Biol115
The Thread of Life
Lecture 6
Getting the message:
DNA transcription
“Understanding how DNA transmits all it knows about cancer, physics, dreaming
and love will keep man searching for some time”
~David R. Brower
Principles of Biology
• Chapter ‘Gene Expression’ (The expression of genes)
• Chapter ‘Transcription’
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Objectives
• Explain the processes that occur during the three phases of transcription.
• Describe the molecular factors that aid in transcription.
• Relate the importance of specific sequences on the DNA molecule to the
process of transcription.
• Describe the differences between eukaryotic and prokaryotic transcription.
• Describe RNA processing.
• Key terms: elongation, initiation, intron, promoter, RNA polymerase, TATA box,
termination, transcription, transcription initiation complex, transcription start site
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Gene expression: the cookbook analogy
http://recipecurio.com/recipecopies/collection3/sour-cherrypudding-recipe.jpg
http://www.bfeedme.com/wpcontent/uploads/2007/01/coo
kbook-clipart.gif
transcription
http://img.timeinc.net/rec
ipes/i/recipes/sl/02/11/ch
ocolate-cake-sl-366569l.jpg
translation
polypeptide/protein
DNA
mRNA
http://www.mun.ca/biology/scarr/Central_Dogma.gif
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Basic principles of transcription
• Transcription is the synthesis of RNA under the direction of DNA
• Transcription is mediated by the enzyme, RNA polymerase
• Transcription produces messenger RNA (mRNA)
RNA polymerase and
transcription
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Basic principles of
transcription
• In prokaryotes, mRNA produced by
transcription is immediately translated without
more processing
• In a eukaryotic cell, the nuclear envelope
separates transcription from translation
• Eukaryotic RNA transcripts are modified
through RNA processing to yield finished
mRNA
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Synthesis of an RNA transcript
The three stages of transcription:
• Initiation
• Elongation
• Termination
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RNA polymerase binding and initiation
of transcription
• Promoters signal the initiation of RNA synthesis
• Transcription factors mediate the binding of RNA polymerase
and the initiation of transcription
• The completed assembly of transcription factors and RNA
polymerase II bound to a promoter is called a transcription
initiation complex
• A promoter called a TATA box is crucial in forming the initiation
complex in eukaryotes
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Elongation of the RNA strand
• As RNA polymerase moves along the DNA, it untwists the double
helix, 10 to 20 bases at a time
• Transcription progresses at a rate of 40 nucleotides per second in
eukaryotes
• A gene can be transcribed simultaneously by several RNA
polymerases
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Termination of transcription
• The mechanisms of termination are different in bacteria and
eukaryotes
• In bacteria, the polymerase stops transcription at the end of the
terminator
• In eukaryotes, the polymerase continues transcription after the
pre-mRNA is cleaved from the growing RNA chain; the
polymerase eventually falls off the DNA
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Transcription in prokaryotes
Multiple RNA polymerases
transcribing the same gene
simultaneously!
Nature 417, 967-970.
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RNA polymerase binding
and initiation of transcription
• In eukaryotes, transcription factors
mediate the binding of RNA polymerase
and the initiation of transcription
• A sequence in the upstream regulatory
region, called a TATA box, is crucial in
forming the initiation complex in eukaryotes
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The eukaryotic promoter
Promoters are the start sites of transcription. Eukaryotic promoters can include the TFIIB
recognition element, the TATA box, the initiator element and the downstream promoter element. Not
all of these are present in every promoter.
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Termination of transcription
RNA polymerase stops transcribing at the end of a gene – ‘stop’
defined by termination signals (example shown)
Termination signals (Gould and Keeton, Biological Sciences)
Hairpin loops and poly-U tails (Gould and Keeton, Biological Sciences)
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Eukaryotic cells modify RNA after
transcription
• Enzymes in the eukaryotic nucleus modify pre-mRNA before
the genetic messages are dispatched to the cytoplasm
• During RNA processing, both ends of the primary transcript
are usually altered
• Also, usually some interior parts of the molecule are cut out,
and the other parts spliced together
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Alteration of mRNA ends
• Each end of a pre-mRNA molecule is modified in a particular
way:
• The 5 end receives a modified nucleotide 5 cap
• The 3 end gets a poly-A tail
• These modifications share several functions:
• They seem to facilitate the export of mRNA
• They protect mRNA from hydrolytic enzymes
• They help ribosomes attach to the 5 end (for translation)
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Insert Fig. 17-9 P336
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Split genes and RNA splicing
• Most eukaryotic genes and their RNA transcripts have long noncoding stretches
of nucleotides that lie between coding regions
• These noncoding regions are called intervening sequences, or introns
• The other regions are called exons because they are eventually expressed,
usually translated into amino acid sequences
• RNA splicing removes introns and joins exons, creating an mRNA molecule with
a continuous coding sequence
• Spliceosomes consist of a variety of proteins and several small nuclear
ribonucleoproteins (snRNPs) that recognise the splice sites
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Example of mRNA splicing
E = exon
I = intron
E1
I1
E2
I2
E3
I3
E4
I4
E5
I5
E6
DNAASDFASISASDFTHECGHDTHREADARETVTOFSRVTLIFE
pre-mRNA
mature mRNA
DNAISTHETHREADOFLIFE
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Insert Fig. 17-10 P337
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RNA splicing
In eukaryotes, coding regions,
or exons, are interspersed
with non-coding introns.
Introns are removed, and
exons are joined together
during RNA splicing to
produce a mature mRNA
molecule.
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mRNA for the egg protein, ovalbumin, being spliced to
remove introns
Gene
Pre-mRNA
mRNA
Factor 8
200,000 nt
10,000 nt
Duchenne
muscular
dystrophy
2,000,000 nt
16,000 nt
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The functional and evolutionary
importance of introns
• Proteins often have a modular
architecture consisting of discrete regions
called domains
• In many cases, different exons code for
the different domains in a protein
• Exon shuffling may result in the evolution
of new proteins
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The functional and evolutionary
importance of introns
• Some genes can encode more than one kind of polypeptide,
depending on which segments are treated as exons during
RNA splicing
• Such variations are called alternative RNA splicing
• Because of alternative splicing, the number of different
proteins an organism can produce is much greater than its
number of genes
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Alternative splicing: expressing several
proteins from a single gene.
Even though it is
translated from a
single gene, the
information within
an mRNA
transcript can be
rearranged during
mRNA splicing.
The final mRNA
would be
translated into a
protein with a
different number or
order of protein
subunits than what
was coded for by
the pre-spliced
mRNA.
The Drosophila Dscam gene, involved in
adhesion between neurons, contains 4
clusters of exons, each with array of
possible exons. These are spliced into the
mRNA in an exclusive fashion, so that
only one of each of the possible exons is
represented. If all combinations of these
exons are used in alternative splicing, the
Dscam gene can produce 38,016 different
proteins.
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You should now be able to:
1. Briefly explain how information flows from gene to protein.
2. Compare transcription in bacteria and eukaryotes.
3. Include the following terms in a description of transcription: mRNA,
RNA polymerase, the promoter, the terminator, the transcription unit,
initiation, elongation, termination, splicing and introns.
4. Explain three types of post-transcriptional processing of eukaryotic
pre-mRNA.
5. Suggest reasons for the occurrence of introns in eukaryotic genes and
the possible evolutionary benefits that they confer.
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