DNA Transcription

Transcription is the process through which a DNA sequence is enzymatically copied by an RNA polymerase to produce a complementary RNA. So to say, it is the transfer of genetic information from DNA into RNA. In the case of protein-encoding DNA, transcription is the beginning of the process that ultimately leads to the translation of the genetic code (via the mRNA intermediate) into a functional peptide or protein. The stretch of DNA that is transcribed into an RNA molecule is called a transcription unit. Transcription has some proofreading mechanisms, but they are fewer and less effective than the controls for copying DNA; therefore, transcription has a lower copying fidelity than DNA replication.[citation needed]
As in DNA replication, transcription proceeds in the 5' → 3' direction (i.e. the old polymer is read in the 3' → 5' direction and the new, complementary fragments are generated in the 5' → 3' direction). Transcription is divided into 3 stages: initiation, elongation and termination.

3 Step of transcription

1. Initiation:Transcription of RNA differs from DNA synthesis in that only one strand of DNA, the template strand, is used to make mRNA. Because transcription only proceeds in the 5' → 3' direction, it follows that the DNA template strand that is used must be oriented in 3' → 5' (complementary) direction. The strand that is not used as a template strand is called the non-template strand or the coding strand. Thus, DNA exists as a double strand, whereas RNA only exists as a single strand. The difference is due to the fact that DNA replication is semi-conservative, while transcription results in de novo production of a single strand of RNA.
   Transcription begins with the binding of RNA polymerase to the promoter in DNA. In prokaryotes, the RNA polymerase is a core enzyme consisting of five subunits: 2 α subunits, 1 β subunit, 1 β' subunit, and 1 ω subunit. At the start of initiation, the core enzyme is associated with a sigma factor (number 70) that aids in finding the appropriate -35 and -10 basepairs downstream of promoter sequences. Transcription initiation is far more complex in eukaryotes, the main difference being that eukaryotic polymerases do not recognize directly their core promoter sequences.
   Unlike DNA replication, transcription does not need a primer to start. The DNA unwinds and produces a small open complex and synthesis begins on only the template strand.

Elongation : Unlike DNA replication, mRNA transcription can involve multiple RNA polymerases, so many mRNA molecules can be produced from a single copy of the gene. This step also involves a proofreading mechanism that can replace an incorrectly added RNA molecule.

Termination : Bacteria use two different strategies for transcription termination: in the "Rho-independent" type, RNA transcription stops when the newly synthesized RNA molecule forms a hairpin loop, followed by a run of Us, which makes it detach from the DNA template. In the "Rho-dependent" type of termination, a protein factor called "Rho" destabilizes the interaction between the template and the mRNA, thus releasing the newly synthesized mRNA from the elongation complex. Transcription termination in eukaryotes is less well understood. It involves cleavage of the nascent transcript, followed by template-independent addition of As at its new 3' end, in a process called Poly-adenylation.

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