Lecture 5 (Murray)

Question 1

Basic Characteristics of Schizosaccharomyces pombe (fission yeast)

Answer 1

Fission Yeast Overview

Division Method:
- Binary fission.
- Cells elongate, form division septum, divide symmetrically.

Cell Cycle Observation:
- Monitor cell cycle via cell morphology.
- Identify cell cycle mutants based on distinct shapes and sizes.

Evolutionary Lineage:
- Early divergence from Saccharomyces cerevisiae.
- Evolutionarily distant yeast model.

Doubling Time:
- ~2 hours.

Question 2

Genetic Characteristics of S. pombe (fission yeast)

Answer 2

Chromosome Number and Size:
- 3 chromosomes.
- Total size: 12 kilobase pairs (KB).

Centromeres:
- Complex structure, similar to higher eukaryotes.
- Contains “tm” sequence for CENP-A binding.
- Essential for chromosome segregation.
- Repetitive sequences flanking CENP-A region.
- Repetitive sequences packaged into heterochromatin.
- Absent in S. cerevisiae.

Question 3

Mating Type genes in Saccharomyces pombe

Answer 3

Mating Type genes in Saccharomyces pombe
- mat1 locus: Active mating type (P or M).
- Silent loci: mat2-P and mat3-M store alternate types.
- Switching: Gene conversion transfers information to mat1.
- Imprinting: DNA nick at mat1 during replication.
- Regulation: Controlled by the cell cycle.
- Purpose: Promotes diversity for mating.

Question 4

Centromeric Silencing in S.pombe

Answer 4

Centromeric Silencing in S.pombe
- Pericentric chromatin: Repetitive sequences partially transcribed.
- Double-stranded RNA: Transcripts processed by Dicer into siRNAs.
- RITS complex: Includes Ago1, guided by siRNAs to target chromatin.
- HMT: Methylates H3K9, recruiting HP1.
- HP1: Spreads heterochromatin, silencing repetitive sequences.
- Outcome: Represses recombination, maintains genome stability.
- Note: RNA interference-based silencing mechanism is conserved in higher eukaryotes.

Question 5

Differences between S. c. and S.p.? Genome organization?

End of Lecture Question

Answer 5

Question 6

Why is mating-type switching evolutionarily advantageous?

End of Lecture Question

Answer 6

Advantages of mating-type switching

1. Facilitates Mating: Yeast mating-type switching ensures that even in clonal populations, individuals can mate with their offspring or nearby cells, increasing reproductive opportunities.
2. Promotes Genetic Recombination: By enabling mating, switching facilitates recombination, enhancing genetic diversity within the population.
3. Adapts to Environmental Stress: Sexual reproduction via switching produces genetically varied offspring, which may include individuals better suited to survive environmental challenges like nutrient depletion.