Transcription
Transcrition is the synthesis of RNA from DNA. It is the first step of gene expression. It occurs in G0, G1 and G2 phases of cell cycle. Unlike DNA replication, it requires only one enzyme to be completed i.e., Rna Polymerase. However, it is a continuous process; for convenience we can divide it.
Initiation phase
Transcription begins with the binding of RNA Polymerase at promotor region. In prokaryotes, there are two binding sites are located in promotor i.e., TATAAT also called -10 sequence and TTGACA also called -35 sequence, whereas in eukaryotes, TATA (TATA box) also called -25 sequence and CAAT (CAAT box) also called -70 sequence. Names of these sequences (-10, -35 or -25, -70) refer to position that these sequences are located before the initiation site of structural region of the gene.
RNA polymerase consists of four subunits : alpha, beta, beta' and sigma; only the first three subunits are required for polymerase activity and are considered the core enzyme while the sigma factor is required for RNA polymerase to bind to the promotor. It is similar to the DNA polymerase in that it also adds nucleotides to the 3' end of the growing polypeptide chain but unlike DNA polymerase it does not require primer to perform polymerase activity. In prokaryotes, only one type of RNA polymerase is found while in eukaryotes, there three types of RNA polymerases, namely RNA polymerase-I, which synthesize rRNA, RNA polymerase-II, which synthesize mRNA, RNA polymerase-III which synthesize tRNA.
As the Rna Polymerase Binds to the promoter, DNA duplex become unwind, base pairs are broken down, and a bubble like structure, the transcription bubble is appeared.
Elongation Phase
As the RNA polymerase binds to promoter, sigma factor is released and remaining core enzyme extends the polymerization of ribonucleoside triphosphate (rNTP). It does not require primer to initiate polymerization. One of the two strand of the gene acts as template for transcription. This template strand is also called antisense because mRNA is complementary to this strand. The other strand of the gene is called coding or sense strand. In elongation phase, RNA polymerase keep on moving from 5' to 3' direction towards the terminator region, beside it transcription bubble also moves along the DNA, leaving the growing RNA strand protruding from the bubble. This event continues till the RNA polymerase reaches the terminator region of the gene.
Termination Phase
The sequence of terminator region of the gene stops the synthesis of mRNA. The terminator region consists of a series of GC base pairs followed by a series of AT base pairs. The part of mRNA which is transcribed in this region, projects to form a loop like structure called GC hairpin followed by a small tail of poly U nucleotides. The GC hairpin causes the RNA polymerase to stop the synthesis of RNA.
