URA3 gene expression allows yeast cells to grow on synthetic media lacking uracil (SC-URA). The diagram shows different cell types with URA3 gene inserted at distinct chromosomal locations. The ability of each cell type to grow on SC-URA in either log phase or stationary phase is provided on the right.

The following interpretations were made:

A. URA3 gene in cell type 1 is probably located in a heterochromatic region.
B. URA3 gene in cell type 2 is located in facultative heterochromatin.
C. URA3 gene in cell type 2 is probably located in a chromosome region silenced in log phase.
D. URA3 gene in cell type 3 is heterochromatinized in log phase.

Which one of the following options are correct interpretations?

Correct option is B

From the growth data on SC-URA medium:

• Cell Type 1 grows in both log and stationary phase (+ , +).
• Cell Type 2 grows only in stationary phase (- , +).
• Cell Type 3 does not grow in either phase (- , -).

Assessing Each Statement:

• Statement A: "URA3 gene in cell type 1 is probably located in a heterochromatic region."

  • Incorrect
  • Since Cell Type 1 grows in both phases, URA3 is actively expressed, meaning it is not in heterochromatin.

• Statement B: "URA3 gene in cell type 2 is located in facultative heterochromatin."

  • Correct
  • Cell Type 2 does not grow in log phase but grows in stationary phase.
  • This indicates that URA3 is silenced in log phase but activated in stationary phase, which is characteristic of facultative heterochromatin.

• Statement C: "URA3 gene in cell type 2 is probably located in a chromosome region silenced in log phase."

  • Incorrect
  • While URA3 is inactive in log phase, the statement is redundant with B.
  • Since B is a more precise characterization, C is not needed as a separate correct answer.

• Statement D: "URA3 gene in cell type 3 is heterochromatinized in log phase."

  • Correct
  • Cell Type 3 does not grow in either phase, meaning URA3 is permanently silenced.
  • This suggests that URA3 is in constitutive heterochromatin or is heavily repressed.
  • The phrase "heterochromatinized in log phase" implies it is already silent in log phase, which aligns with the data.

Option 2 (B and D) is correct.

Information Booster

1. Euchromatin – Open, transcriptionally active chromatin.
2. Facultative heterochromatin – Can switch between active and silent states (e.g., Cell Type 2).
3. Constitutive heterochromatin – Always silenced (e.g., Cell Type 3).
4. Position-effect variegation (PEV) – Gene silencing based on chromatin positioning.
5. Chromatin modifications – H3K9 methylation promotes heterochromatin, while H3K9 acetylation promotes euchromatin.