Transcription in prokaryotes
Transcription is the process by which DNA is copied into RNA. In prokaryotes, transcription occurs in the cytoplasm, where the DNA is located and is carried out by a single RNA polymerase enzyme. The process of transcription in prokaryotes can be divided into three stages: initiation, elongation, and termination.
Initiation: During initiation, the RNA polymerase enzyme binds to a specific site on the DNA called the promoter region. The promoter region is usually located just upstream of the gene that is being transcribed and contains specific sequences that are recognized by the RNA polymerase enzyme. Once the RNA polymerase has bound to the promoter region, it begins to unwind the DNA double helix to expose the template strand.
Elongation: Once the RNA polymerase has bound to the promoter region and unwound the DNA, it begins to add nucleotides to the growing RNA strand. The RNA polymerase reads the template strand of DNA in the 3′ to 5′ direction and synthesizes the RNA strand in the 5′ to 3′ direction. As the RNA polymerase moves along the DNA, it continues to unwind the double helix in front of it and re-form the double helix behind it. This process of RNA synthesis continues until the RNA polymerase reaches a termination sequence.
Termination: When the RNA polymerase reaches the termination sequence, it stops adding nucleotides to the RNA strand and releases the RNA molecule. In prokaryotes, there are two types of termination sequences: intrinsic termination and rho-dependent termination. Intrinsic termination occurs when the RNA polymerase reaches a sequence on the DNA that forms a hairpin loop in the RNA molecule, causing the RNA polymerase to pause and fall off the DNA. Rho-dependent termination requires a protein called Rho factor, which binds to the RNA molecule and causes the RNA polymerase to pause and release the RNA molecule.
After transcription is complete, the RNA molecule undergoes further processing, including the addition of a 5′ cap and a poly(A) tail, as well as splicing to remove introns in eukaryotes. The resulting mRNA molecule is then transported out of the nucleus and into the cytoplasm, where it serves as a template for protein synthesis.
Transcription in eukaryotes
Transcription in eukaryotes is similar to prokaryotes in many respects, but there are some important differences in the process. In eukaryotes, transcription occurs in the nucleus, where the DNA is located and is carried out by three different RNA polymerases: RNA polymerase I, RNA polymerase II, and RNA polymerase III. Each of these polymerases is responsible for transcribing different types of RNA molecules.
Initiation: The process of transcription in eukaryotes begins with the binding of transcription factors to specific DNA sequences called promoters. These transcription factors recruit RNA polymerase II to the promoter region, where it forms a transcription initiation complex with several other proteins. The initiation complex then unwinds the DNA double helix to expose the template strand.
Elongation: Once the RNA polymerase II has bound to the promoter region and unwound the DNA, it begins to add nucleotides to the growing RNA strand. As the RNA polymerase moves along the DNA, it continues to unwind the double helix in front of it and re-form the double helix behind it. RNA polymerase II also undergoes a process of phosphorylation, which allows it to elongate the RNA molecule.
Termination: When RNA polymerase II reaches the end of the gene being transcribed, it continues to transcribe for several hundred nucleotides beyond the end of the gene. This process is called transcriptional read-through and results in the synthesis of a long RNA molecule called a pre-mRNA. The pre-mRNA undergoes further processing, including the addition of a 5′ cap and a poly(A) tail, as well as splicing to remove introns.
After processing, the mature mRNA molecule is transported out of the nucleus and into the cytoplasm, where it serves as a template for protein synthesis.
In summary, transcription in eukaryotes is a highly regulated process that involves the recruitment of RNA polymerase II to the promoter region of a gene, elongation of the RNA molecule, and the processing of the pre-mRNA molecule to produce a mature mRNA molecule that can be transported out of the nucleus for translation.