Nucleus — Core Principles
Core Principles
The nucleus is the defining organelle of eukaryotic cells, serving as the cell's control center. It is typically spherical or oval and enclosed by a double-membraned nuclear envelope, which is perforated by nuclear pores.
These pores regulate the transport of molecules between the nucleus and the cytoplasm. Inside the nucleus, the genetic material, DNA, is organized with proteins (histones) into a complex called chromatin.
Chromatin exists in two states: euchromatin (less condensed, transcriptionally active) and heterochromatin (highly condensed, transcriptionally inactive). During cell division, chromatin condenses further to form visible chromosomes.
The nucleoplasm is the jelly-like substance filling the nucleus. A prominent, non-membrane-bound structure within the nucleus is the nucleolus, which is responsible for synthesizing ribosomal RNA (rRNA) and assembling ribosomal subunits.
The primary functions of the nucleus include storing and protecting the cell's genetic information, controlling gene expression, facilitating DNA replication and repair, and initiating ribosome biogenesis.
Its integrity is crucial for cell viability and heredity.
Important Differences
vs Euchromatin vs. Heterochromatin
| Aspect | This Topic | Euchromatin vs. Heterochromatin |
|---|---|---|
| Condensation Level | Less condensed, loosely packed | Highly condensed, tightly packed |
| Transcriptional Activity | Transcriptionally active (genes expressed) | Transcriptionally inactive (genes silenced) |
| Appearance (EM) | Appears lighter or electron-lucent | Appears darker or electron-dense |
| Location | Distributed throughout the nucleoplasm, often central | Often found at nuclear periphery, around nucleolus, or centromeres/telomeres |
| DNA Content | Contains actively transcribed genes | Contains repetitive DNA, structural DNA, or silenced genes |