THE GENE
What information does the gene
possess?
The genetic approach
Before the discovery of
the importance of DNA
geneticists managed to
determine what
information is
represented in the gene
Image Credits: Nobelprize.org
Beedle and Tatum (1940)
The red bread mould Neurospora
crassa
 This mould is haploid so all the
genes are expressed
 Different individuals can be
separated because the spores form
in long thin “pods” called ascs
 Its requirements are simple
 It grows quickly and produces
results in a few days
Neurospora crassa
Image Credits:
University of Edinburgh, School of Biological Sciences
Wild type feeds on minimal medium
(nitrates, sulphate, phosphate other inorganic salts, sugar and biotin)
Synthesis of enzymes
20 amino acids
9 vitamins
other organic
compounds
© 2010 Paul Billiet ODWS
Genes ?
Producing mutant strains
Alter the genes by mutation and you will alter the
dietary requirements
Wild type mould
Mutagens
(ionising radiation)
Separate spores
Which ones are mutant moulds?
© 2010 Paul Billiet ODWS
Finding the mutants
Mutant moulds?
Grow up on complete medium = Minimal
medium
PLUS
Select for deficient strains which will grow on
minimal medium plus ONE supplementary
nutrient
© 2010 Paul Billiet ODWS
Cross breeding strains
Deficient strain (n)
X
Wild type (n)
Fusion
Zygospore (2n)
Meiosis I and II
4 haploid nuclei
Mitosis
8 haploid spores
Separate spores and grow on complete medium
Screen for nutrient requirements
© 2010 Paul Billiet ODWS
Image Credit:
Neurospora ascs containing haploid spores
Mutant alleles are fluorescing
Results
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Four of the daughter spores gave rise to colonies
that are like the deficient parent and four give
colonies that are like the wild type parent
The condition is hereditary
It is caused by the mutation of a single gene
© 2010 Paul Billiet ODWS
Conclusion
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Enzymes are needed by this mould to synthesise
vitamins and amino acids
The production of these compounds is affected by
mutation (damaged genetic material)
The inheritance pattern of these defective
enzymes is the same as that of single genes
Therefore ONE GENE corresponds to ONE
ENZYME
© 2010 Paul Billiet ODWS
Generalising
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Enzymes are all proteins….
but not all proteins are enzymes
Proteins are all affected by mutations in the same
way
Therefore ONE GENE corresponds to ONE
PROTEIN
© 2010 Paul Billiet ODWS
More precisely….
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Some proteins are made of subunits (quaternary
structure)
e.g. Haemoglobin = 4 subunits (2 x α and 2 x β)
Each subunit is independently affected by
mutations
Therefore ONE GENE corresponds to ONE
POLYPEPTIDE
© 2010 Paul Billiet ODWS
That’s not all…
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The hypothesis was established by 1962 there was still
another refinement when gene expression was worked out
Genes are transcribed to make RNA molecules
mRNA molecules are translated into polypeptides but…
not all RNA is mRNA, genes are also transcribed into
tRNA molecules and rRNA molecules
tRNA and rRNA is not translated (though they are used in
the translation process)
So a gene is does not necessarily correspond to a
polypeptide at all
© 2010 Paul Billiet ODWS
It gets better (or worse!)…
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Immunoglobulins (antibodies) are made of polypeptides
with two domains (constant C and variable V)
It seems two genes are needed to construct the
polypeptide of an immunoglobulin, one for each domain
The gene for the variable part is transcribed as pre-RNA
This pre-RNA is then spliced in different places by
snRNPs to take out introns
Different introns can be cut out to make different
immunoglobulins from the same pre-RNA (alternative
splicing)
So a single gene can code for more than one protein
© 2010 Paul Billiet ODWS
So what is a gene?
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It contains information
That may code for a protein
Or part of protein
Or more than one protein
Or RNA that does not translate into a protein
The smallest physical unit of heredity encoding
a molecular cell product
(Penguin Dictionary of Biology).
© 2010 Paul Billiet ODWS