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Cellular Life

Growing in the bottom of a good bottle of home-brewed beer is a tiny, microscopic, single celled fungus that has been solely responsible for the liquid above it. But apart from its ability to convert grains into palatable drinks, the yeast cells have something else to teach us about the benefits of "variation by summing biological information".

In its simplest form, each individual yeast is a single eukaryotic cell, protected by a cell wall, and holding in its nucleus a single set of all the genes it needs to grow, metabolize maltose sugar into alcohol and divide by mitosis.

making amino acids (or not!)

One task carried out on a daily basis by each yeast cell is the synthesis and production of the amino acid tryptophan. This amino acid is an important part of many proteins, and the yeast cells need a steady supply of this critical substance if it is to survive, be able to make its proteins, and grow.

The amino acid tryptophan has a relatively complex molecular structure, and is made by the yeast is a series of steps or stages. At each step an enzyme in the cytoplasm of the yeast cell changes a simpler substrate into the next, more complex, form of the molecule, and after enough steps, the final product is the needed amino acid.

Each enzyme in this team of enzymes (called a metabolic pathway) is the product of a single gene on the DNA of the yeast. If all these genes are coding for the correct protein, all the enzymes will be made properly and the yeast will be able to make all the tryptophan it needs.

But, if any of these genes are damaged, a critical protein/enzyme will not be made properly. The damaged enzyme will not be able to perform its appointed task, and the production line that produces the tryptophan will be broken. Without help, the damaged yeast cell will die from a lack of a vital amino acid ingredient. All because and important piece of biological information was lost.


Any cell, including the simple yeast cell, that only holds one copy of all its chromosomes, DNA and genes in its nucleus is called a haploid. Most prokaryotes, and a lot of eukaryotic organisms exist, live and reproduce in this genetic state without any problems, but, they are very vulnerable to any mutational event that damages one of their genes.

If a haploid yeast is damaged in any of the genes that produces any of the team of enzymes needed for the proper production of any of its vital ingredients, it is in immediate danger of dying of starvation. It is a risky business being a haploid cell or organism.

Cell biologists and geneticists have a way of representing a cell or organism that cannot make its own tryptophan; they call such cells try-, (which is pronounced "trip minus").

Such damaged cells can only grow if the experimenter adds extra tryptophan, the missing amino acid, to the nutrient growth liquid (or solid).

This makes it easy to see if a cell has been damaged in this way; it cannot grow without the addition of extra tryptophan.

Haploid cells damaged in any of the genes of a metabolic pathway can only grow if the missing ingredient is added to their growth medium.

different causes -
same result

Let us assume that there are three critical enzymes necessary for the final stages in the production of the amino acid tryptophan. [Of course there are a lot more stages, and enzymes, needed for the full production of the amino acid]. Each enzyme converts a simpler substrate into a more complex product, and in the last enzyme catalyzed chemical reaction the complete amino acid is produced.

A mutation in gene 1, results in the loss of function of enzyme #1, which in turn means that molecule #1 cannot be converted into molecule #2.

This mutant yeast cell cannot make any tryptophan, so unless this amino acid is added to its growth medium it cannot survive. It is try-.

A different mutation in a different gene, gene 3, results in the loss of function of enzyme #3, which in turn means that molecule #3 cannot be converted into the final product molecule #4 - the essential amino acid.

This mutant yeast cell also cannot make any tryptophan, so unless this amino acid is added to its growth medium it cannot survive. It is also try-, and in many ways is very similar and indistinguishable from the first mutant yeast.

Survival for haploids depends on keeping all their genes intact and functioning. Mutations are nearly always lethal for the simple reason that there is only one gene coding for each protein and function.

Long ago in the evolution of eukaryotic cells and organism, this dangerous situation was resolved by a simple process; two cells joined forces, doubling their compliment of genes, and thus doubling their chances of survival if any of these genes were damaged by mutation.

© 2002, Professor John Blamire