Transcription is the process by which genetic information flows from DNA to RNA, representing the initial step in gene expression. This molecular event is a central tenet of the “central dogma” of molecular biology, outlining how genetic information moves from DNA to RNA, and finally to proteins. This flow of information is observed across all forms of life, from bacteria to complex organisms.
The Essential Players
Transcription relies on several molecular components. At its core is the DNA molecule, specifically a gene, which contains instructions for building a protein or functional RNA molecule. Within this gene, one strand serves as the template strand, guiding the synthesis of the new RNA molecule. RNA polymerase is the enzyme that constructs the RNA strand using the DNA template.
Ribonucleotide building blocks are also necessary for this synthesis, including adenine (A), uracil (U), guanine (G), and cytosine (C). These molecules are the individual units that form the RNA chain. Specific DNA sequences, such as promoter regions, indicate where transcription should begin. Conversely, terminator sequences signal RNA polymerase to stop the transcription process.
Unpacking the Transcription Process
Transcription unfolds in a three-phase process: initiation, elongation, and termination. Initiation begins when RNA polymerase recognizes and binds to a specific DNA sequence called the promoter. This binding positions the enzyme and causes the DNA double helix to unwind, separating the two DNA strands and exposing the template strand.
Once the DNA strands are separated, the process moves into elongation. RNA polymerase moves along the template DNA strand, reading its nucleotide sequence in the 3′ to 5′ direction. As it moves, the polymerase synthesizes a complementary RNA molecule by adding ribonucleotides to the growing RNA chain in the 5′ to 3′ direction. This synthesis follows complementary base pairing: adenine (A) in DNA pairs with uracil (U) in RNA, thymine (T) in DNA pairs with adenine (A) in RNA, guanine (G) in DNA pairs with cytosine (C) in RNA, and cytosine (C) in DNA pairs with guanine (G) in RNA.
The final phase, termination, occurs when RNA polymerase encounters a specific terminator sequence on the DNA. This sequence signals the RNA polymerase to halt its movement and detach from the DNA template. The newly synthesized RNA molecule is then released from the enzyme and the DNA. In eukaryotes, this RNA transcript often undergoes further processing to become a mature, functional RNA molecule.
The RNA Outcome
After transcription, the newly synthesized RNA molecule is released. For protein-coding genes, this RNA molecule is known as messenger RNA (mRNA). Messenger RNA carries genetic instructions copied from DNA, acting as an intermediate message. In eukaryotic cells, mRNA travels from the nucleus, where transcription occurs, to the cytoplasm, where ribosomes are located.
Once in the cytoplasm, mRNA serves as a template for protein synthesis, a process called translation. Ribosomes read the genetic code on the mRNA to assemble amino acids into a specific protein. Transcription enables the genetic information stored in DNA to be expressed as functional proteins that carry out diverse roles within the cell.