Transcription

Transcription (RNA synthesis)

Transcription is the process by which a single-stranded RNA is synthesized from DNA. In another way, the transfer of genetic information from DNA to RNA is called transcription. In this process, one strand of DNA so-called template DNA gives rise to a new RNA strand. It is also called RNA synthesis.

DNA (TRANSCRIPTION)→ RNA

Basic requirements for Transcription:

1)Template DNA-A single strand of DNA acts as a template to direct the formation of complementary RNA during transcription. In RNA synthesis, only one strand of DNA transcribes RNA.

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

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

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

1)Initiation:

It is the beginning of synthesis of RNA strand. This process involves:

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

• By the action of RNA polymerase- sigma complex, two DNA strands uncoil or unwind and separate at a specific point. (No primer is required for RNA synthesis.)

2) Elongation:

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

•As the DNA strand unwinds, both DNA strands are separated. Among two DNA strands, the 3'-5' strand acts a template or master strand for the formation of RNA, whereas another strand (5'-3') remains dormant or does not participate, hence called as anti-sense strand.

•Template or master strand has a promoter or initiation site and a terminator site. RNA synthesis begins at promoter site and ends at the terminator site.

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

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

3)Termination:

It is the last step of transcription. It signifies the end of transcription. It involves:

•The synthesis of RNA is terminated as soon as RNA polymerase reaches the termination site.

•As the transcription is over, DNA strands rewind. The newly formed RNA is called transcript and the process is called transcription.

•RNA do not remain connected with DNA template, but separate out as a single strand and move out of the nucleus through the nuclear pore into the cytoplasm.

•Several copies of RNA transcripts are released from each DNA template.

Thus, the process of RNA synthesis is completed.

Reference

Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.

Mehta, Krishna Ram. Principle of biology. 2nd edition. Kathmandu: Asmita, 2068,2069.

Jorden, S.L. principle of biology. 2nd edition . Kathmandu: Asmita book Publication, 2068.2069.