The central dogma of molecular biology describes how genetic information flows within a biological system. Proposed by Francis Crick in 1958, this concept outlines how instructions encoded in our genes build a cell’s functional components. Information moves from DNA to RNA, and then from RNA to protein.
DNA Copying
Before cell division, DNA must be accurately copied. This process, DNA replication, ensures each new daughter cell receives a complete and identical set of genetic instructions. DNA exists as a double helix, resembling a twisted ladder, where two strands are linked by specific pairs of chemical bases.
During replication, the two strands of the DNA double helix unwind and separate. Each separated strand serves as a template for building a new complementary strand. Enzymes like helicase help unwind the DNA, creating a Y-shaped replication fork. DNA polymerase adds new matching nucleotides to each template, forming two new DNA molecules. This method is termed semi-conservative replication because half of the original molecule is conserved in each new copy.
From DNA to RNA
Transcription copies a specific DNA segment into an RNA molecule. This process is essential because DNA, the master genetic code, remains protected in the nucleus, while proteins are assembled outside. Messenger RNA (mRNA) acts as an intermediary, carrying the genetic message from DNA to the protein-making machinery.
Transcription begins when RNA polymerase binds to a gene’s promoter region on the DNA. This binding signals the DNA to unwind, exposing the sequence of bases to be copied. RNA polymerase then reads one DNA strand, the template strand, and synthesizes a complementary RNA molecule. As it moves along the DNA, it adds ribonucleotides, creating a growing mRNA strand. This process copies the DNA’s information into an RNA message, which detaches from the DNA once the gene’s end is reached.
From RNA to Protein
After transcription, the mRNA molecule carries genetic instructions out of the nucleus to the ribosomes, cellular structures responsible for protein synthesis. Translation converts the mRNA nucleotide sequence into a protein’s amino acid sequence. Proteins are the cell’s functional workhorses, performing tasks from catalyzing reactions to providing structural support.
Translation involves several components, including mRNA, ribosomes, and transfer RNA (tRNA) molecules. The mRNA sequence is read in groups of three nucleotides, called codons, each specifying a particular amino acid. Ribosomes act as protein synthesis factories, moving along the mRNA strand and facilitating the interaction between mRNA codons and tRNA molecules. Each tRNA molecule has a specific anticodon that base-pairs with a complementary mRNA codon and carries the corresponding amino acid. As the ribosome moves, tRNAs deliver their amino acids in the correct sequence, forming a chain that grows longer with each addition.
Expanding the Dogma
The central dogma provides a framework for understanding genetic information flow, but scientific discoveries have revealed exceptions and refinements to Crick’s original formulation. One exception is reverse transcription, where genetic information flows from RNA back to DNA. This is observed in retroviruses, such as the human immunodeficiency virus (HIV).
Retroviruses possess reverse transcriptase, which allows them to use their RNA genome as a template to synthesize a DNA copy. This newly synthesized viral DNA can integrate into the host cell’s DNA, becoming a permanent part of its genetic material and enabling the virus to replicate. Another exception involves RNA replication, seen in some RNA viruses, where RNA is directly copied into new RNA molecules without a DNA intermediate. These viruses utilize an RNA-dependent RNA polymerase to replicate their genomes. These pathways demonstrate the flexibility of genetic information transfer, particularly in viruses, without invalidating the central dogma’s general principles in most cellular processes.