Question 1

What are the three phases of a yeast growth curve?

Accepted Answer

The yeast growth curve has three major phases. During lag phase, cells acclimate to the environment and grow in size but do not divide. In log phase, cells actively divide, leading to cell doubling. Once nutrients deplete, cells enter stationary phase, where division slows and the population remains constant.

Question 2

How is optical density used to measure yeast cell growth?

Accepted Answer

Optical density, or OD, measures the logarithmic ratio of light intensity entering a yeast suspension to light passing through it. A spectrophotometer measures OD at 600 nm wavelength. Plotting OD values over time generates a growth curve that tracks population changes and helps determine cell viability and doubling time.

Question 3

What are the main components of yeast growth media?

Accepted Answer

Rich yeast growth media contains three major components: yeast extract, which provides amino acids, vitamins, and carbohydrates; peptone, the primary amino acid source derived from animal meat or milk; and glucose, a sugar source providing energy. After dissolving these in water, the mixture is sterilized using an autoclave before use.

Question 4

How do you start a pure yeast culture from a plate?

Accepted Answer

Streak yeast cells onto an agar plate using a sterile instrument to spread the culture. Individual colonies form around the streaks, each derived from a single cell. Pick one colony and inoculate it into growth media by mixing briefly. Incubate at 30°C in a shaker for 48 hours to establish a pure culture.

Question 5

What is doubling time and why does it matter for yeast experiments?

Accepted Answer

Doubling time is the period required for a yeast cell population to double in number, calculated using OD measurements from log phase. Wild-type yeast typically has a doubling time of approximately 90 minutes. Knowing doubling time allows researchers to plan experiments throughout the day and predict when cultures reach desired cell densities.

Question 6

How should yeast strains be stored for long-term use?

Accepted Answer

Yeast grown on plates can be stored at 4°C for up to three months, though re-streaking on fresh plates is recommended before inoculating new cultures. For long-term storage, aliquot cells in cryovials containing glycerol and store at -80°C, where they remain viable for several years.

Question 7

What research applications use yeast maintenance techniques?

Accepted Answer

Stationary phase yeast cells study genes regulating genomic DNA instability during aging by plating on selective media where only mutated cells form colonies. Chronological aging studies analyze cell viability loss over time using liquid cultures and growth curves. Log phase cells, representing healthy normal cells, are used to study eukaryotic organelles like ribosomes through reproduction of saccharomyces cerevisiae sexual and asexual processes.

Yeast Maintenance - Assessment Quiz