Transcription is the process of synthesizing RNA from DNA and involves four main stages - initiation, elongation, termination, and post-transcription processing. It occurs differently in prokaryotes and eukaryotes. In prokaryotes, transcription and translation are coupled and occur in the cytoplasm, while in eukaryotes transcription occurs separately in the nucleus. The document provides details on the mechanisms and factors involved in each stage of transcription for both prokaryotes and eukaryotes.

1. 1
Transcription in Prokaryotes and Eukaryotes
Overview
 Introduction
 TranscriptioninProkaryotesandEukaryotes
 Pre-Initiation
 Initiation
 Elongation
 Termination
 Post-Transcription
Transcription
DNA → RNA
The formation or synthesis of single RNA from DNA is called Transcription.
Four Stages are involved :
1. Pre-Initiation
2. Initiation
3. Elongation
4. Termination
Introduction
 Synthesis of single stranded RNA
 5’→3’ direction
 RNA Polymerase is used
 No primers needed
 Only a part of the genome is transcribed
 First stage of gene expression and the principle conservation step.
RNA Polymerase
 Nuclear Polymerases

2. 2
 RNA PolI-Produces rRNA(28S, 5.8S, 18S)
 RNA PolII-Produces mRNA, snRNA, siRNA, miRNA
 RNA PolIII-Produces tRNA, mRNA (5S), SRPRNA
In Prokaryotes & Eukaryotes
Prokaryotes
 Occurs in cytoplasm
 Coupled transcription & translation
 No definite phase of occurrence
 A single RNAP synthesizes mRNA, tRNA& rRNA
 No initiation factors required
 Polycistronic
Eukaryotes
 Occurs in nucleus
 No coupling of transcription & translation
 Occurs in the G1 & G2 phases
 RNAP I, II & III synthesize rRNA, mRNA & tRNA
 TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIH recognize TATA box
 Monocistronic
Polycistronic& Monicistronic
Monocistronic
An mRNA molecule is said to be monocistronic if it contains genetic information to translate
only a single protein.
In the end only one polypeptide chain coding for one protein is obtained from a single gene with
an operator & promoter region.
Polycistronic
PolycistronicmRNA contains information for several genes which are translated into several
proteins.

3. 3
mRNA contains several ORFs (open reading frames), each of which is translated in proteins. The
coding is grouped and all of the genes are translated together with a common promoter &
operator region (like in operons)
Pre-Initiation
 First step of transcription.
 The Pre-Initiation Complex (PIC) includes RNA Polymerase II and six transcription
factors-TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIH
 Other co-activators and chromatin remodeling complexes also comprise of PIC
 TATA Binding Protein (TBP) is a subunit of TFIID and binds to the promoter, creating a
sharp bend.
 TBP-TFIIA interact; TBP-TFIIB interact; TFIIB-TFIIF interact & TFIIF recruits RNA
PolII; TFIIE joins the group and recruits TFIIH
 Subunits within TFIIH that haveATPaseandhelicaseactivity create
negativesuperhelicaltension in the DNA.
Initiation
Negative superhelical tension causes approximately one turn of DNA tounwindand form
thetranscription bubble. Promoter melting requires hydrolysis by ATP and is mediated by TFIIH.
TFIIH pulls the double stranded DNA into the cleft of RNA Polymerase and helps in transition
from closed to open state. The two strands get separated.

4. 4
Abortive Initiation
 Before entering elongation phase, The polymerase may terminate prematurely.
 This produces a truncated polypeptide chain.
 Many cycles of abortive initiation may occur before actually producing a growing
polypeptide chain.
 This helps in providing a scrunching kind of motion.
Elongation
 The polypeptide chain is elongated with the help of Elongation Factors.
 RNA Polconveniently adds nucleotides to the 3’ end. The template strand for this is
known as the sense strand and the other anti-sense strand.
 There are different classes of elongation factors. Some factors can increase the overall
rate of transcribing, some can help the polymerase through transient pausing sites, and
some can assist the polymerase to transcribe through chromatin

5. 5
Transcription Fidelity
 RNA polymerases select correctnucleoside triphosphate(NTP) substrate to prevent
transcription errors. Only the NTP which correctly base pairs with the coding base in the
DNA is admitted to the active center.
 RNA polymerase performs two known proof reading functions to detect and remove
misincorporatednucleotides: pyrophosphorylyticediting and hydrolytic editing.
Pausing and Backtracking
 RNA polymerase does not transcribe through a gene at a constant pace. Rather it pauses
periodically at certain sequences, sometimes for long periods of time before resuming
transcription.
 Promoter-proximal pausing during early elongation is a commonly used mechanism for
regulating genes poised to be expressed rapidly or in a coordinated fashion. The blockage
is released once the polymerase receives an activation signal.
Pausing and Backtracking

6. 6
Termination
Two Types:
1. Factor Dependent
2. Factor Independent
o Factor dependent requires Termination Factors along with RNA PolI.
o Factor Independent termination can be done by RNA PolIII. A stretch of
Thyminesalong a hair pin loop causes disintegration of complexes.
Post Transcriptional Modifications
 5’ end Capping
 A guanine nucleotide linked to the 5’ end triphosphate
 Polyadenylation
 Poly Adenine units added to 3’ end of the Ribonucleotidechain.