Within the nucleus of every plant and animal cell is a substance called chromatin, a complex of DNA and proteins. Its primary purpose is to package a vast amount of genetic material into a compact form that fits inside the microscopic nucleus, preventing it from tangling. If you were to stretch out the DNA from a single human cell, it would be about six feet long. This organization can be compared to winding a long piece of thread around spools to keep it neat and accessible.
The Building Blocks of Chromatin
Chromatin is composed of two primary components: DNA and proteins called histones. DNA carries the genetic instructions for a cell’s function, while histones act as the structural framework for packaging this DNA. These proteins are positively charged, allowing them to bind tightly to the negatively charged DNA molecule. This interaction forms the basic repeating unit of chromatin, known as the nucleosome.
A single nucleosome consists of a segment of DNA wrapped around a core of eight histone proteins. This structure is often visualized using the “beads on a string” analogy, where each “bead” is a nucleosome. The “string” connecting these beads is a short stretch of DNA called linker DNA. This initial level of organization is the first step in a multi-level packing process.
Packaging DNA into the Nucleus
The “beads on a string” structure represents the first tier of DNA compaction. To fit inside the nucleus, chromatin undergoes several more layers of folding. The string of nucleosomes is coiled into a more condensed structure, traditionally referred to as the 30-nanometer fiber. The formation of this fiber involves the histone H1 protein, which helps pull the nucleosomes closer together.
This fiber is then organized into a series of loops that are further compressed and folded, creating an even denser structure. While the precise architecture of these higher-order structures is an active area of research, this hierarchical system of coiling and looping allows the cell to manage its genome. This organization is not static; the cell can selectively decondense certain regions of chromatin to access the genetic information as needed.
Functional States and Gene Regulation
Chromatin exists in two primary functional states within the nucleus related to gene activity. These states, known as euchromatin and heterochromatin, differ in their level of condensation. This structural difference determines whether the genetic information on the DNA is accessible to the cellular machinery for reading genes.
Euchromatin is a loosely packed form of chromatin. This “open” configuration allows transcription enzymes to access the DNA and read the instructions within genes, turning those genes “on.” It is the predominant form of chromatin in a cell, making up about 90% of the human genome. Regions of DNA that contain frequently used genes are found in a euchromatic state.
In contrast, heterochromatin is a tightly packed, condensed form of chromatin. In this “closed” state, the DNA is largely inaccessible to transcription machinery, meaning the genes within these regions are “turned off.” Heterochromatin often contains repetitive DNA sequences and is found at the centromeres and telomeres of chromosomes. The cell can dynamically convert regions between euchromatic and heterochromatic states to regulate gene expression.
Chromatin’s Role in Cell Division
The structure of chromatin undergoes its most significant changes in preparation for and during cell division, or mitosis. For most of a cell’s life, during a phase called interphase, chromatin remains in its less condensed forms. When a cell prepares to divide, its chromatin must be organized to ensure that each new daughter cell receives an exact copy of the genome.
To accomplish this, the long chromatin fibers undergo extreme condensation. This process begins during prophase, where the chromatin coils and folds upon itself to become progressively more compact. This supercoiling results in the formation of distinct, visible structures known as chromosomes, which are highly condensed versions of a single chromatin fiber.
This level of compaction is necessary to prevent the DNA strands from becoming tangled or broken as they are segregated into the two new cells. The condensed chromosomes are more manageable and can be efficiently moved by the cell’s mitotic spindle. Once cell division is complete, the chromosomes in the new daughter cells decondense, returning to their less compact interphase state.