GENE EXPRESSION
by A.Arputha Selvaraj
Two steps are required
1. TranscriptionThe synthesis of mRNA uses the gene on the DNA molecule as a templateThis happens in the nucleus of eukaryotes
2. TranslationThe 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).
© 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).
© 2010 Paul Billiet ODWS
mRNA
A long molecule 1 million Daltons Ephemeral Difficult to isolate mRNA provides the plan for the
polypeptide chain
© 2010 Paul Billiet ODWS
rRNA Coiled Two subunits:
a long molecule 1 million Daltonsa short molecule 42 000 Daltons
Fairly stable Found in ribosomes Made as subunits in the nucleolus rRNA provides the platform for protein
synthesis© 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
© 2010 Paul Billiet ODWS
Transcription: The synthesis of a strand of mRNA (and other RNAs)
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
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
Transcription
DNA
messenger RNA
Gene
Nucleus
© 2010 Paul Billiet ODWS
Translation plan
TRANSLATION
Complete protein
Polypeptide chain
Ribosomes
Stop codon Start codon
© 2010 Paul Billiet ODWS
Translation
Location: The ribosomes in the cytoplasm that provide the environment for translation
The genetic code is brought by the mRNA molecule.
© 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.
© 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
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.
© 2010 Paul Billiet ODWS
The Central Dogma 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:
DNARNAProtein© 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
Thus the central dogma is modified:
DNA↔RNAProtein This has helped to explain an important paradox
in the evolution of life.
Image Credit: Reverse transcriptase
The paradox of DNA 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 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
Thank You