates into the host cell’s chromosome.
Other exceptions involve RNA replication (RNA \(\rightarrow\) RNA), seen in viruses like influenza and coronaviruses. These viruses utilize an RNA-dependent RNA polymerase to synthesize new RNA molecules, bypassing the DNA stage entirely for replication.
A more radical departure is seen in prions, infectious agents composed entirely of misfolded protein. Prions replicate by inducing normal versions of the same protein to change into the infectious form (Protein \(\rightarrow\) Protein). This mechanism of protein-only inheritance transmits biological information without DNA or RNA, challenging the original constraint that information cannot flow from protein to nucleic acid.