GENE
EXPRESSION
Two steps are required
1.
2.
Transcription
The synthesis of mRNA uses the gene
on the DNA molecule as a template
This happens in the nucleus of
eukaryotes
Translation
The synthesis of a polypeptide chain
using the genetic code on the mRNA
molecule as its guide.
© 2010 Paul Billiet ODWS
RIBONUCLEIC ACID (RNA)
Found all over the cell
(nucleus, mitochondria, chloroplasts,
ribosomes and the soluble part of the
cytoplasm).
© 2010 Paul Billiet ODWS
Types
Messenger RNA (mRNA) <5%
 Ribosomal RNA (rRNA)
Up to 80%
 Transfer RNA
(tRNA) About 15%
 In eukaryotes small nuclear
ribonucleoproteins (snRNP).
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© 2010 Paul Billiet ODWS
Structural characteristics of RNA
molecules
Single polynucleotide strand which may
be looped or coiled (not a double helix)
 Sugar Ribose (not deoxyribose)
 Bases used: Adenine, Guanine, Cytosine
and Uracil (not Thymine).
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© 2010 Paul Billiet ODWS
mRNA
A long molecule 1 million Daltons
 Ephemeral
 Difficult to isolate
 mRNA provides the plan for the
polypeptide chain
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© 2010 Paul Billiet ODWS
rRNA
Coiled
 Two subunits:
a long molecule 1 million Daltons
a short molecule 42 000 Daltons
 Fairly stable
 Found in ribosomes
 Made as subunits in the nucleolus
 rRNA provides the platform for protein
synthesis
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© 2010 Paul Billiet ODWS
tRNA
Short molecule about 25 000 Daltons
 Soluble
 At least 61 different forms each has a
specific anticodon as part of its structure.
 tRNA “translates” the message on the
mRNA into a polypeptide chain
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© 2010 Paul Billiet ODWS
Transcription: The synthesis of a strand
of mRNA (and other RNAs)
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Uses an enzyme RNA polymerase
Proceeds in the same direction as replication (5’
to 3’)
Forms a complementary strand of mRNA
It begins at a promotor site which signals the
beginning of gene is not much further down the
molecule (about 20 to 30 nucleotides)
After the end of the gene is reached there is a
terminator sequence that tells RNA
polymerase to stop transcribing
NB Terminator sequence ≠ terminator codon.
© 2010 Paul Billiet ODWS
Editing the mRNA
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In prokaryotes the transcribed mRNA goes
straight to the ribosomes in the cytoplasm
In eukaryotes the freshly transcribed mRNA in
the nucleus is about 5000 nucleotides long
When the same mRNA is used for translation at
the ribosome it is only 1000 nucleotides long
The mRNA has been edited
The parts which are kept for gene expression
are called EXONS (exons = expressed)
The parts which are edited out (by snRNP
molecules) are called INTRONS.
© 2010 Paul Billiet ODWS
Transcription plan
Nucleus
Gene
DNA
Transcription
messenger
RNA
© 2010 Paul Billiet ODWS
Translation plan
Complete protein
Polypeptide chain
TRANSLATION
Ribosomes
Stop codon
© 2010 Paul Billiet ODWS
Start codon
Translation
Location: The ribosomes in the cytoplasm
that provide the environment for
translation
 The genetic code is brought by the mRNA
molecule.
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© 2010 Paul Billiet ODWS
What is the genetic code?
The genetic code consists of the sequence
of bases found along the mRNA molecule
 There are only four letters to this code (A,
G, C and U)
 The code needs to be complex enough to
represent 20 different amino acids used
to build proteins.
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© 2010 Paul Billiet ODWS
How many combinations?
If one base represented one amino acid this would only
be able to produce
4 different combinations. (A, C, G and U)
 If pairs of bases represented each amino acid this
would only be able to produce
4 x 4 = 16 combinations. (AA, AC, AG, AU, CA, CC, CG,
CU etc)
 If triplets of bases represented each amino acid, this
would be able to produce
4 x 4 x 4 = 64 combinations
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This is enough combinations to code for the 20 amino
acids but is the code actually made of triplets?
© 2010 Paul Billiet ODWS
Nature is logical!
Over 10 years biochemists synthesised
bits of mRNA with different combinations
 Then they used them to synthesise
polypeptides
 The results proved the logical answer was
correct
 The genetic code is made of triplets of
bases called codons.
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© 2010 Paul Billiet ODWS
The Central Dogma
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Proposed by Francis Crick 1958
DNA holds the coded hereditary information in
the nucleus
This code is expressed at the ribosome during
protein synthesis in the cytoplasm
The protein produced by the genetic information
is what is influenced by natural selection
If a protein is modified it cannot influence the
gene that codes for it
Therefore there is one way flow of information:
DNARNAProtein
© 2010 Paul Billiet ODWS
An important discovery
Retro viruses (e.g.
HIV) carry RNA as
their genetic
information
 When they invade
their host cell they
convert their RNA into
a DNA copy using
reverse
transcriptase
Image Credit:
 Thus the central dogma is modified:
Reverse transcriptase
DNA↔RNAProtein
 This has helped to explain an important paradox
in the evolution of life.
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The paradox of DNA
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DNA is a very stable molecule
It is a good medium for storing genetic material
but…
DNA can do nothing for itself
It requires enzymes for replication
It requires enzymes for gene expression
The information in DNA is required to synthesise
enzymes (proteins) but enzymes are require to
make DNA function
Which came first in the origin of life DNA or
enzymes?
© 2010 Paul Billiet ODWS
RIBOZYMES: Both genetic and
catalytic
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Certain forms of RNA have catalytic properties
RIBOZYMES
Ribosomes and snRNPs are ribozymes
RNA could have been the first genetic
information synthesizing proteins…
…and at the same time a biocatalyst
Reverse transcriptase provides the possibility of
producing DNA copies from RNA
© 2010 Paul Billiet ODWS
The ribosome a ribozyme
Image Credit:
Ribosome