Question

Describe the steps in the process of transcription of DNA to RNA.

Answer 1

• Formation of RNA transcript from DNA is known as transcription and the process is catalysed by enzyme RNA polymerase.
• Transcription is a primer independent, unidirectional and selective process.
• The mRNA transcript formed is identical to the one strand of DNA which is known as the coding strand and complementary to the second one which is known as the template strand.
• A transcription unit is defined as as the DNA stretch which is transcribed as a single RNA and consists of a transcription start site and terminator site.

• 	Prokaryotic Transcription	Eukaryotic Transcription
  RNA polymerase	RNA polymerase of prokaryotes contains 5 subunits: α : facilitates assembly of coenzyme and recognition of promoter β : catalytic activity β' : catalytic activity ω : assembles RNA polymerase σ : recognises promoter and initiates transcription.	Eukaryotes contain three types of RNA polymerases: RNA pol I : transcribes rRNA(except 5S RNA). RNA pol II : transcribes mRNA, Sno/sn/miRNA. RNA pol III : transcribes tRNA, 5sRNA and 7SL-RNA
  Promoter	the optimal promoter in prokaryotes consists of three consensus sequences TTGACA(recognition domain, recognised by σ factor), a 17 base pair gap(critical for proper binding of RNA polymerase), TATAAT(Pribnow box or unwinding domain)	eukaryotes contain three different promoters for RNA polymerase I, II and III. Promoter for RNA polymerase I contains upstream control element(UCE) and the initiator sequences to which upstream control element binding factor (UBF), selectivity factor(SL-1) and transcription intermediary factor 1A (TIF-1A) binds. Promoter for RNA polymerase II contains the core promoter and the upstream elements for example GC box, CAAT box and Oct box. Promoter for RNA polymerase III contains initiative sequence box A and box C to which transcription factors bind.
  Initiation of transcription	in prokaryotes the DNA binding protein(σ factor) have only one type of DNA binding domain that is helix turn helix, without association of coenzyme sigma factor do not mines to DNA but when it joins coenzyme already loaded on DNA it brings that to the promoter. Sigma factor makes critical contacts in - 35 and -10 region of promoter does it decreases coenzymes binding to non specific DNA sequences and increases to promoter sequence. When the transcript reaches more than 10 rNTP's in size sigma factor is released from coenzyme that is it only acts in transcription initiation not elongation.	Transcription in eukaryotes begins with recruitment of chromatin remodelling autonomously or with the help of activators and coactivators. Pre initiation complex is formed in eukaryotes before initiation of transcription which involves binding of TF IID, TF-IIA, TF-IIB, TF-IIF/RNA polymerase, TF-II E and TF IIH (general transcription factors).
  Elongation	Process of elongation begins when coenzyme is able to leave the promoter and continue RNA synthesis, the time taken for RNA polymerase to the promoter is known as promoter clearance time. Core enzyme moves along template strand at an even pace i.e. it moves over 7-8 base pairs of DNA in one go.	Elongation begins after the release of general transcription factors TF-IIE and TF-IIH. After release of transcription factors RNA polymerase slides in 5'-->3' direction elongating the RNA transcript.
  Termination	Termination in prokaryotes is of two types: Rho dependent termination which requires Rho protein. Rho protein is a helical which depends on ATP and disrupts the DNA RNA complex it binds to RNA at Rho utilisation or rut site and it moves along RNA in 5' --> 3' direction, and when the complex reaches RNA- DNA hybrid, it releases RNA from DNA. Intrinsic or Rho independent termination, in which RNA polymerase is dissociated from DNA by destabilizing the attachment between DNA and growing RNA transcript.	In eukaryotes termination of transcription by RNA polymerase 1 occurs by termination factor similar to that of prokaryotes. RNA polymerase encounters poly A signal in mRNA and induces a change in conformation of RNA-DNA complex, therefore terminating transcription. Transcription by RNA polymerase III is terminated after it transcribes the termination sequence.