Transcription is the process by which genetic information encoded in DNA is copied into RNA, which serves as a template for protein synthesis. Transcription can be divided into three main stages: initiation, elongation, and termination.
- Initiation: The first step in transcription is initiation, during which RNA polymerase, the enzyme responsible for synthesizing RNA, binds to a specific DNA sequence called the promoter. The promoter is located near the beginning of the gene that is to be transcribed and serves as a recognition site for the RNA polymerase. Once RNA polymerase is bound to the promoter, it begins to unwind the double-stranded DNA and create a transcription bubble.
- Elongation: The second stage of transcription is elongation, during which RNA polymerase moves along the DNA template strand, synthesizing a complementary RNA strand in the 5′ to 3′ direction. As RNA polymerase moves along the DNA, it unwinds the double helix and exposes the template strand. The exposed template strand is used as a template for RNA synthesis, with RNA polymerase adding nucleotides to the growing RNA chain according to the base-pairing rules of A-U and G-C. As the RNA polymerase moves along the DNA, the DNA double helix re-forms behind it.
- Termination: The final stage of transcription is termination, during which RNA polymerase reaches the end of the gene and the newly synthesized RNA molecule is released. In bacteria, termination occurs when RNA polymerase encounters a specific DNA sequence called the terminator. The terminator causes RNA polymerase to detach from the DNA template and release the newly synthesized RNA molecule. In eukaryotes, termination is more complex and involves the addition of a poly(A) tail to the 3′ end of the RNA molecule and the cleavage of the RNA from the DNA template.
In addition to the three main stages of transcription, there are several other factors that can affect the efficiency and accuracy of the process. These include regulatory proteins that bind to specific DNA sequences and control the rate of transcription, RNA processing factors that modify the RNA molecule after transcription, and chromatin structure that can either facilitate or inhibit access to the DNA template. Together, these factors ensure that the process of transcription is tightly regulated and responsive to the needs of the cell.