Lesson 2 - Yeast Morphology and Evaluation

Most cells multiply asexually through simple division. Some yeast species do this, however brewing yeasts do not. They multiply by budding, which is a very different process. This section will explore budding.

All cells must grow and proliferate; in other words, they must utilise nutrients to create more cellular mass. This allows them to grow physically and also to multiply in number, in order to generate new cells. This is called asexual reproduction, which is identical to what happens in our bodies when older cells die and are replaced by new ones. In our cells, and those of organisms such as bacteria, one cell divides and produces two exact copies, each indistinguishable from the original.

Brewing yeast are different. They use a process called budding:    

Figure: The process of budding in yeast

Budding is the process of asexual reproduction in Saccharomyces yeasts. To bud, the yeast cell carries out a series of activities that we call the cell cycle. The figure below shows the cell cycle.

Figure: Cell cycle

We will follow the cycle as shown and explain what occurs during each stage.

G1

G1 is the phase in which yeast cells in an active fermentation spend the majority of their time. The G stands for gap because although there is plenty going on, the cell is not in the process of budding. During G1, cells increase in size, and gather the resources required to start budding. Towards the end of G1, the cell reaches a decision point. In order to commence budding it needs to be satisfied that the following conditions are met:

• Cell is sufficiently large
• Previous round of cell division is complete
• Cell contains sufficient sterols
• Cell is not subject to environmental stresses (osmotic pressure, high temperature, high pressure etc.)
• Sufficient nutrients are available

START

If these conditions are met, a milestone called START is reached and budding commences. Once START is passed, the cell is committed to bud and cannot stop. If the START conditions are not met, for example if an essential nutrient is absent, the cell moves into a holding phase called G0.

G0

In G0 the cell’s metabolism is adjusted to shut down all non-essential pathways, and others are switched on. This gives the cell the best chance of withstanding a period of starvation. Yeast cells held in storage vessels are in G0.

S (not to be confused with START)

The S phase gets its name from the fact that it is the phase in which DNA synthesis occurs. During S, an area of the cell wall is broken down. New wall material and the underlying membrane are synthesised, and a bud begins to emerge. There is a hive of activity underneath the area where the bud is being formed. Microtubules in the cell form a skeleton along which vesicles containing enzymes and building blocks of the growing bud are transported. This requires energy. An osmotic effect causes the membrane to push against the wall. As the wall becomes weaker, this pushes the emerging bud out. As the bud grows in size, the DNA in the mother cell’s nucleus is replicated, as is the spindle body, which is a structure made up of microtubules in the cell nucleus. The spindle body holds the chromatids (the replicated strands of each chromosome) and will physically moves them when required.

G2

G2 is also known as the post synthetic gap phase because DNA synthesis is complete. G2 extends from S to M and during this phase the cell prepares for mitosis by synthesising proteins. The yeast’s bud also grows significantly.

As we can see on the cell cycle figure G1, S and G2 are the stages of interphase.

M

M stands for mitotic phase which includes mitosis and cytokinesis. M can be broken down into four stages:

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

During mitosis the chromatids are moved apart by the spindle bodies, the nucleus divides and the new nucleus moves into the bud.

Cytokinesis

In cytokinesis, the mother and bud separate. The name cytokinesis translates roughly from the Latin as cell movement. Some scientists do not class cytokinesis as part of the M phase.

Immediately before the mother and daughter cell separate, a ring-like structure develops, which eventually closes the aperture between the two cells. This structure is rich in a polysaccharide called chitin. The chitin leaves a characteristic bud scar on the surface of the mother cell. The new daughter cell also bears a mark where it was attached to the mother cell, in this case it is known as the birth scar. The birth scars also contain chitin, but these are less persistent than bud scars and expand and fade as the daughter cell grows.

Under optimal growth conditions for brewing yeast (around 30^°C), the budding process takes around 90 minutes from start to finish. Under the much cooler conditions of a brewery fermentation, three to six hours are needed to complete the cell cycle.

Daughter cells are much smaller than the mother cells. They must increase in size before they can bud themselves. Unlike most cells undergoing mitosis (where parents and offspring are identical), this method of replication is asymmetrical. This means that the daughter cells are different to their mothers both in size and their metabolic capabilities. 

Yeast cells can only bud a finite number of times. It varies a great deal between strains. It can be as few as 10 and as many as 50. When the limit is reached, the cells will die unless they have already been killed by an applied stress, such as starvation.

The budding method of cell replication used by Saccharomyces yeasts has many important ramifications for brewers. What might these ramifications be?

Figure: Cell multiplication by budding

Give yourself 5-10 min to think about this, and then click ‘Next’ to continue with this lesson.