1. What information does the gene possess?
© 2016 Paul Billiet ODWS

2. The genetic approach
Before the discovery of the importance of DNA, geneticists managed to determine what information is represented in the gene
© 2016 Paul Billiet ODWS

3. Beedle and Tatum (1940) The red bread mould Neurospora crassa
A haploid organism, 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
© 2016 Paul Billiet ODWS

4. Synthesis of enzymes Genes ?
Wild type feeds on minimal medium
(nitrates, sulphate, phosphate other inorganic salts, sugar and biotin)
Synthesis of enzymes
Genes ?
20 amino acids
9 vitamins
other organic compounds
© 2016 Paul Billiet ODWS

5. 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?
© 2016 Paul Billiet ODWS

6. Grow up on complete medium
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
© 2016 Paul Billiet ODWS

7. 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
© 2016 Paul Billiet ODWS

8. Neurospora ascs containing haploid spores
Mutant alleles are fluorescing
© 2016 Paul Billiet ODWS

9. 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
© 2016 Paul Billiet ODWS

10. Therefore ONE GENE corresponds to ONE ENZYME
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 genes
Therefore ONE GENE corresponds to ONE ENZYME

11. Therefore ONE GENE corresponds to ONE PROTEIN
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
© 2016 Paul Billiet ODWS

12. Therefore ONE GENE corresponds to ONE POLYPEPTIDE
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
© 2016 Paul Billiet ODWS

13. So a gene does not necessarily correspond to a polypeptide at all
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 are not translated (though they are used in the translation process)
So a gene does not necessarily correspond to a polypeptide at all

14. It gets better (or worse!)…
Immunoglobulins (antibodies) are made of a polypeptide with two domains (constant C and variable V)
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 spliceosomes 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
© 2016 Paul Billiet ODWS

15. 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).
© 2016 Paul Billiet ODWS

16. The genome

17. The proteome
Total set of proteins expressed by a cell, tissue or organ
Information to construct a protein lies in genes
Proteome ≠ genome
Proteome > total genes in genome
© 2016 Paul Billiet ODWS

18. From genome to proteome

19. Proteomics Study of the structure and function of proteins
Human Proteome Project (HPP)
Proteome and cancer research
© 2016 Paul Billiet ODWS