Claiming beliefs that one does not really possess or follow; falseness
THE GENE What information does the gene possess?.
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Transcript of THE GENE What information does the gene possess?.
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 daysNeurospora 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 Genes ?
20 amino acids9 vitamins
other organic compounds
© 2010 Paul Billiet ODWS
Producing mutant strains
Alter the genes by mutation and you will alter the dietary requirements
Wild type mould
Mutagens(ionising radiation)
Which ones are mutant moulds?
Separate spores
© 2010 Paul Billiet ODWS
Finding the mutants
Mutant moulds?
Select for deficient strains which will grow on minimal medium plus ONE supplementary
nutrient
Grow up on complete medium = Minimal medium PLUS
© 2010 Paul Billiet ODWS
8 haploid spores Separate spores and grow on complete medium
Screen for nutrient requirements
Cross breeding strainsDeficient strain (n) X Wild type (n)
Fusion
Zygospore (2n)
Meiosis I and II4 haploid nuclei
Mitosis
© 2010 Paul Billiet ODWS
Image Credit: Neurospora ascs containing haploid sporesMutant alleles are fluorescing
Results
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
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 genesTherefore ONE GENE corresponds to ONE
ENZYME
© 2010 Paul Billiet ODWS
Generalising
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….
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…
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!)… 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?
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