The central dogma of molecular biology describes the flow of genetic information within a biological system. It explains how the instructions encoded in DNA are converted into functional products, primarily proteins, which carry out most cellular activities. This principle outlines a core mechanism by which life’s blueprint is put into action, ensuring that genetic information is expressed to build and maintain an organism.
The Molecular Players
The central dogma involves three main types of molecules: DNA, RNA, and proteins. Deoxyribonucleic acid (DNA) serves as the genetic blueprint for an organism, storing instructions needed for its development, survival, and reproduction. DNA is found in the nucleus of eukaryotic cells, organized into chromosomes, and contains genes that specify the sequences for proteins.
Ribonucleic acid (RNA) acts as an intermediate messenger, carrying specific genetic instructions from DNA to the sites of protein synthesis. This molecule is crucial because it can move out of the nucleus in eukaryotic cells, unlike DNA, making the information accessible for protein production. Proteins are the functional workhorses of the cell, performing diverse tasks, including catalyzing metabolic reactions, providing structural support, transporting molecules, and responding to signals. Their diverse structures and functions are determined by the specific sequence of amino acids, dictated by the genetic information.
Information Transfer: Transcription
Transcription is the initial step in gene expression, where the genetic information stored in a segment of DNA is copied into a messenger RNA (mRNA) molecule. This process creates a portable RNA copy of a gene for protein synthesis. During transcription, the DNA double helix unwinds, exposing the nucleotide sequence of the gene.
An enzyme called RNA polymerase plays a central role, binding to a specific region on the DNA called the promoter, which signals the start of a gene. RNA polymerase then moves along one of the DNA strands, known as the template strand, reading its nucleotide sequence. As it reads, the enzyme synthesizes a complementary RNA strand by adding individual ribonucleotides. This process follows specific base-pairing rules: adenine (A) in DNA pairs with uracil (U) in RNA, while guanine (G) pairs with cytosine (C). The RNA strand grows in a 5′ to 3′ direction until the RNA polymerase encounters a termination sequence, signaling the end of the gene and releasing the mRNA molecule.
Information Transfer: Translation
Translation is the next major step in gene expression, where the genetic information carried by the messenger RNA (mRNA) molecule is decoded to synthesize a specific protein. This process converts the nucleotide language of mRNA into the amino acid language of proteins, the functional components of the cell. Translation occurs in the cytoplasm on structures called ribosomes, composed of ribosomal RNA (rRNA) and proteins.
The mRNA sequence is read in sequential sets of three nucleotides, known as codons. Each codon specifies a particular amino acid or acts as a stop signal. Transfer RNA (tRNA) molecules are crucial for this decoding; each tRNA carries a specific amino acid and has an anticodon that base-pairs with a complementary codon on the mRNA. The ribosome moves along the mRNA, facilitating binding of the correct tRNA to each codon. As amino acids are brought into position, peptide bonds form, creating a growing polypeptide chain. This process continues until a stop codon is reached, releasing the completed polypeptide chain from the ribosome, ready to fold into its functional three-dimensional structure.
Beyond the Standard Flow
While the central dogma describes the flow of information from DNA to RNA to protein, biological systems exhibit exceptions and extensions. One significant deviation is reverse transcription, observed in certain viruses, such as retroviruses like HIV. These viruses utilize an enzyme called reverse transcriptase to synthesize DNA from an RNA template, reversing the usual flow of genetic information. This allows the viral genetic material, initially RNA, to integrate into the host cell’s DNA.
Another extension involves RNA molecules themselves, which can have diverse functions beyond carrying genetic messages or acting as ribosomal and transfer RNAs. Some RNA molecules can regulate gene expression, modify other nucleic acids, or act as enzymatic catalysts. These instances highlight that while the central dogma provides a foundational understanding of information flow, biological processes are dynamic and can involve alternative pathways for genetic information transfer and utilization.