Biology · Cell: The Unit of Life · NEET
Euchromatin is the loosely packed one. Because it is loose, the staining dye does not collect much, so euchromatin stains LIGHT. Heterochromatin is densely (tightly) packed, so it stains DARK. NCERT says exactly this: loosely packed and light-stained = euchromatin; densely packed and dark-stained = heterochromatin. NEET loves to swap these two, so lock it in: light and loose go together (euchromatin).
Euchromatin is transcriptionally ACTIVE. "Transcriptionally active" means the genes in that region can be read to make RNA. Because euchromatin is loose, the cell machinery can reach the DNA and copy it. Heterochromatin is packed too tightly for the machinery to reach, so it is transcriptionally INACTIVE. Simple link: loose = active = euchromatin; tight = inactive = heterochromatin.
Chromatin contains DNA, basic proteins called histones, some non-histone proteins, and also RNA. It is the loose, indistinct network of nucleoprotein fibres seen in the interphase nucleus (the nucleus of a cell that is not dividing). During cell division this same chromatin condenses into the compact structures we call chromosomes. So chromatin and chromosome are the same material at different levels of packing.
They are the same DNA-protein material, just packed differently. In interphase (non-dividing cell) it is loose and thread-like and is called chromatin. During cell division the chromatin condenses tightly and becomes visible as distinct chromosomes. Rule to remember: chromatin = uncondensed (interphase); chromosome = condensed (dividing cell). Chromosomes are visible only in dividing cells.
The material of the nucleus that gets stained by basic dyes was named chromatin by Flemming. The nucleus itself was first described by Robert Brown in 1831. Two different scientists, two facts NEET can ask together: Robert Brown = nucleus (1831); Flemming = named chromatin.
Staining depends on how tightly the DNA is packed. Basic dyes bind DNA. In heterochromatin the DNA is packed very densely, so a lot of dye collects in a small area and it looks DARK. In euchromatin the DNA is spread out loosely, so less dye collects per area and it looks LIGHT. So dark colour under a microscope tells you the region is heterochromatin (dense, inactive).
Match List-I with List-II. List-I: A. Alfred Hershey and Martha; B. Euchromatin; C. Frederick Griffith; D. Heterochromatin List-II: I. Streptococcus pneumoniae; II. Densely packed and dark-stained; III. Loosely packed and light-stained; IV. DNA as genetic material confirmation Choose the correct answer:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Heterochromatin is transcriptionally inactive. Its DNA is packed too tightly for genes to be read. Euchromatin is the active form.
Euchromatin stains light because it is loosely packed. Heterochromatin stains dark because it is densely packed.
Chromatin contains basic proteins called histones, plus non-histone proteins, along with DNA and some RNA.
Chromatin (the loose thread-like form) is seen in the interphase nucleus of a non-dividing cell. In a dividing cell it condenses into visible chromosomes.
Flemming gave the name chromatin to the nuclear material that is stained by basic dyes.
