The genetic information of a living organism is stored in deoxyribonucleic acid, or DNA. To fit the approximately two meters of DNA found in a single human cell into a microscopic nucleus, the DNA must be packaged. This packaging material is called chromatin, a complex of DNA and associated proteins. Chromatin organizes the genetic material and condenses the DNA while ensuring the cell can still access specific genes. This structure is visible inside the nucleus of eukaryotic cells, appearing as a dense, fibrous material under a microscope.
The Core Components of Chromatin
Chromatin’s structure is built from two main components: the DNA molecule itself and specialized proteins called histones. Histones are rich in positive electrical charges, allowing them to bind tightly to the negatively charged backbone of the DNA helix. This binding initiates the first level of DNA compaction, creating a structure that resembles a string of beads.
The individual “beads” on this string are known as nucleosomes. Each nucleosome is formed when a segment of DNA, typically about 147 base pairs long, wraps nearly two times around a core of eight histone proteins. This initial winding step achieves a compaction ratio six times greater than the naked DNA helix.
The nucleosome core is composed of two copies each of four different histone proteins: H2A, H2B, H3, and H4. The short stretch of DNA that links one nucleosome bead to the next is referred to as linker DNA. This entire assembly of DNA and histones creates a fiber that is approximately 11 nanometers in diameter.
Hierarchy of Chromatin Organization
The “beads-on-a-string” structure is the least condensed form of chromatin and is referred to as the 10-nanometer fiber. This structure is the starting point for the folding required to fit the entire genome into the cell nucleus. The next level of compaction involves the coiling of this 10-nanometer fiber into a thicker, more tightly packed structure known as the 30-nanometer fiber.
The 30-nanometer fiber is often described by models such as the solenoid or zigzag structure. A fifth histone protein, called H1, plays a role in stabilizing this higher-order folding by binding to the linker DNA region. This coiling further compacts the DNA, achieving a forty-fold reduction in length compared to the original DNA molecule. While the 30-nanometer fiber is a key concept, its consistent presence in its classic form inside a living cell remains a subject of ongoing research.
The 30-nanometer fiber then folds and coils further into large, irregular loops and domains. These loops are often anchored to a non-histone protein scaffold within the nucleus. These looped domains can be up to 100,000 base pairs long. This looping and tethering of the fiber helps to partition the genome into functional territories, even when the cell is not actively dividing.
The Functional States of Chromatin
The appearance of chromatin under the microscope is not uniform across the nucleus because its structure is dynamic, reflecting its activity. Chromatin exists in two primary functional states, known as euchromatin and heterochromatin, which represent different degrees of DNA compaction.
Euchromatin is the less condensed form of chromatin, appearing as lightly stained or diffuse areas when observed with a microscope. Its open, relaxed structure allows the cellular machinery to easily access the underlying DNA sequence. Genes located within euchromatin are typically active, meaning they are being transcribed to produce proteins. This state resembles the 10-nanometer “beads-on-a-string” structure, making the genetic information readily available.
In contrast, heterochromatin is the highly condensed, tightly packed form of chromatin. It appears as dense, dark-staining clumps, often near the periphery of the nucleus. This closed structure physically blocks access to the DNA, effectively silencing the genes located within these regions. Heterochromatin often incorporates the more compact 30-nanometer fiber and serves a structural role, notably forming parts of the centromeres and telomeres.
Distinguishing Chromatin and Chromosomes
The distinction between chromatin and chromosomes is based primarily on the cell’s life cycle. Chromatin is the working form of the genetic material, existing during the cell’s normal growth phase, known as interphase. During this time, the DNA is dispersed throughout the nucleus in a tangled, fibrous network that allows for gene reading and DNA replication.
Chromosomes, however, are the highly organized, distinct structures that chromatin condenses into when the cell prepares to divide. As the cell enters mitosis or meiosis, the loose chromatin fibers undergo maximum compaction, folding down by up to 10,000 times.
This condensation transforms the diffuse chromatin network into the familiar rod-shaped structures known as chromosomes. These compact packages ensure that the duplicated genetic material is accurately segregated to the two new daughter cells. Once cell division is complete, the tight chromosome structures decondense, returning to the less compact, working state of chromatin for the next interphase period.