Transcription
Transcription is defined as the transfer of genetic information from deoxyribonucleic acid (DNA) to ribonucleic acid (RNA). The process of transcription in prokaryotic cells (e.g., bacteria) differs from the process in eukaryotic cells (cells with a true nucleus) but the underlying result of both transcription processes is the same, which is to provide a template for the formation of proteins.
The use of DNA as a blueprint to manufacture RNA begins with an enzyme called RNA polymerase. The enzyme is guided to a certain region on the DNA, called the promoter, by association with molecules known as sigma factors. There are many promoters on DNA, located just before a region of DNA that codes for a protein. The promoter serves to position the RNA polymerase so that transcription of the full coding region is accomplished.
Once the polymerase has bound to a promoter, the sigma factors detach and can serve another polymerase. The attached polymerase then begins to move along the DNA, unwinding the two strands of DNA that are linked together and using the sequence on one of the strands as the blueprint for RNA manufacture. The strand from which RNA is made is known as the template or the antisense strand, while the other strand to which it is complimentary is called the sense or the coding strand.
As the polymerase moves along the DNA, the strands link back together behind the polymerase. The effect is somewhat similar to a zipper with a bulge, where the two links of the zipper have come apart. The bulging region can move along the zipper, with separation and reannealing of the strands occurring continuously with time. The promoter can accommodate the binding of another polymerase as soon as the region is free. Thus, the same stretch of DNA can be undergoing several rounds of transcription at the same end, with polymerase molecules positioned all along the DNA.
The RNA that is produced is known as messenger RNA (or mRNA). The species derives its name from its function. It is the tangible form of the message that is encoded in the DNA. The mRNA in turn functions as a template for the next step in the genetic process, that of translation. In translation the mRNA information is used to manufacture protein.
Termination of transcription occurs when the RNA polymerase reaches a signal on the DNA template strand that signals the polymerase to stop and to end the association with the DNA.
Some microorganisms have variations on the basic transcription mechanism. For example, in yeast cells the mRNA can be "capped" by the addition of specialized pieces of nucleic acid called telomeres to either end of the transcribed molecule. The telomeres function to extend the life of the mRNA and provide a signal of the importance of the information contained within.
The intricate and coordinated transcription process in bacteria is also a rapid process. For example, measurements in Escherichia coli have established that the RNA polymerase moves along the DNA at a speed of 50 nucleotides per second.
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