Q.42 Given below are two statements, one is labelled as Assertion (A) and other one
labelled as Reason (R).
Assertion (A) : Euchromatin is transcriptionally active and contains most of the
protein-coding DNA.
Reason (R) : Heterochromatin is light staining and less condensed portion of the
chromatin.
In light of the above statements, choose the correct answer from the options given
below.
1. Both (A) and (R) are true and (R) is the correct explanation of (A).
2. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
3. (A) is true but (R) is false.
4. (A) is false but (R) is true.
Euchromatin is transcriptionally active and gene-rich, while heterochromatin is densely packed and mostly inactive, making Reason (R) incorrect about its staining and condensation. The correct answer for this assertion-reason question is option 3: (A) is true but (R) is false.
Correct Answer
Option 3: (A) is true but (R) is false.
Assertion (A) correctly states that euchromatin—loosely packed chromatin—is transcriptionally active and holds most protein-coding genes, allowing easy access for RNA polymerase. Reason (R) wrongly describes heterochromatin as “light staining and less condensed”; it actually stains darkly and remains highly condensed, repressing transcription.
Term Explanations
Euchromatin Features
Euchromatin appears light under microscopy due to its decondensed state during interphase. It replicates early in S-phase and contains active genes with histone marks like H3K4me3 and acetylation, promoting transcription of housekeeping and developmental genes.
Heterochromatin Features
Heterochromatin is dark-staining and tightly coiled, either constitutive (always inactive, like centromeres) or facultative (conditionally silent). It silences genes via H3K9 methylation and repeats, protecting genome stability but limiting transcription.
| Feature | Euchromatin | Heterochromatin |
|---|---|---|
| Condensation | Loose | Dense |
| Staining | Light | Dark |
| Transcription | Active | Inactive |
| Gene Content | Protein-coding rich | Repeat-rich, gene-poor |
Options Analysis
| Option | Description | Correct? | Reason |
|---|---|---|---|
| 1 | Both true; (R) explains (A) | No | (R) false; heterochromatin is dark/condensed, unrelated to (A) |
| 2 | Both true; (R) not explanation | No | (R) false about staining/condensation |
| 3 | (A) true; (R) false | Yes | Matches definitions: euchromatin active; heterochromatin dark/dense |
| 4 | (A) false; (R) true | No | (A) true; euchromatin is gene-rich/active |
Exam Tip
Recall staining opposites: euchromatin light/loose/active; heterochromatin dark/tight/silent. Practice AR questions by verifying each statement independently before linkage.
