What is the role of the histone proteins of the chromatin?
a) They contain the instructions to build a protein.
b) They act as a spool around which the DNA is wrapped.
c) They help assemble ribosomes.
d) They maintain the nuclear membrane
The Correct Answer and Explanation is:
Correct answer: b) They act as a spool around which the DNA is wrapped.
Explanation
Histone proteins play a fundamental role in the packaging and organization of DNA within the nucleus of eukaryotic cells. The human genome contains about 3 billion base pairs of DNA, which, if stretched out, would measure approximately 2 meters in length. To efficiently fit this DNA inside the microscopic nucleus of a cell, it must be tightly and precisely packed. This is where histone proteins come in.
Histones are a family of basic proteins that associate with DNA to form a complex known as chromatin. The core unit of chromatin is the nucleosome, which consists of DNA wrapped around a histone octamer—eight histone proteins (two each of H2A, H2B, H3, and H4). Approximately 147 base pairs of DNA coil around this histone core, much like thread wrapped around a spool. This structure not only compacts the DNA but also plays a crucial role in gene regulation.
The arrangement of DNA around histones allows the cell to control which segments of DNA are accessible for transcription, replication, or repair. This regulation occurs through chemical modifications to the histones, such as methylation or acetylation, which can either condense the chromatin (making genes less accessible) or relax it (making genes more accessible).
Thus, histones are not just structural elements—they also contribute to the epigenetic regulation of gene expression. The accessibility of genes influences cell function and identity and can be altered in response to environmental factors or developmental cues.
To clarify the incorrect options:
- (a) Refers to DNA, not histones.
- (c) Involves ribosomal RNA and proteins, not histones.
- (d) Nuclear membrane structure is maintained by nuclear lamins and membranes, not histones.
Therefore, option (b) correctly identifies the essential role of histone proteins in chromatin structure and DNA packaging.
