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What is transcription?
Transcription In general, transcription is the process of synthesizing a sequence of mRNA from a complementary template strand of DNA which is read from the 3' → 5' directio…n but synthesized in the 5' → 3'. It takes place in the nucleus of the cell in eukaryotes and the cytoplasm or nucleoid (if present) in prokaryotes. A more detailed explanation can be found below including its three stages: initiation, elongation, and termination. Initiation The enzyme that does the most work in transcription is RNA polymerase, which is similar to DNA polymerase in DNA replication. RNA polymerase first attaches to the promoter region of the DNA strand, a few nucleotides downstream from the starting point. In eukaryotes, RNA polymerase will bind to a "TATA box," which is a sequence (TATA) of nitrogen bases on the promoter of the template strand. Eukaryotes also require "transcription factors" for RNA polymerase to bind to the promoter. The transcription factors are various proteins which aid the binding through protein-protein interactions. These transcription factors become important in the expression of genes due to their presence or lack of presence; control elements and associated activators also play a role. When the transcription factors catalyze the binding of RNA polymerase to the DNA, the RNA polymerase and transcription factors form what is called the transcription initiation complex. Elongation The RNA polymerase then proceeds to unwind the DNA and form a RNA transcript from the DNA template strand in the 5' → 3' direction. RNA nucleotides are paired to the template strand and is ligated, then released at a rate of 40 nucleotides per second in eukaryotes. Many polymerases may be transcribing one gene at one time. After RNA polymerase moves on, DNA recoils into its natural double helical shape. Termination In bacteria, RNA polymerase stops transcription when it passes through the termination sequence. The resulting RNA transcribed acts as a termination signal which detaches the RNA strand. In eukaryotes, RNA transcribes a termination sequence called the polyadenylation signal sequence (AAUAAA). Various proteins aid in cutting the pre-mRNA (hnRNA) off the template strand, though the polymerase actually continues transcribing for a few hundred nucleotides downstream. The RNA produced is degraded by an enzyme which follows up to the polymerase, releasing it from the strand. (MORE)
