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Transcription in Prokaryotes and EukaryotesOverview

Introduction

Transcription in Prokaryotes and Eukaryotes

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

RNA PolI-Produces rRNA(28S, 5.8S, 18S)

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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.

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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.

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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

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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

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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.