Lesson 2 - Yeast Morphology and Evaluation
This lesson explores what yeast cells are, how they grow and multiply, what internal organelles they contain, and the functions that these organelles perform. This knowledge will equip us well, so that we can examine some of the practical methods that we use to characterise and evaluate yeast in the brewery.
Yeast Genetics
Genes make any organism unique. However, you must keep in mind that fruit flies share 60% of human genes, yet do not resemble humans at all. Of course, it is these dissimilar genes that make the difference! Let us look at what is different about the genetic make-up of yeast.
Yeast cells contain 16 chromosomes, which are located in the nucleus. They are made up of around 6,600 genes, or open reading frames (ORF). We do not yet know the functions of many of these genes, however advancements are being made every day and this situation is rapidly changing. The genes are collectively known as the genome. Funnily enough, in 1996, a Saccharomyces yeast cell genome was the first eukaryotic cell to be completely sequenced.
The genome describes everything that a cell is capable of doing. Gene activity is tightly regulated. This means that not all genes will be active (expressed) at any given time, only those that are needed under a particular set of conditions. For example, external conditions such as the availability of oxygen or fermentable sugars, cause particular sets of genes to be switched on or off. This is called up- or down-regulation, respectively. The phenotype is the name given to the cell’s physiological state, produced by the total of all gene activity under any particular set of conditions.
The 16 individual chromosomes of S. cerevisiae vary in length and range from just over 200 to more than 1,500 kilobases. Small differences in the genes present, or even their absence, explains the unique nature of individual yeast strains. This underpins all traits such as cropping behaviour, alcohol tolerance, production of particular spectra of flavour compounds, and so on.
We can measure or visualise differences in chromosome size and composition. We use this for many modern methods of strain identification, or ‘genetic fingerprinting’. We will return to this later in the lesson.
Orphan genes are genes without a known function. Yeast contain a lot of these genes, however, the number is decreasing as time goes by. If you are interested in how this is changing day by day (and would like access to a wealth of other information), do follow the link below:
Chromosomes come in sets. A cell that multiplies by asexual mitosis usually has a single set of chromosomes in its nucleus. Scientifically, this is called a haploid cell. Brewing strains are different, and usually contain more than a single set of chromosomes per nucleus.
The number of sets of chromosomes is called the ploidy (n):
Haploid n = 1
Diploid n = 2
Triploid n = 3
Polyploid n = > 1
Domesticated strains (usually polyploid) are yeast strains that have been used in biotechnological processes for a long time. Brewing strains are usually triploid or tetraploid. As ever, it is a bit more complicated than this. Many brewing strains do not contain exact numbers of copies of every gene. This is called aneuploidy, as we will discuss later in more detail.
What difference does it make to a cell if it has more than one copy of its chromosomes? Why do you think this is quite common amongst industrial strains?
Give yourself 5-10 min to think about this, and then click ‘Next’ to continue with this lesson.