Yeast are single-celled fungi used in activities like baking and brewing for millennia. The species Saccharomyces cerevisiae, or budding yeast, has cells measuring about 5–10 micrometers in diameter. It reproduces through mitosis, where one cell divides to create two genetically identical daughter cells. This process of cell division is a hallmark of eukaryotic organisms.
Yeast as a Model Organism for Cell Division
Scientists use Saccharomyces cerevisiae to study cell division because it provides a simplified, yet representative, system. As a unicellular eukaryote, its cellular processes are less complex than in humans but share remarkable similarity. Many proteins and pathways governing cell life are conserved between yeast and human cells, meaning discoveries in yeast often have direct relevance to human biology.
Yeast also has practical advantages. Its cells grow and divide rapidly, with a generation time of about 90 minutes, allowing for quick experiments. They are also inexpensive to cultivate and their genetics are easy to manipulate, contributing to research that has unraveled the cell cycle’s basic mechanisms.
The Stages of Yeast Mitosis
The process of cell division in budding yeast begins with the formation of a small bud on the mother cell during the late G1 phase of the cell cycle. This bud will become the new daughter cell. The cell cycle is a sequence of events categorized into four main phases: G1, S, G2, and M.
Following the G1 phase, the cell enters the S phase to duplicate its DNA, creating identical copies of each chromosome known as sister chromatids. These are held together as the cell progresses into the G2 phase for growth and preparation. The M phase, or mitosis, is when chromosome segregation occurs.
This process begins with the assembly of the mitotic spindle, a bipolar array of protein filaments called microtubules. The spindle captures the duplicated chromosomes and aligns them. Once all chromosomes are properly attached to microtubules from opposite poles, the connections holding the sister chromatids together are severed.
The spindle then pulls the separated chromatids to opposite ends of the cell, ensuring each new cell receives a complete set of chromosomes. This is followed by cytokinesis, the physical division of the cytoplasm, resulting in two separate, though not identically sized, cells.
Unique Features of Mitosis in Yeast
Yeast mitosis has several distinctive characteristics. The most prominent is “closed mitosis,” where chromosome segregation happens inside a nuclear envelope that remains intact. In contrast, animal cells undergo “open mitosis,” which involves the breakdown and reassembly of the nuclear envelope.
Another distinction is in the structures that organize the mitotic spindle. In yeast, these are Spindle Pole Bodies (SPBs) embedded within the nuclear envelope, which are the functional equivalents of centrosomes in animal cells.
The division process is also asymmetric. Because yeast reproduces by budding, the new daughter cell is physically smaller than the mother cell at separation, though it receives a full set of genetic material.
Controlling the Yeast Cell Cycle
The progression through the yeast cell cycle is a regulated process governed by a series of molecular checkpoints. These checkpoints are quality-control mechanisms that monitor the cell’s status and can halt the cycle if something is amiss. For instance, checkpoints ensure that DNA replication is complete before mitosis begins and that all chromosomes are properly attached to the mitotic spindle.
The engine driving the cell from one phase to the next is a family of enzymes called cyclin-dependent kinases (CDKs). CDKs are only active when bound to their regulatory partners, proteins called cyclins. The levels of different cyclins rise and fall in a predictable pattern, and the sequential activation of different cyclin-CDK complexes orchestrates the events of each phase.
Relevance to Understanding Human Disease
Studying yeast mitosis has implications for human health, particularly in cancer research. Cancer is a disease of uncontrolled cell division, often arising from defects in the cell cycle control mechanisms first identified in yeast. Because these regulatory genes are conserved in humans, scientists can use yeast to understand what goes wrong when these control systems fail.
Errors in chromosome segregation during mitosis can lead to aneuploidy, a state where cells have an incorrect number of chromosomes, a common feature of tumor cells. Studying how chromosome segregation fails in yeast provides insights into the genomic instability that fuels cancer progression and helps identify targets for new therapies.