Lesson 10 - Yeast Unit Final SAQ

**What is the difference between the genotype and phenotype of a cell? Describe the underlying principles that explain how metabolism is controlled at the level of the genotype and phenotype.

Response:

Metabolism has a hierarchy. At the top is the genome; this is the total of all the genes present in any cell. The genome controls all the properties that any cell possesses. If it does not have the requisite set of genes, it can never carry out the functions that these genes code for. An example would be the ability to use pentoses – sugars with five carbon atoms.

Some yeast strains can do this and have the genes that make enzymes can then transport pentoses into cells and break them down into simpler sugars. Other yeasts, including Saccharomyces yeasts, do not possess these genes and cannot use pentoses.

The phenotype describes the activities of a cell at any moment in time. In other words, it is the total of all the products from all the genes that are active at any given instant. The molecular basis of the Crabtree effect, the repression of respiratory pathways by glucose, illustrates how the phenotype varies according to the spectrum of genes that are active or inactive at any instant.

There are two levels of metabolic control:

1. Genes can either be switched on (up-regulated or derepressed) or switched off (down-regulated or repressed). The switches are usually the presence or absence of triggering molecules. In the case of yeasts and other microorganisms, these are often nutrients.

We have seen that sugar uptake in yeast during fermentation is an ordered process. Simple sugars are used first in preference to longer chain sugars. Typically, the presence of the simple sugar represses the genes that produce the transporters used to take up the more complex sugars.

2. The second level of control is via regulation of enzyme activity. Enzymes are the products of active genes. Several methods are used to achieve this. Enzyme activity can be regulated by either binding to a molecule that changes its shape (known as its conformation) so that it is more or less able to bind to its substrate and release the product.

Alternatively, compounds can bind to the active site of the enzyme and block access for the usual substrate. This is called competitive inhibition. A common example of this is where the presence of the product of a pathway inhibits the activity of the enzyme that catalyses the first step. This is called feed-back inhibition.

Control of metabolism at the gene level is quite slow since, for the effect to be seen, it requires the DNA code to be read and enzymes to be built then transported to the part of the cell where they are needed.

Control of metabolism at the enzyme level is much faster. We can appreciate, therefore, that overall control is delicately balanced. Often control at the higher gene level brings about big global changes such as the Crabtree effect. Control at the enzyme level allows fine tuning of the activities of the products made as directed by the genes.