The genetic material within the nucleus of every human cell is deoxyribonucleic acid (DNA). Although the DNA from a single cell measures nearly two meters in length, it is contained within a microscopic space. This requires DNA to be highly organized and packaged. The structure of the DNA must change dynamically depending on what the cell is doing, allowing the cell to read its instruction manual.
The Name and Basic Definition
The stringy, uncoiled form of DNA, which is the standard state when the cell is not actively dividing, is called chromatin. Chromatin is a complex substance composed of DNA tightly bound to various proteins, found throughout the nucleus of eukaryotic cells. This relaxed structure is characteristic of the cell’s interphase, the period of cell growth and normal function. The primary role of chromatin is to package the DNA into a smaller volume while also allowing the genetic information to be accessed.
The Structural Components of Chromatin
Chromatin is a highly ordered complex built around specialized proteins called histones. Histones are small, positively charged proteins that facilitate binding to the negatively charged DNA molecule. The fundamental repeating unit of chromatin structure is the nucleosome, which is often described using the visual analogy of “beads on a string.”
Each nucleosome consists of a core particle formed by an octamer of histone proteins (H2A, H2B, H3, and H4). A segment of DNA, approximately 146 base pairs long, is tightly wrapped around this histone octamer. These nucleosomes are connected by a segment of DNA called linker DNA, forming the 10-nanometer fiber. This 10-nanometer fiber represents the most basic level of chromatin organization, which then coils into a thicker 30-nanometer fiber.
Why DNA Needs to Be Uncoiled
The organization of DNA into chromatin is directly related to the cell’s ability to use its genes. For a gene to be expressed, the DNA must be partially unwound so that necessary enzymes, like RNA polymerase, can access and read the genetic code in a process called transcription. The uncoiled state provides the necessary gene accessibility for the cell to manufacture proteins and perform its functions. This state is also necessary for DNA repair mechanisms, as the enzymes responsible for fixing damage need to reach the DNA sequence.
The chromatin state is not uniform throughout the nucleus. Scientists distinguish between two main forms based on their level of packing and activity. Euchromatin is the more loosely packed form, which appears lighter when stained and contains genes that are actively being transcribed. Conversely, heterochromatin is densely packed, appears dark when stained, and is largely transcriptionally inactive, often containing structural elements like centromeres and telomeres. This differential packaging acts as a mechanism for regulating which genes are turned “on” and “off.”
The Dynamic Shift: Chromatin to Chromosomes
The relaxed chromatin structure is suitable for the daily operations of the cell, but it is not ideal for cell division. When a cell prepares to divide, the stringy chromatin undergoes a dramatic condensation into distinct, rod-shaped structures known as chromosomes. This transition occurs during the prophase stage of mitosis or meiosis. The loops of the 30-nanometer chromatin fibers fold upon themselves further, resulting in a condensation factor of nearly 10,000-fold.
This highly condensed form serves the specific function of ensuring the genetic material is safely and efficiently transported to the two new daughter cells. Chromosome formation prevents the long, delicate DNA strands from becoming tangled or damaged during the physical separation process. Once cell division is complete, the chromosomes decondense and revert back to the relaxed, functional chromatin state, allowing transcription and other processes to resume.