1. Initiation:
- RNA polymerase, along with other transcription factors, binds to a specific region of DNA called the promoter, located upstream of the gene to be transcribed.
- The RNA polymerase unwinds the DNA double helix, creating a transcription bubble.
- Nucleotides in the form of ribonucleotides (ATP, GTP, UTP, and CTP) start to pair with the exposed DNA template strand based on complementary base pairing (A with U, G with C, etc.).
2. Elongation:
- RNA polymerase adds ribonucleotides one by one to the growing RNA chain, forming phosphodiester bonds.
- The RNA molecule elongates in a 5' to 3' direction, synthesizing a complementary copy of the DNA template strand.
- As the RNA polymerase moves along the DNA template, the transcription bubble continues to unwind ahead of it, and the DNA behind it reanneals.
3. Termination:
- Transcription proceeds until a specific termination signal is reached on the DNA template. Termination signals can be sequences of nucleotides (termination sequences) or structures (hairpin loops).
- Once the termination signal is encountered, RNA polymerase dissociates from the DNA template, and the RNA transcript is released.
- The RNA molecule undergoes further processing, such as addition of a 5' cap, removal of introns (non-coding regions), and addition of a poly-A tail at the 3' end, to become a mature messenger RNA (mRNA).
The resulting mRNA transcript carries the genetic information from the DNA sequence and serves as a template for protein synthesis during the process of translation. Transcription is a crucial step in gene expression, enabling the conversion of genetic information encoded in DNA to functional RNA molecules.
