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.
Template Strand
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.
Figure 1 Overview of transcription. (a) Transcription of two genes in opposite directions. Gene 1 is transcribed from the bottom strand. The RNA polymerase migrates to the left, reading the template strand in a 3-to-5 direction and synthesizing RNA in a 5-to-3 direction. Gene 2 is transcribed in the opposite direction, to the right, because the top strand is the template. As transcription proceeds, the 5 end of the RNA is displaced from the template as the transcription bubble closes behind the polymerase.
Coding Strands:
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.
Promoter:
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.
Figure2 Promoter sequence. (a) The promoter lies “upstream” (toward 5′ end) of the initiation point and coding sequences. (b) Promoters have regions of similar sequences,as indicated by the yellow shading in seven different promoter sequences in E. coli. Spaces (dots) are inserted in the sequences to optimize the alignment of the common sequences. Numbers refer to the number of bases before (-) or after (+) the RNA synthesis initiation point. The consensus sequence for all E. coli promoters is at the bottom.
RNA POLYMERASE
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).
Figure 3Transcription initiation in prokaryotes and the subunit composition of prokaryotic RNA polymerase. (a) Binding of the subunit to the -10 and -35 regions positions the other subunits for correct initiation. (b) Shortly after RNA synthesis begins, the subunit dissociates from the other subunits, which continue transcription.
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ELONGATION
The elongation phases of RNA synthesis, which begins after the formation of the first bond, is therefore carried by core enzyme.
Fig 4: Elongation: Synthesis of an RNA strand complementary to the single-strand region of the DNA template strand is in the 5-to-3 direction. DNA that is unwound ahead of RNA polymerase is rewound after it has been transcribed.
TRANSCRIPTION BUBBLE:
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.
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TERMINATION
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.
Fig: 5 Termination: The intrinsic mechanism shown here is one of two ways used to end RNA synthesis and release the completed RNA transcript and RNA polymerase from the DNA. In this case, the formation of a hairpin loop sets off their release. For both the intrinsic and the rho-mediated mechanism, termination first requires the synthesis of certain RNA sequences.
Figure 6 The structure of a termination site for RNA polymerase in bacteria. The hairpin structure forms by complementary base pairing within a GC-rich RNA strand. Most of the RNA base pairing is between G and C, but there is one single A–U pair.
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