Transcription in Prokaryotes and Eukaryotes.pptx

This presentation includes a brief note on transcription, the process by which genetic information from DNA is copied into RNA. In prokaryotes, transcription occurs in the cytoplasm and involves a single RNA polymerase that binds directly to the DNA promoter. In eukaryotes, transcription takes place in the nucleus and is more complex, involving multiple RNA polymerases, transcription factors, and post-transcriptional modifications like splicing, capping, and polyadenylation.

1. TRANSCRIPTION

2. INTRODUCTION The process of synthesis of RNA by copying the template strand of DNA is called transcription. Here only the selected portion of genome is copied. The enzyme involved in transcription is RNA polymerase.   It does not require primase. More exactly it is a DNA dependent RNA polymerase.

3. TRANSCRIPTION IN PROKARYOTES INITIATION ELONGATION TERMINATION

4. INITIATION Sigma factor ( σ )binds to promoter sequence (-10,-35 sequence)  Core enzyme binds to the sigma factor  and promoter site but DNA is still closed. This is called the closed promoter complex.   Holoenzyme untwist the double strands of DNA . Formation of Transcription Bubble Unwounded promoter is called the open promoter complex.   Sigma factor is released so that core enzyme can go forward transcribing.

5. ELONGATION RNA polymerase (core enzyme) moves along to transcribe the DNA sequence into a single strand RNA of the coding gene in the direction 3'-5'  When transcribing, the RNA polymerase interact with DNA sequence forming transcription bubble.   DNA double helix is reformed as the RNA polymerase moves forward.  Few RNA nucleotides (newly synthesized) form an RNA/DNA hybrid within RNA polymerase.   As transcription proceed, single strand RNA gets out of the RNA polymerase.(5'-3') RNA polymerase has proofreading capabilities  but not efficient .

6. TERMINATION It occurs at a specific DNA sequence, which serves as a "stop signal" for RNA polymerase.  When the core enzyme of the RNA polymerase reaches this sequence, the transcription bubble collapses, the transcription complex dissociates, the core enzyme and the mRNA transcript are released, and the DNA returns to its double helical state. TYPES OF TERMINATION A. Rho-dependent termination The rho-dependent terminator received its name because it is dependent on a specific protein called a rho factor.  The rho factor is thought to bind to the end of the RNA chain and slide along the strand towards the open complex bubble. When the factor catches the polymerase, it causes the termination of transcription.

7. B. Rho-independent termination   The rho-independent signal is found on the DNA template strand and consists of a region that contains a section that is then repeated a few base pairs away in the inverted sequence. It is followed by a short string of adenines.   When this stretch is transcribed into an RNA sequence, the RNA can fold back and base pair with itself forming a hairpin loop. The string of adenines in the DNA sequence are transcribed into  uracils  in the RNA sequence. Because the uracil bases will only pair weakly with the adenines, the RNA chain can easily be released from the DNA template, terminating transcription.

8. TRANSCRIPTION IN EUKARYOTES INITIATION  ELONGATION  TERMINATION

9. TRANSCRIPTION IN EUKARYOTES There are three types of RNA polymerases, I, II & III (only one type in prokaryotes). mRNA synthesis is catalysed by RNA-polymerase II.  There are three different types of promotes for the three classes of RNA polymerases (only one type of promoters in prokaryotes).  Initiation of transcription is more complex than in prokaryotes.  Termination of transcription does not involve stem-loop structure (stem-loop is present in prokaryotes).  Binding of RNA polymerase II to its promoter involves a number of proteins, called transcription factors (no transcription factors in prokaryotes). 

10. Nearly 50% or more of the promoters contains a specific sequence (TATAAT), called TATA box or Goldberg-Hogness box (absent in prokaryotes).  In some genes, other sequences, such as CAAT box, (GCCCAATCT), may be present beyond the TATA box (absent in prokaryotes). They act as binding sites for the transcription factors which stimulate or repress transcription initiation.  Many genes may contain sequences, such as enhancers and silencers, located outside the promoter (absent in prokaryotes). Enhancers induce and silencers inhibit transcription.

11. INITIATION The transcription of the mRNA begins with the binding of RNA polymerase II with the promoter of the DNA template.    TATA box serves as a recognition site for several transcription factors - helps in positioning the initiation complex exactly at the very start site with unerring precision. TATA box is recognized by initiation factors, such as TATA-binding protein (TBP) and a variety of TBP-associated factors (TAFs). They helps to respond to transcriptional activators and silencers.   TATAS-less promoters - initiation is controlled by a CT-rich sequence, called initiator element ( Inr ), and a down-stream promoter element (DPE).   Then the incorporation of the first ribonucleotide, mediated by RNA polymerase II in association with transcription initiation factors.  The enzyme moves forward from the promoter site, freeing the promoter for further initiation. This is called promoter clearance, and it marks the end of transcription initiation. 

12. ELONGATION  Once successful initiation has been completed, RNA polymerase II moves forward along the DNA template, catalysing the incorporating of more and more ribonucleotides and synthesising a nascent mRNA in the 5'-3' direction.   During this, RNA polymerase II gets associated with a series of transcription elongation factors which prevent its dissociation from the DNA template until it reaches the end of the protein coding gene and elongation is over.  In eukaryotes transcription elongation is always coupled with the co-transcriptional RNA modification.  During transcription elongation, the double-stranded DNA gets unwound ahead of the elongation complex and rewound behind it. This results in a transcription bubble,

13. TERMINATION   It involves the dissociation of the enzyme and the elongation factors from DNA and the liberation of the nascent mRNA transcript.  In eukaryotes, termination of transcription by RNA polymerase II occurs at a point beyond the end of the protein - coding gene.  It is believed that transcription termination probably occurs when RNA polymerase encounters either a chemical impediment or receives a stop signal from a termination codon of the DNA template.   the dissociation of the elongation factors at some point destabilizes the transcription complex. This, in turn, leads to the dissociation of RNA polymerase II, termination of transcription and the release of mRNA transcription.

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