Biotech Brief Wine and biotechnology

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Biotech Brief
Wine and biotechnology

Gustav Styger, Institute for Wine Biotechnology
In the previous column we discussed the classic ways of generating new yeast strains with unique or novel wine-making capabilities via the techniques of sporulation, mating and hybridisation. However, these techniques are time consuming, not always successful and sometimes require specialised equipment like a micro-manipulator for handling the yeast spores. Hybridisation is useful when one wants to combine traits from two parental strains into a new strain, but sometimes it is easier to try and improve a trait already present in a single strain. In this case, mutagenesis is commonly used.
Mutagenesis

Schematic representation of the five types of chromosomal mutations. (Source: http://en.wikipedia.org/wiki/Image:Types-of-mutation.png)

Although mutation seems like a nasty word it occurs continuously in nature and is in fact one of the main reasons for the diversity we see in animal and plant species. Mutations are simply changes in the genetic source code, or DNA of an organism. These changes can be caused by a variety of factors, such as Ultra Violet (UV) radiation, exposure to chemicals or even infection with viruses. The changes can be small, involving the substitution of one nucleotide for another on the DNA strand (also called a point mutation), to large scale rearranging of the chromosome structure. The various ways that mutations can cause genetic changes are shown in the figure.
It is thus clear that mutations cause variations in the total population gene pool. Mutations in important genes, which cause the specific gene to function less than optimally, will be lost over time due to natural selection or the principal of survival of the fittest. In nature, most mutations are neutral, having no significant effect on the organism’s survival and the cell’s own DNA repair mechanisms can rectify the mutation. Sometimes, however, a mutation can cause a gene to behave in a different way than before, leading the cell to have an evolutionary advantage over its nearby competitors.
In the laboratory two methods of mutagenesis are frequently used, i.e. UV radiation and treatment with ethyl methanesulfonate (EMS). UV radiation can indirectly damage DNA by generating harmful free radicals and reactive oxygen species, but also directly by causing adjacent cytosine nucleotides to form dimers, interfering with the DNA replication process and leading to point mutations. EMS also leads to point mutations, but its mode of action is by targeting the guanine nucleotide for alkylation.
By using mutagenesis, researchers can generate millions of different mutations in various genes in a population of yeast strains. The majority of mutations will have no effect on the target gene(s), and some may even be deleterious to the yeast cell, but even if only a few thousand yeast cells survive the treatment, they all have the possibility that the specific mutation contained in their genome might cause them to have a novel function or feature. The challenge then is how to differentiate between strains carrying a positive mutation and the rest of the population? This is known as the selection strategy and in the next column we will investigate this further.

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