Yeast Biology: Genetics, Reproduction, and Mutations

What are yeast?

Unicellular fungi

What species is commonly used in genetics labs?

Saccharomyces cerevisiae

What group of fungi do yeast belong to?

Ascomycetes

What does "yeast" refer to?

A unicellular lifestyle, not a strict classification

What do yeast produce under anaerobic conditions?

Ethanol and CO2

What do yeast produce under aerobic conditions?

CO2 and water

Why is yeast useful in baking?

CO2 production makes dough rise

What carbon sources can yeast use?

Sugars, acetate, ethanol

What vitamin is required by yeast?

Biotin

What happens if yeast lack oxygen in liquid culture?

They grow anaerobically

What are the two yeast mating types?

a and alpha

Which mating combination produces diploid cells?

a + alpha

Can a + a mate?

No

What is formed when two mating types fuse?

Diploid zygote

What structures form before yeast mating?

Shmoos

What causes shmoo formation?

Pheromones from opposite mating type

What happens to nuclei during mating?

They fuse into a diploid nucleus

What color are haploid ade mutants?

Red

What color are diploid or wild-type colonies?

White or cream

What causes red colony color?

Buildup of AIR intermediate

How are dominant alleles written?

CAPITAL letters

How are recessive alleles written?

lowercase letters

What is a genotype?

Genetic makeup

What is a phenotype?

Observable traits

What happens if ADE genes are mutated?

Cell cannot make adenine

What is an auxotroph?

A mutant requiring a nutrient to grow

Which genes cause red colonies?

ade1 and ade2

What intermediate accumulates in ade mutants?

AIR

What is YPD media?

Rich media with all nutrients

What grows on YPD?

All yeast

What is MV media?

Minimal media

What grows on MV?

Only prototrophs

What is sporulation media?

Nutrient-poor media that induces meiosis

What happens to ade mutants when adenine is added?

Colonies turn white

Why does adenine change colony color?

It bypasses the blocked pathway

How do yeast reproduce asexually?

Budding (mitosis)

When does sexual reproduction occur?

When mating types meet or under stress

What are the two ploidy states of yeast?

Haploid and diploid

What triggers sporulation?

Nutrient limitation

What does meiosis produce in yeast?

Four haploid spores

What is an ascus?

A sac containing spores

What type of cells can sporulate?

Diploid cells

What conditions cause sporulation?

Starvation or low nutrients

What is an allele?

A version of a gene

What is a dominant allele?

Expressed in heterozygote

What is a recessive allele?

Expressed only when homozygous

What is epistasis?

One gene masks another

What is complementation?

Two mutants restore function when combined

Give an example of a physical mutagen

UV radiation

Give an example of a chemical mutagen

Ethidium bromide

How does ethidium bromide cause mutations?

It inserts into DNA

Which is more dangerous: single or double strand break?

Double-strand break

Why are double-strand breaks dangerous?

Hard to repair, can cause mutations or death

What does the Siamese cat example show?

Environment affects gene expression

Why are Siamese cats darker on extremities?

Cooler temperatures activate pigment enzyme

What is ATP used for?

Energy

What is ADP?

Lower-energy form of ATP

What is AMP?

Even lower-energy molecule

What do ATP, ADP, AMP, and DNA all contain?

Adenine

What are HA1, HA2, HB1, HB2?

Different yeast strains with known genotypes and mating types

What must you know about HA1, HA2, HB1, HB2?

Mating type, genotype, and phenotype

What is the main purpose of using HA and HB strains?

To study mating, complementation, and genotype-phenotype relationships

If two strains do not mate, what is the likely reason?

Same mating type

If two strains mate successfully, what must be true?

They are opposite mating types (a and alpha)

What forms when HA and HB strains mate?

A diploid cell

What color is typically produced after successful mating of ade mutants?

White (complementation restores pathway)

What does it mean if HA1 x HB1 produces white colonies?

The mutations complement (different genes affected)

What does it mean if HA1 x HB1 produces red colonies?

No complementation (same gene mutated)

What does complementation tell you about HA and HB strains?

Whether mutations are in the same or different genes

If two red haploid strains produce white diploids, what happened?

Each strain supplies a functional copy of the missing gene

If two red haploid strains produce red diploids, what happened?

Both strains have mutations in the same gene

What is the genotype of a diploid formed from two different ade mutants?

Heterozygous at both loci (e.g., ADE1/ade1 ADE2/ade2)

Why are diploids usually white even if parents are red?

Functional allele restores adenine pathway

What is being tested when HA strains are crossed with HB strains?

Complementation and mating type

What happens if HA1 is crossed with HA2?

Depends on mating type; may not mate if same type

What is the first thing to check before predicting a cross result?

Mating type compatibility

What is the second thing to check after mating type?

Genotype (which genes are mutated)

What determines colony color after mating?

Adenine pathway functionality

What does a white colony after mating indicate?

Functional adenine pathway

What does a red colony after mating indicate?

Blocked adenine pathway

What does it mean if a strain grows on MV media?

It is prototrophic (no nutrient requirement)

What does it mean if a strain does not grow on MV?

It is auxotrophic

How are HA/HB strains used with MV media?

To test nutrient requirements and mutations

What is the relationship between HA/HB strains and ade genes?

They often carry mutations in ade1 or ade2

What key exam skill involves HA and HB strains?

Predicting outcomes of crosses (color, growth, genotype)