PG1005
Gene Transcription
Lecture 17
Dr Neil Docherty
My Teaching Objectives
• To summarise the gene-RNA-protein central dogma of
Gene expression
• To outline the various types of RNA molecule found within the
cell
• To explain the mechanism of action of RNA polymerase II
• To describe the key steps involved in mRNA maturation
• To introduce the concept of transcriptional regulation
Genetic Information Directs
Protein Synthesis From Messenger RNA
5’
Gene code (DNA)
3’
3’
5’
RNA polymerase II
Intermediate (mRNA) 5’
TRANSCRIPTION
3’
NUCLEUS
CYTOPLASM
ribosome (+rRNAs)
membrane
rER-Golgi
TRANSLATION
/TARGETTING
organelle
secretory
vesicle
tRNA (+ amino acid)
free ribosomes
cytoplasmic protein
RNA Structure
-Linear polymer composed of
4 nucleotide subunits
--Nucleotides are
ribonucleotides (ribose sugar)
-AGCU (adenine, guanine,
cytosine and uracil)
-Intramolecular complimentary
sequences found in RNA
Can form intramolecular
bonds permitting folding and
generation of precise 3D
structures
RNA
DNA
Principal Type of RNA (1)
There are 5 types you need to know about
All have a role in the control of gene expression
Not all are intermediate “central dogma” codes however
1) mRNA
-Messenger RNA (<2000bp) is the direct coding intermediate for the
production of proteins. Passes through various steps of processing
before mature form is translated on the ribosome
2) rRNA
-ribosmal RNA forms complex with multimolecular protein machinery to
form the ribosome. Central to some of the processing steps involved in
the production of mature mRNA
Principal Type of RNA-2
3) tRNA
-Transfer RNAs couple complimentary base pairing with mRNA to the
delivery of specific amino acids to the ribosome to provide each
monomer of the polypeptide chain
4) snRNA
-Small nuclear RNAs, in association with proteins form small
ribonucleoprotein particles are involved in the spliceosome which
removes stretches of non-coding sequence from immature mRNA
5) miRNA
-Small transcripts which are cleaved intracellularly to give rise to short
sequences of complimentary RNA which act in concert with protein
complexes to directly degrade target mRNAs and/or inhibit
translation
RNA Polymerases
•  RNA polymerase I Synthesizes a 45S rRNA which is
cleaved to give rise to the 28S, 18S and 5.8S rRNAs,
all components of the ribosome
•  RNA polymerase II
Synthesizes mRNA, snRNA and miRNA.
•  RNA polymerase III
Synthesizes tRNA and 5S rRNA
Stages of Transcription
Three steps1) Initiation of transcription
at start site
2) Elongation of nascent
RNA
3) Termination of
transcription
Transcriptional Initiation (RNApol II)
1)  Sequential assembly of
general transcription factors
around the TATA box
TBP-TATA binding protein
TAF-TBP associated factors
N.B. Binding of TBP leads to a
pronounced bend in the DNA.
2) TFIIB binds at consensus sites
around the TATA box and directs
entry of the polymerase
3)TFIIH binds and exercises 2
key functions
1.  Helicase activity
2.  Kinase activity
The pre-initiation complex
The Mediator Complex
Helps RNApol II to locate at promoters and helps
secure assembly
Elongation
New nucleotides are added
to the free 3’ OH end of the
growing immature mRNA
5’ Capping
Once nascent mRNAs
have reached 25bp in length
They are capped at the 5’ end by
The addition of methylated
guanosine nucleotide
Termination
Introns and Exons
•  Newly synthesised mRNA is immature and contains
transcribed regions of spacer DNA (introns) and
protein coding DNA (exons)
•  As part of maturation the introns are spliced out
The Spliceosome
Intron recognition and catalysis driven by snRNA in complex
with protein (snRNPs)
Export of mRNA
•  Binding proteins recognise
-The 5’ cap
-The poly-A tail
These then allow traffic through the nuclear pores
Modulation of mRNA Transcription
Many layers;
1. DNA binding
-Enhancers
-Repressors
-Coactivators
-Corepressors
-Specific transcription factors
(re:next slide)
-Cytosine methylation
2. Histone modification
-acetylation
-deacetylation
Modulation of mRNA Transcription by
Cell Signals
Ligand-receptor
Nuclear hormone
receptors
Ligand second
messenger systems
Desequestration of
Transcription factors
Your Learning Objectives
Your learning from today should focus on being able to;
•  Define transcription as the key event in gene
expression during which the genetic code is read and
transcribed into a portable template set of instructions
for protein synthesis
•  Detail the sequence of events occurring during the
initiation, elongation and termination of transcription
•  Describe the reactions catalysed by the spliceiosome
and define what introns and exons
•  Provide examples of how and why the transcription
rates of genes are often under exquisite regulation