MECHANISM OF TRANSCRIPTION prashant.pptx

MECHANISM OF TRANSCRIPTION
Transcription is the first step of gene expression, in which a particular segment of DNA is copied into RNA by the enzyme RNA polymerase. During transcription, a DNA sequence is read by RNA polymerase, which produces a complementary RNA strand.

1. MECHANISM OF TRANSCRIPTION INTRODUCTION : Transcription  is the first step of gene expression, in which a particular segment of DNA is copied into  RNA by the enzyme RNA polymerase. During transcription, a DNA sequence is read by RNA polymerase, which produces a complementary RNA strand . PRESENTED BY PRASHANT VC DEPT OF ZOOLOGY GUK

2. MECHANISM OF TRANSCRIPTION IMPORTANT POINTS : Synthesis of single stranded RNA 5’ 3’ direction No primers needed Only a part of genome is transcriped Occurs in nucleus THERE ARE 3 RNA POLYMERASES IN EUKARYOTES (animals) : RNA POLYMERASE I – synthesizes rRNAs RNA POLYMERASE II – synthesizes all protein coding genes & mostly mRNAs RNA POLYMERASE III – synthesizes tRNAs , snRNAs , scRNAs

3. STAGES OF TRANSCRIPTION INITIATION : In the first step, transcription factors(TF) bind to the promoter region present in the DNA. After this RNA polymerase II binds. (TRANSCRIPTION INITIATION COMPLEX = TF + RNA POLYMERASE ON PROMOTER) The most common type of core promoter in eukaryotes is a short DNA sequence known as a  TATA BOX. In the TATA BOX ,the attachment of TFs in the sequence- TBP and TFII D binds first TFII A and TFII B bind TFII D TFII F-RNA-pol complex binds TFII B TFII F and TFII E open the dsDNA (helicase and ATPase) TFII H: completion of PIC DNA helicase, has helicase activity and so is involved in the separating of opposing strands of double-stranded DNA to provide access to a single-stranded DNA template.

4. ELONGATION : RNA polymerase unwinds DNA exposing 10 to 20 bases, followed by the synthesis of RNA molecule in the 5' to 3' direction, using one of the DNA strands as a template. In complementary base pairing, A, T, G, and C on the template DNA strand specify U, A, C, and G, respectively, on the RNA strand being synthesized. RNA transcript separates from template as RNA polymerase continues elongation.

5. TERMINATION : The last stage of transcription is termination, which leads to the dissociation of the complete transcript and the release of RNA polymerase from the template DNA . The process differs for each of the three RNA polymerases- For polymerase I, transcription is stopped using a termination factor. For polymerase III, transcription ends after transcribing a termination sequence that includes a polyuracil stretch. For polymerase II, it continues for hundreds of nucleotides past termination signal. The pre-mRNA, carrying the signal as AAUAAA, is then cleaved by a special endonuclease that recognizes the signal and cuts at a site 11 to 30 residues to its 3' side. RNA PROCESSING- Capping at the 5’-end; P olyadenylation at the 3′-end; Splicing- intron excision.

6. IMPORTANCE : Transcription is important because the information that is contained in DNA can not be transferred to other cells in the body because DNA can not leave the nucleus. This means that the information is carried over to RNA that can transfer the information outside the nucleus. CONCLUSION : From the above mechanism of transcription it is very explicit that transcription is a very complex process but it is very important for gene expression and transferring information which can be realized, analyzed and studied for scientific purposes for generations. REFERENCE : Karp – Cell and Molecular Biology 6 th edition Lehninger_Principles_of_BIOCHEMISTRY 6 th edition www.ncbi.nlm.nih.gov/NBK9904 www.boundless.com/textbooks