Gene Function and Structure

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Until 1940, the gene was considered as the basic unit of genetic information as defined by three criteria.

The unit of function, controlling the inheritance of one “character” or phenotypic attribute.

The unit of structure, operationally defined by recombination and by mutation.

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Oliver (1940) - intergenic recombination at lozenge

Mutations in lozenge affect eye shape in Drosophila.

Two mutations, lzs and lzg, were considered alleles of the same gene because lzs/lzg heterozygotes have lozenge, not wild-type, eyes.

But when lzs/lzg females are crossed to lzs or lzg males, about 0.2% of the progeny are wild-type!

These must result from recombination between lzs and lzg , because the wild-type progeny always had recombinant flanking markers. Also, the frequency of 0.2% is much higher than the reversion rate of the mutations.

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Further studies of intergenic recombination in bacteriophage and bacteria (where billions, instead of thousands, of progeny can be analyzed) showed that recombination occurs between adjacent nucleotide pairs.

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So the nucleotide, not the gene, is the basic unit of genetic structure.

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The coding sequence of a gene and its polypeptide product are colinear

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Summary

Mendel’s work established the concept of the gene.

This concept has evolved from:

the unit that can mutate to cause a specific block in metabolism…

to the unit specifying one enzyme…

to the sequence of nucleotide pairs in DNA encoding one polypeptide chain.