In eukaryotic cells, DNA, the genetic blueprint, is an exceptionally long molecule. To fit this immense length, approximately two meters in a human cell, into the microscopic confines of a cell’s nucleus, a sophisticated packaging system is employed. This organization ensures DNA is compact and accessible for cellular processes. The coiling and folding of DNA around specific proteins allow it to be stored and managed within the nucleus.
What is a Nucleosome?
A nucleosome is the fundamental, repeating structural unit for DNA packaging within eukaryotic cells. It consists of a segment of DNA wound around a core of eight histone proteins. This core, known as a histone octamer, is made up of two copies each of four distinct histone proteins: H2A, H2B, H3, and H4. These histones are positively charged proteins, which helps them interact with the negatively charged DNA.
About 146 to 147 base pairs of DNA wrap around the histone octamer in a left-handed superhelical turn, completing about 1.67 turns. This compact structure forms a disc-like particle, measuring about 11 nanometers in diameter and 5.5 nanometers in height. Under an electron microscope, a chain of nucleosomes resembles “beads on a string,” with the DNA acting as the string and each nucleosome forming a bead. Linker DNA connects adjacent nucleosome core particles, varying in length from about 10 to 80 base pairs depending on the cell type.
What is Chromatin?
Chromatin is the complex of DNA and various proteins that makes up the chromosomes within the nucleus of eukaryotic cells. Nucleosome units undergo further levels of compaction and folding to create this higher-order structure. This organization is necessary to condense the vast amount of DNA into a manageable size.
The “beads-on-a-string” arrangement of nucleosomes further coils and folds into a more compact structure known as the 30-nanometer fiber. This compaction is stabilized by an additional histone protein, H1, which binds to both the nucleosome and the linker DNA. Beyond the 30-nanometer fiber, chromatin undergoes looping and folding, leading to more condensed structures, such as looped domains and, eventually, the highly compacted chromosomes visible during cell division.
Chromatin exists in two forms within the nucleus, reflecting different levels of compaction and activity. Euchromatin is a less condensed form, associated with active gene transcription. Its looser arrangement allows the cellular machinery to access the DNA for gene expression. In contrast, heterochromatin is highly condensed and located in regions like centromeres and telomeres, where DNA is transcriptionally inactive, making it less accessible for gene expression.
Nucleosome and Chromatin Relationship
Nucleosomes are the basic building blocks of chromatin. This relationship can be visualized by thinking of nucleosomes as individual bricks, with chromatin representing the entire wall constructed from them. Chromatin is the overarching structure resulting from the assembly and compaction of nucleosomes and various non-histone proteins.
The DNA double helix, about 2 nanometers in diameter, is the initial level of genetic material organization. This double helix wraps around histone proteins to form nucleosomes, increasing compaction. These nucleosomes coil and fold into the next level of organization, forming the 30-nanometer chromatin fiber. This fiber undergoes further looping and folding, leading to increasingly condensed structures, such as the 300-nanometer wide interphase chromatin. Ultimately, this process culminates in the formation of the highly compact mitotic chromosome.
Importance in Cell Function
The organized structure of nucleosomes and chromatin is important for several cellular functions. Their primary role involves packaging the extensive length of DNA, approximately two meters, into the nucleus, which measures about 10 micrometers in diameter. This compaction ensures the entire genome can fit efficiently within the confined space.
Beyond packaging, the dynamic structure of chromatin plays a role in regulating gene expression. The degree of chromatin compaction directly influences DNA accessibility to the cellular machinery involved in transcription. Adjustments in nucleosome positioning and transitions between condensed heterochromatin and less condensed euchromatin activate or silence specific genes, controlling when and where genes are expressed.
Chromatin also facilitates processes like DNA replication and repair. Its flexible structure allows for temporary unwinding and loosening when these processes need to occur, providing access for enzymes and proteins to the DNA. During cell division, chromatin condenses into distinct chromosomes, regulated by nucleosome and chromatin interactions. This organized condensation ensures accurate and efficient segregation of genetic material to daughter cells, preventing tangles and damage during this complex process.