Transcription

Transcription: an enzymatic system that converts the genetic information of a segment of @@DNA into RNA@@ strand with a base sequence @@complementary to one of the DNA strands@@ \n

Template strand (minus -): strand that serves as a template for RNA synthesis

}}Basic features of RNA synthesis:}}

  1. Precursors of RNA synthesis 4 ribonucleotide ==5’-triphosphate (rNTPs)==: ATP, GTP, CTP, UTP
  2. RNA chain grows: ==5’ --> 3’ direction== (DNA synthesis), RNA strand and DNA strand @@antiparallel@@
  3. RNA polymerases can initiate RNA synthesis (@@no primer needed@@), unlike DNA polymerases
  4. Only @@one strand is transcribed@@ (unlike DNA replication), unwinding limited to transcribed gene segment

}}Transcription:}}

  1. Initiation
  2. Elongation
  3. Termination
  4. Post transcriptional processing

<<Initiation of transcription<<

Promotor

  • first step in transcription, binding of RNA polymerase to DNA at a particular region called a promotor
  • a specific sequence in DNA template which is responsible for directing RNA polymerase to initiate transcription at a particular point
  • Position 1: the first point transcribed
  • usually a @@purine@@
  • Upstream: sequence proceeding the first base are numbered negatively

==prokaryotic promotors==: Pribnow box

==eukaryotic promotors==: GC box, CAAT box, TATA box (Hogness box)

Initiation factors

==prokaryotic initiation factor:== sigma factor (σ*) (one subunit of RNA polymerase holoenzyme)*

different σ factors help to identify gene sequences.

once the transcription process is initiated by formation of a few phosphodiester bonds, the σ subunit dissociates.

==eukaryotic initiation factors:== several

the gene to be transcribed will attain a @@looser@@ conformation than the transcriptionally inactive chromatin

this will help the accessibility of certain promotor sequence to the transcription factors

DNA may be transcriptionally inactivated by %%methylation%%

Enhancers and silencers

Enhancers: specific sequences present from 200 to 300 bp upstream from the transcription initiation point

How enhancers can activate transcription from a distance:

  1. act as binding site for a sequence - specific topoisomerase that introduces negative supercoils
  2. serve as an entry for RNA polymerase
  3. bind to a group of transcription protein factors that can form a loop between distant sites of DNA and thus can stimulate transcription

Silencers: other specific sequences located on the same chromosome either upstream or downstream of transcription point.

  • ==decrease== rate of transcription

Transcription frequency signal

Transcription frequency signal: specific sequences further far upstream in DNa

on binding to specific proteins they determine:

  • the frequency of transcription
  • how many times the RNA polymerase should work on that particular gene

<<Elongation process of transcription<<

  • on binding of RNA polymerase to the promotor a conformational change occurs = segment of DNA is unwound and RNA polymerase is positioned at the polymerization start site
  • RNA polymerase moves along DNA template (one by one)
  • transcription bubble: RNA polymerase, DNA, nascent RNA
  • RNA polymerases ==don’t need primer==, no other enzymatic activities other than ==transcription==
  • RNA polymerases ==no proofreading==:

- fidelity less

- mistake rate more

- less serious (not transmitted to daughter cells or next generation)

%%Eukaryotic cells 3 kinds of RNA polymerases%% (+ mitochondrial RNA polymerase):

  1. RNA polymerase I (pol I): synth. of only one type of RNA “pre-ribosomal RNA”
  2. ^^RNA polymerase II (pol II):^^ central function of synthesizing ^^pre-mRNA + special function RNAs^^
  3. @@RNA polymerase III (poll III)@@: makes @@tRNA, 5S rRNA, other snRNA@@

<<Termination<<

In prokaryotes

  1. Factor (Rho) independent termination:

specific inverted repeat sequence that is transcribed into self-complementary sequence: hairpin configuration

  1. Factor ρ (Rho) independent termination:
  • depends on specific protein ==rho “ρ”== can bind to specific sequence having ==“regularly spaced C-residues”==
  • nascent RNA wraps around rho factor
  • dissociation of the transcript from the template requires ==energy== produced from ATP hydrolysis by ATPase activity of rho factor

<<Post transcriptional processing<<