Yeast Biology & Yeast as a Model for Aging Research

Saccharomyces cerevisiae • Lifespan • Budding • Metabolism • Why Yeast Helps Us Study Aging

Yeast (Saccharomyces cerevisiae) is one of the most widely used model organisms in biology.
It is simple, safe, easy to grow, and shares many core cellular processes with humans.

This page introduces yeast biology, how they divide, why they age, and why scientists study yeast to understand aging in humans.

1. What Is Yeast?

Yeast are unicellular eukaryotes — meaning they have:

• a nucleus
• chromosomes
• mitochondria
• membrane-bound organelles

They are far simpler than human cells but use the same basic machinery:

DNA → RNA → protein
ATP → energy
Chromosomes → genes
Mitochondria → respiration

Yeast cells are:

• ~5–8 µm in diameter (barely visible under light microscopy)
• non-pathogenic (safe for home labs)
• able to grow on simple sugar sources

2. How Yeast Divide: Budding

Yeast replicate through asymmetric budding.

2.1. Steps of budding

1. A small bud forms on the mother cell
2. DNA replicates
3. One nucleus migrates into the bud
4. Bud enlarges
5. Bud detaches → daughter cell

2.2. Key features

• Daughter cells are born young
• Mother cells accumulate aging factors
• Bud scars remain on the mother cell (visible with special stains)

2.3. Optical visibility

• Budding: visible with light microscopy
• Nucleus movement: not visible
• Organelles: not visible

3. Two Lifespans in Yeast

Yeast are unique: they have two different types of lifespan.

3.1. Replicative Lifespan (RLS)

How many daughters a mother cell can produce before it “senesces”.

• Typical lifespan: 20–30 buds
• Each division adds a bud scar
• Mother cells get larger, slower, and accumulate damage

This mirrors aging in dividing human cells.

3.2. Chronological Lifespan (CLS)

How long a non-dividing yeast cell survives in stationary phase.

This mirrors aging in non-dividing human cells such as:

• neurons
• heart cells
• muscle cells

4. What Makes Yeast Age?

Several factors contribute to yeast aging:

4.1. Accumulation of damaged proteins

Misfolded or oxidized proteins accumulate in the mother cell.

4.2. Extrachromosomal rDNA Circles (ERCs)

Small DNA loops form from ribosomal DNA repeats.
They accumulate in the mother and accelerate aging.

4.3. Mitochondrial decline

Declining respiration leads to:

• lower ATP
• higher ROS (reactive oxygen species)
• decreased metabolic flexibility

4.4. Lower NAD⁺ levels

Reduces sirtuin activity (SIR2 in yeast), important for genome stability.

4.5. Epigenetic changes

Chromatin becomes disorganized → transcription “noise” increases.

4.6. Increased vacuole size

Older yeast often have massive vacuoles.

5. Why the Daughter Cell Is Young

When a mother cell divides:

• Damaged proteins remain in the mother
• ERCs remain in the mother
• Old mitochondria remain in the mother
• The daughter receives new or high-quality mitochondria
• Daughter starts with a “reset” transcriptome and chromatin state

This asymmetric distribution is one of the reasons yeast are ideal for aging studies.

6. Yeast Metabolism and Aging

Yeast can switch between:

• fermentation (low ATP, fast)
• respiration (high ATP, slow)

Metabolism strongly influences lifespan.

6.1. Caloric restriction extends yeast lifespan

Low glucose → more respiration → fewer ERCs → longer lifespan.

6.2. NAD⁺ precursors increase lifespan

Yeast respond to:

• NR (nicotinamide riboside)
• NMN (to a lesser degree)
• CR mimetics

These increase SIR2 activity.

6.3. Mitochondrial health = longevity

Mutations that improve mitochondrial function increase lifespan.

7. Why Yeast Are Used to Study Aging in Humans

Even though yeast are unicellular, many aging mechanisms are highly conserved:

This makes yeast an excellent, inexpensive model for:

• testing longevity compounds
• studying DNA repair
• analyzing mitochondrial biology
• mapping epigenetic changes
• high-throughput screening

They allow rapid experiments because:

• generation time is short (~90 min)
• lifespan is ~1–3 days (RLS) or ~weeks (CLS)
• easy gene editing (CRISPR/SIR2 studies)
• affordable and safe

8. What You Can See Under a Light Microscope

Although internal aging mechanisms are invisible, you can observe phenotypes:

Visible:

• Budding pattern
• Cell size differences (older mothers are larger)
• Bud scars (with certain stains)
• Vacuoles (sometimes visible as clear bubbles)

Not visible:

• DNA or chromosomes
• Mitochondria
• Proteins or epigenetic changes
• ERCs
• ROS levels

9. Yeast in Your Home Lab: Useful Experiments

Beginners can easily explore yeast biology:

9.1. Observe budding pattern

Simple wet mount → look at mother/daughter pairs.

9.2. Growth curves

Measure yeast growth over time.

9.3. Stress testing

Expose yeast to: - heat
- cold
- low glucose
- mild salt

Observe growth differences.

9.4. Simple lifespan experiments (RLS-style)

Track budding in single cells across several generations
(advanced for beginners but possible).

10. Summary

Yeast are:

• simple
• safe
• fast-growing
• genetically similar in core pathways to humans

They are one of the most powerful models for studying:

• aging
• metabolism
• mitochondrial function
• gene regulation
• epigenetics
• lifespan interventions

Understanding yeast biology provides a strong foundation for exploring aging science at both beginner and advanced levels.