The process of formation of RNA from DNA template.
The ﬁrst step of information transfer from gene to protein is to produce an RNA strand whose base sequence matches the base sequence of a DNA segment, sometimes followed by modiﬁcation of that RNA to prepare it for its speciﬁc cellular roles.
Hence RNA is produced by a process that copies the nucleotide sequence of DNA. Because this process is reminiscent of transcribing (copying) written words, the synthesis of RNA is called transcription.
The DNA is said to be transcribed into RNA,and the RNA is called a transcript.
First, the two strands of the DNA double helix separate locally, and one of the separated strands acts as a template for RNA synthesis. In the chromosome overall, both DNA strands are used as templates; but, in any one gene, only one strand is used, and, in that gene, it is always the same strand.
the non template strand is called as coding strand because the mRNA is just like it except T on DNA is replaced by U in mRNA.
It is a short sequence of nucleotides on DNA upstream from the gene.
Each gene has two sites; one is present towards left side of gene, called promoter or promoter gene area, while other is present towards right side of the gene and is called structural gene area, or gene sequence. Nucleotides of promoter are shown by negative sign whereas nucleotide of gene sequence are represented by positive sign.
The consensus region frequently occur 35 base pair (bp) and 10 (bp) upstream from the start of transcription. These two regions are called as -35 box and -10box (pribnow box).
In eukaryotes, a similar sequence called TATA box lies about 25 bp upstream from the start of transcription.
In E.coli, single large RNA polymerase carry out the synthesis of mRNA, rRNA and tRNA with the subunit structure α2ββ’Ϭ.
This complete enzyme is termed as holoenzyme, which is needed to initiate transcription.
Core Enzyme: The Ϭ factor is essential for correct recognition of the promoter and it is removed after binding. The RNA polymerase without sigma subunit is called core enzyme.
In eukaryotes there are three RNA polymerases RNA polymerase-I (synthesize rRNA), Pol-II (synthesize mRNA), Pol-III (synthesize tRNA).
The elongation phases of RNA synthesis, which begins after the formation of the first bond, is therefore carried by core enzyme.
A portion of DNA double strand is unwound (12-17 bp) transcription takes place, after the transcription complex has passed, DNA rewinds. The transcription bubble moves along the DNA, leaving the growing strand protruding from the bubble. The RNA polymerase copies the DNA sequence accurately at the rate of 30 -40 nucleotides/second.
Transcription continues until a termination single is reached. The simplest termination
single is as GC-rich region that is a palindrome, followed by an AT-rich sequence .The RNA made from the DNA palindrome, is self- complementary and so base pairs internally to form a hairpin structure followed by a few U residues. However, not all termination sites have this hair pin structure. Those that lack such a structure require an additional protein, called rho (ρ), to allow recognition of the termination site and stop transcription.
ADDITION OF A CAP
Capping of pre-mRNA occurs immediately after synthesis while transcription is in progress. The cap is in the form of a modified GTP (7-methyl guanosine) . The ribose of 7-methyl guanosine and the first nucleotide of mRNA are linked by 5’ to 5’ tri- phosphate bridge .
ADDITION OF A TAIL
Most eukaryotic pre-mRNAs undergo polyadenylation, which involves cleavage of the RNA at its 3’ end and the addition of up to 250 A residues to form a poly (A) tail