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Transcription (RNA synthesis)



The process of coping genetic information from one strand of  DNA into RNA is termed as Transcription.Here also, the principle of complementarity governs the process of transcription, except the adenosine complements now forms base pair with uracil instead of thymine. However, unlike in the process of replication, which once set in, the total DNA of an organism gets duplicated, in transcription only a segment of DNA and only one of the strands is copied into RNA. This necessitates defining the boundaries that would demarcate the region and the strand of DNA that would be transcribed. In this process, the anti-sense strand/ master strand/ Template strand i.e. 3'-5' strand of DNA gives rise to RNA. strand. The other strand which has the polarity (5'→3') and the sequence same as RNA (except thymine at the place of uracil), is displaced during transcription. Strangely, this strand (which does not code for anything) is referred to as coding strand.

Basic requirements for Transcription

1. Template DNA

A single strand of DNA (3'-5') acts as a template to direct the formation of complementary RNA during transcription.

2. Substrates

The substrates for RNA synthesis are the four ribonucleoside triphosphates - adenosine triphosphate (rATP), guanosine triphosphate (rGTP), cytidine triphosphate (rCTP) and uridine triphosphate (rUTP). Cleavage of high-energy phosphate bond between the alpha and phosphate provides the energy for the addition of nucleotides to the growing RNA chain.

3. Enzymes

In prokaryotes, only one RNA polymerase synthesizes all three types of RNAs. In eukaryotes, there are three types of RNA polymerases responsible for the synthesis of all three types of RNAs (mRNA, rRNA and tRNA).

a. RNA polymerase I :     responsible for the synthesis of rRNA

b. RNA polymerase II :   responsible for the synthesis of mRNA

c. RNA polymerase III :   responsible for the synthesis of tRNA

4. Promoter and Terminator

Fig : Schematic structure of a transcription unit

Promoter sequences are responsible for directing RNA polymerase to initiate transcription at a particular point. Sigma factor recognizes the start signal or promoter region of DNA. Similarly, a termination factor called Rho factor is required for termination of terminator region.

Process of Transcription

The transcription process is similar to that of replication of DNA. It requires a promoter region and a terminator region. It is completed in three steps:

1. Initiation

2. Elongation

3. Termination

Fig: Process of Transcription in Bacteria

1. Initiation

It is the beginning step of synthesis of RNA strand. It involves:

a.  The RNA polymerase binds to a specific region of DNA known as the promoter region. In prokaryotes, it requires as initiation sigma factor which recognizes the start signal. The RNA polymerase- sigma complex binds to the promoter region and initiates transcription.

b. By the action of RNA polymerase-sigma complex, two DNA strands uncoil or unwind and separate at a specific point. Primer is not required for RNA synthesis.

2. Elongation

It is the step of formation of RNA strand.It involves:

a. As the DNA strand unwinds, both DNA strands are separated. Among them the 3'-5' DNA strand acts as template strand for the formation of RNA while the 5'-3' DNA strand remains dormant and does not participate.

b. The template strand has a promoter and a terminator site. RNA synthesis begins at the promoter site and ends at the terminator site.

c. Formation of new ribonucleotide chain proceeds by the addition of new bases. Base pairing occurs on the basis of their specificity i.e. A pairs with U and C pairs with G. These bases remain available in nucleoplasm in the form of ATP, CTP, UTP and GTP. This polymerization process is catalyzed by ab enzyme known as DNA-dependent RNA polymerase.

d. The RNA polymerase progressively moves forward and progresses the formation o RNA strand. As the polymerase reaches the termination site, it triggers the end of transcription.

3. Termination

It is the step of termination of RNA synthesis. The synthesis of RNA is terminated as soon as RNA polymerase reaches to the termination site. As the transcription is over, DNA strands rewind. The newly formed RNA is called Transcript. RNA does not remain connected with the DNA template, but separate out as single strand and move out of the nucleus through the nuclear pore into cytoplasm. Several copies of RNA transcripts are released from each DNA template.

In eukaryotes,  transcripts contain both the exons (coding segments of gene) and the introns (non-coding segments) and are non-functional. Hence, it is subjected to a process called splicing where the introns are removed and exons are joined in a defined order by enzyme spliceosome. RNA undergoes additional processing called as capping and tailing. In capping an unusual nucleotide (methyl guanosine triphosphate) is added to the 5'-end of RNA. In tailing, adenylate residues (200-300) are added at 3'-end in a template independent manner. It is the fully processed RNA now that is transported out of the nucleus for translation.

Fig: Process of splicing, capping and tailing in eukaryotes

In this way the genetic information flows from DNA to RNA.