Question

Which can be observed in an interphase nucleus under the light microscope?
A. Chromosome
B. Nucleosome
C. Centrosome
D. Heterochromatin

Answer 1

Hint: Interphase is the portion of the cell cycle in which the cell grows and makes a copy of its DNA. This portion of the cell cycle is not accompanied by observable changes under the microscope and it includes G1, S and G2 phases.

Complete answer:
Let us analyse each given option to find out the correct answer.
Option A: Chromosome is an organized package of DNA found in the nucleus of the cell. Chromosomes are electron-dense structures in a transmission electron microscope which can only be seen in prometaphase, metaphase, and anaphase of the mitosis. The chromosomes are visible under a light microscope, and they change in shape as the DNA is duplicated and separated into two cells.
Therefore, this is an incorrect option.

Option B: Nucleosome is a section of DNA that is wrapped around a core of proteins. It can be observed under the electron microscope. Therefore, this is an incorrect option.

Option C: Centrosome is a cellular structure involved in the process of cell division. During the cell division, the centrosome duplicates, and then division begins, the two centrosomes move to opposite ends of the cell. Centrosome can be seen under the electron microscope. Therefore, this is an incorrect option.

Option D: In interphase, nucleus heterochromatin can be seen under the light microscope. The heterochromatin is condensed in the secondary constriction I and II of the chromosome. Heterochromatin is a tightly packed form of DNA, which comes in different varieties. These varieties lie on a continuum between the two extremes of constitutive and facultative heterochromatin. Both play a role in the expression of genes. Chromatin is found in two varieties: euchromatin and heterochromatin. Therefore, this is the correct option.

Thus, the correct option is D, Heterochromatin.

Note: It is to be noted that heterochromatin has several functions:
- Gene regulation to the protection of the integrity of chromosomes.
- Dense packing of DNA.
- Bind DNA or its associated factors.