Transcription is the process in which a segment of DNA is copied into a newly synthesized strand of messenger RNA with the use of enzyme RNA polymerase. Both prokaryotes and eukaryotes perform fundamentally the same process of transcription, an important difference is that in eukaryotes transcription occurs in the nucleus, and mRNA is transported to the cytoplasm whereas in the case of prokaryotes transcription occurs in the cytoplasm.
RNA POLYMERASE
RNA polymerase is the enzyme that produces the mRNA molecule. Prokaryotes use the same RNA polymerase to transcribe all of their genes. It is composed of 5 polypeptide subunits i.e α,α,β,β`,σ. These subunits assemble every time a gene is transcribed and disassemble once the transcription is complete.
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INITIATION
- Transcription requires the DNA double helix to partially unwind in the region of mRNA synthesis. The region of unwinding is called a transcription bubble.
- The DNA sequence onto which the proteins and enzymes involved in transcription bind initiate the process called a promoter.
- One important sequence in the prokaryotic promoter is located 10 bases before the transcription start site and is commonly called the TATA box.
- To begin transcription, RNA polymerase holoenzyme assembles at the promoter. Dissociation of σ allows the core enzyme to proceed along the DNA template, synthesizing mRNA by adding nucleotides, in the 3’ to 5’ direction.
- Only one of the 2 DNA strands is transcribed, known as the template strand because it is the template for mRNA production. Another strand is called the non-template strand.
ELONGATION
- As elongation proceeds, the DNA is continuously unwound ahead of the core enzyme as the hydrogen bonds that connect the complementary base pairs in the DNA double helix are broken.
- The DNA is rewound behind the core enzyme as the hydrogen bonds are reformed. The base pairing between DNA and RNA is not stable enough to maintain the stability of the mRNA synthesis components. Instead, the RNA polymerase acts as a stable linker between the DNA template and the newly forming RNA strand to ensure that elongation is not interrupted prematurely.
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TERMINATION
- Once a gene is transcribed, the RNA polymerase needs to be instructed to dissociate from the DNA template and liberate the newly made mRNA.
- Depending on the gene being transcribed, there are two kinds of termination signals. One is protein-based and the other is RNA-based. Both termination signals rely on specific sequences of DNA near the end of the gene that causes the polymerase to release the mRNA.
- In a prokaryotic cell, by the time transcription ends, the transcript would already have been used to begin making copies of the encoded protein because the processes of transcription and translation can occur at the same time since both occur in the cytoplasm. In contrast, transcription and translation cannot occur simultaneously in eukaryotic cells since transcription occurs inside the nucleus and translation occurs outside in the cytoplasm.
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