Q.32 Which one of the following is not true for euchromatin?
- They are transcriptionally active
- They are less condensed regions of chromosomes
- Barr body is an example of euchromatin
- All of these
Barr body is an example of euchromatin is not true; the Barr body represents inactivated heterochromatin. Euchromatin is transcriptionally active and less condensed, unlike the condensed, inactive Barr body.
Introduction
Euchromatin Barr body not true highlights a common exam misconception: Barr bodies represent X-chromosome inactivation as heterochromatin, not euchromatin. This guide analyzes all MCQ options, defines euchromatin traits, and clarifies chromatin differences for biology students.
Euchromatin Definition
Euchromatin forms loosely packed “beads on a string” nucleosomes (11 nm diameter) with wide spacing between H2A/H2B/H3/H4 cores linked by H1. This structure enables RNA polymerase and transcription factors access for active gene expression, mainly containing housekeeping genes replicated in S phase.
Found throughout eukaryotic nuclei (92% human genome), it stains lightly in microscopy vs. dark heterochromatin. Prokaryotes contain only euchromatin.
Option Analysis
Option Statement True for Euchromatin? Explanation 1 Transcriptionally active Yes Looser packing allows transcription machinery access; produces mRNA from structural genes. 2 Less condensed regions Yes Nucleosomes widely spaced vs. tightly packed heterochromatin; appears light in staining. 3 Barr body is euchromatin No Barr body = condensed, inactive X-chromosome (heterochromatin) via Xist RNA coating; transcriptionally silent. 4 All of these No Fails due to option 3; only 1-2 describe euchromatin. Chromatin Comparison
Euchromatin vs. Heterochromatin:
Feature Euchromatin Heterochromatin (e.g., Barr body) Condensation Loose Highly condensed Transcription Active Inactive/repressed Location Distal chromosome arms Centromeres, telomeres, Barr body Staining Light Dark Genome % ~92% human ~8% Barr body forms in female mammals for dosage compensation, marked by H3K27me3, not euchromatic marks like acetylation. Vital for NEET cell biology.
