Transcription is the name given to the chemical synthesis of RNA from a DNA template. In other words, DNA is transcribed in order to make RNA, which is then decoded to produce proteins.



Transcription is the first stage of the expression of genes into proteins. In transcription, a mRNA (messenger RNA) intermediate is transcribed from one of the strands of the DNA molecule. The RNA is called messenger RNA because it carries the 'message' or genetic information from the DNA to the ribosomes, where the information is used to make proteins. RNA and DNA use complementary coding, where base pairs match up, similar to how the strands of DNA bind to form a double helix. One difference between DNA and RNA is that RNA uses uracil in place of the thymine used in DNA. RNA polymerase mediates the manufacture of an RNA strand that complements the DNA strand. RNA is synthesized in the 5' -> 3' direction (as seen from the growing RNA transcript). There are some proofreading mechanisms for transcription, but not as many as for DNA replication. Sometimes coding errors occur.

Comparison of Transcription in Prokaryotes Versus Eukaryotes
There are significant differences in the process of transcription in prokaryotes versus eukaryotes.
In prokaryotes (bacteria), transcription occurs in the cytoplasm. Translation of the mRNA into proteins also occurs in the cytoplasm. In eukaryotes, transcription occurs in the cell's nucleus. mRNA then moves to the cytoplasm for translation.
DNA in prokaryotes is much more accessible to RNA polymerase than DNA in eukaryotes. Eukaryotic DNA is wrapped around proteins called histones to form structures called nucleosomes. Eukaryotic DNA is packed to form chromatin. While RNA polymerase interacts directly with prokaryotic DNA, other proteins mediate the interation between RNA polymerase and DNA in eukaryotes.
mRNA produced as a result of transcription is not modified in prokaryotic cells. Eukaryotic cells modify mRNA by RNA splicing, 5' end capping, and addition of a polyA tail.


Steps of Transcription
Transcription may be broken into five stages: pre-initiation, initiation, promoter clearance, elongation and termination

Transcription - Pre-Initiation
The first step of transcription is called pre-initiation. RNA polymerase and cofactors bind to DNA and unwind it, creating an initiation bubble. This is a space that grants RNA polymerase access to a single strand of the DNA molecule.

Transcription - Initiation
The initiation of transcription in bacteria begins with the binding of RNA polymerase to the promoter in DNA. Transcription initiation is more complex in eukaryotes, where a group of proteins called transcription factors mediate the binding of RNA polymerase and the initiation of transcription.

Transcription - Promoter Clearance
RNA polymerase must clear the promoter once the first bond has been synthesized. Approximately 23 nucleotides must be synthesized before RNA polymerase loses its tendency to slip away and prematurely release the RNA transcript

Transcription - Elongation
One strand of DNA serves as the template for RNA synthesis, but multiple rounds of transcription may occur so that many copies of a gene may be produced.

Transcription - Termination
Termination is the final step of transcription. Termination results in the release of the newly synthesized mRNA from the elongation complex.