Gene Expression: Transcription, Translation, and the Central Dogma
The Molecular Basis of Gene Expression
Transcription: Synthesizing RNA from DNA
Transcription is the process where genetic information encoded in DNA is copied into an RNA molecule. This process involves three main stages:
Initiation
The enzyme RNA polymerase binds to the promoter region of the DNA near the beginning of the gene. In different cells, RNA polymerase binds to promoters of different genes, depending on internal and external cellular conditions. The binding of RNA polymerase requires the DNA double helix near the promoter to separate, forming a transcription bubble.
Elongation
The RNA polymerase moves along the template strand of DNA, catalyzing the addition of complementary RNA nucleotides to the growing RNA molecule. As the polymerase passes, the DNA double helix is wound back up a short distance from the point of synthesis.
Termination
Transcription ends when the RNA polymerase reaches a specific DNA sequence called the termination signal. Upon reaching this signal, the RNA polymerase releases both the DNA template and the newly synthesized RNA molecule.
Translation: Decoding mRNA into Polypeptides
Translation occurs in the cytoplasm and is the process where messenger RNA (mRNA) is decoded by ribosomes to produce a specific polypeptide chain, following the rules specified by the genetic code. Ribosomes are composed of small and large subunits that surround the mRNA.
Preparation for Translation: Aminoacylation (tRNA Charging)
Although not technically a step of translation itself, preparation is necessary: the correct amino acid is attached to the corresponding transfer RNA (tRNA). When the tRNA is bound to an amino acid, it is referred to as being "loaded" or aminoacylated.
Initiation
Initiation involves the association of the small and large ribosomal subunits with the mRNA. The loaded initiator tRNA, carrying the first amino acid, pairs its anticodon with the initiation codon (usually AUG) on the mRNA.
Elongation (Lengthening)
A tRNA carrying its respective amino acid (aa) enters the A site (Aminoacyl site). A catalytic chemical reaction, carried out by the enzyme peptidyl transferase, forms a peptide bond between the amino acid in the A site and the growing polypeptide chain held by the tRNA in the P site (Peptidyl site). This process requires energy, supplied by molecules like GTP.
The tRNA in the P site releases its amino acid and moves to the E site (Exit site), while the tRNA now holding the polypeptide chain moves from the A site into the P site. The A site is then empty, ready for the next aminoacyl-tRNA. This cycle repeats, forming an extensive polypeptide chain until the ribosome encounters a stop codon.
Termination
Termination occurs when one of the stop codons enters the A site. These codons are not recognized by any tRNA. Instead, they are recognized by proteins called release factors, which cause the separation of the small and large ribosomal subunits and the release of the completed polypeptide chain into the cytoplasm.
Beadle and Tatum: The One Gene, One Enzyme Hypothesis
The foundational experiments conducted by George Wells Beadle and Edward Lawrie Tatum involved exposing the bread mold, Neurospora crassa, to X-rays to induce mutations. Through a series of experiments published in 1941, they demonstrated that these mutations caused specific changes in enzymes involved in metabolic pathways.
These findings led them to propose a direct link between genes and enzymatic reactions, famously known as the hypothesis: "One gene, one enzyme."