SCYM Cell Cycle & DNA Content Analysis 2 — Questions and Answers
Question 1: What is the primary reason RNase A is added to samples before propidium iodide staining for DNA content analysis?
- To improve cell membrane permeabilization
- To eliminate RNA that would otherwise contribute to PI fluorescence (Correct answer)
- To prevent cell clumping during staining
- To stabilize nuclear DNA structure
Correct answer: To eliminate RNA that would otherwise contribute to PI fluorescence
PI intercalates into both double-stranded DNA and RNA; RNase A digests cellular RNA so that measured fluorescence reflects DNA content exclusively.
Question 2: Which fixation method is most commonly used to prepare cells for propidium iodide-based DNA content analysis?
- 4% paraformaldehyde at room temperature
- 2.5% glutaraldehyde
- Ice-cold 70% ethanol (Correct answer)
- 10% neutral buffered formalin
Correct answer: Ice-cold 70% ethanol
Ice-cold 70% ethanol permeabilizes cells by precipitating membrane proteins, allowing PI uptake while preserving nuclear integrity and DNA content for accurate quantification.
Question 3: Aneuploid tumor cells are identified in DNA content analysis primarily by which characteristic?
- CV greater than 10% on the G1 peak
- DNA index significantly different from 1.0 (Correct answer)
- Sub-G1 fraction exceeding 5%
- S phase fraction above 20%
Correct answer: DNA index significantly different from 1.0
Aneuploid cells carry an abnormal chromosome complement, placing their G0/G1 peak at a position different from the diploid reference, yielding a DNA index significantly different from 1.0.
Question 4: The proliferation index in cell cycle analysis is calculated using which cell cycle compartments?
- G0/G1 fraction only
- S and G2/M fractions relative to all cells (Correct answer)
- G2/M fraction only
- S phase fraction only
Correct answer: S and G2/M fractions relative to all cells
The proliferation index = (S + G2/M) / (G0/G1 + S + G2/M), representing the proportion of cells that have committed to active cycling beyond the G1 gap phase.
Question 5: EdU (5-ethynyl-2'-deoxyuridine) is preferred over BrdU for S phase detection because EdU detection:
- Requires heat-induced DNA denaturation for antibody access
- Uses click chemistry that does not require DNA denaturation (Correct answer)
- Produces significantly lower background fluorescence
- Is incorporated into RNA rather than DNA
Correct answer: Uses click chemistry that does not require DNA denaturation
EdU contains a terminal alkyne that reacts with azide-fluorophore conjugates via copper-catalyzed click chemistry, eliminating the harsh acid or heat denaturation required to expose BrdU epitopes for antibody staining.
Question 6: DAPI (4',6-diamidino-2-phenylindole) binds preferentially to which type of DNA sequence?
- GC-rich regions in the major groove
- RNA secondary structures
- AT-rich regions in the minor groove (Correct answer)
- Single-stranded DNA overhangs
Correct answer: AT-rich regions in the minor groove
DAPI inserts into the minor groove of double-stranded DNA with high selectivity for AT-rich sequences, providing highly specific nuclear DNA staining.
Question 7: In a BrdU/PI dual-parameter assay, what two cell properties are simultaneously measured?
- Cell viability and apoptosis
- Active DNA synthesis and total DNA content (Correct answer)
- Mitochondrial membrane potential and cell size
- Surface antigen expression and dead cell exclusion
Correct answer: Active DNA synthesis and total DNA content
BrdU incorporation identifies cells actively synthesizing DNA (BrdU+), while PI staining reveals total DNA content, allowing precise assignment of BrdU+ cells to their cell cycle position.
SCYM Cell Cycle & DNA Content Analysis 2
What is the primary reason RNase A is added to samples before propidium iodide staining for DNA content analysis?