Within every eukaryotic cell is a nucleus filled with a gel-like substance known as the nucleoplasm. Floating within this substance is deoxyribonucleic acid (DNA), the molecule holding the genetic instructions for a cell’s development, function, and reproduction. The nucleoplasm provides a stable environment that protects the DNA from metabolic processes in the rest of the cell. All activities involving DNA, from storage to its use as a template, occur within this protected space.
DNA Packaging as Chromatin
A single human cell contains about two meters of DNA that must fit inside a nucleus only 10 to 20 micrometers in diameter. To solve this spatial problem, the DNA is intricately packaged with positively charged proteins called histones. These proteins act as spools for the negatively charged DNA to wrap around.
This winding creates a structural unit called a nucleosome, consisting of DNA wrapped around a core of eight histone proteins. These nucleosomes are then further organized and compacted. A linker histone helps stack the nucleosomes into a more condensed, helical fiber.
This DNA-protein complex is known as chromatin, the default state of DNA in a non-dividing cell. The structure is dynamic and can be modified to allow or restrict access to genetic information. Less condensed regions (euchromatin) are accessible for cellular machinery, while more densely packed regions (heterochromatin) are inactive.
DNA’s Role in Gene Expression
DNA’s information is used through gene expression, which leads to the production of proteins. The first step, transcription, occurs within the nucleoplasm, where genetic information in a gene is rewritten into a mobile copy. For transcription to begin, a segment of chromatin must unwind, exposing the DNA sequence of a gene.
An enzyme called RNA polymerase binds to a “promoter” region on the DNA, signaling the gene’s start. The enzyme then moves along the DNA strand, synthesizing a complementary molecule of messenger RNA (mRNA). This mRNA transcript exits the nucleus and travels into the cytoplasm for the next stage of protein synthesis. The original DNA does not leave the nucleoplasm and rewinds into its chromatin structure after transcription is complete.
DNA Replication Before Cell Division
Before a cell can divide, its DNA must be duplicated through replication to ensure each new daughter cell receives a complete copy of the genetic material. This process occurs during the synthesis (S) phase of the cell cycle. Replication begins at multiple sites called origins of replication, where an enzyme called helicase unwinds and separates the two DNA strands.
Each separated strand then serves as a template for building a new partner strand. An enzyme complex moves along each template, adding matching nucleotides to create a new complementary half. The result is two identical DNA double helices formed from the original one. Each new molecule contains one original “parental” strand and one newly synthesized “daughter” strand, preparing the cell for division.
Condensation into Chromosomes
After DNA replication, the cell prepares for division (mitosis). To manage the separation of the two identical DNA copies, the chromatin undergoes further compaction into structures known as chromosomes. This condensation prevents the long chromatin threads from tangling during cell division.
The process involves the chromatin fibers looping and folding upon themselves, resulting in a highly compact structure. Each replicated chromosome consists of two identical sister chromatids joined at a central point called the centromere, forming a temporary X-shape for division.
During mitosis, the nuclear membrane breaks down, and the condensed chromosomes are captured by the mitotic spindle. The spindle then pulls the sister chromatids apart, delivering one complete set to each emerging daughter cell. Once division is complete, the chromosomes decondense back into their less compact chromatin form.