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


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


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


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


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


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


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


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


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


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).


the gene what information does the gene possess?

Documents