The Endomembrane System and the Nucleus

The cytoplasm of a eukaryotic cell contains a group of organelles -- the endoplasmic reticulum, the Golgi apparatus, lysosomes, and vacuoles -- that are grouped together and studied as a single functional unit called the endomembrane system. They are grouped in this way because their membranes are related in origin, in overall chemical composition, and, most importantly, because these organelles very often work together, passing material from one to the next in a coordinated sequence, to synthesise, modify, package, and route proteins and lipids to their correct destinations within or outside the cell. Organelles such as mitochondria, chloroplasts, and peroxisomes, although also membrane-bound, are deliberately excluded from the endomembrane system, because their functions are not coordinated with those of the ER-Golgi-lysosome-vacuole group in the same direct, sequential way.

The nucleus, first identified by Robert Brown in 1831, is usually the single most conspicuous organelle within a eukaryotic cell, generally spherical in outline and, in most cells, located roughly centrally. It is bound by a nuclear envelope made up of two concentric membranes, an outer and an inner membrane, together enclosing the fluid-filled interior of the nucleus known as the nucleoplasm. The outer nuclear membrane is typically continuous with the endoplasmic reticulum described in the next section, and it may itself bear ribosomes on its outer, cytoplasmic-facing surface, underlining how closely the nucleus and the rest of the endomembrane system are physically linked.

At regular intervals over its surface, the nuclear envelope is perforated by nuclear pores, formed at points where the outer and inner nuclear membranes fuse together to leave a channel through the envelope. These pores are not simple open holes but are built from complex protein assemblies that regulate and facilitate the passage of large molecules -- including messenger RNA and ribosomal subunits moving out of the nucleus, and regulatory proteins moving in -- across the otherwise impermeable double membrane.

Within the nucleoplasm lies the cell's genetic material, organised as a diffuse, thread-like network called chromatin during the periods when the cell is not actively dividing. Chromatin consists of DNA wound around, and closely associated with, a set of proteins called histones, forming a nucleoprotein fibre. When a cell prepares to divide, this diffuse chromatin network progressively condenses and coils up into the compact, well-defined, rod-shaped structures known as chromosomes, each with a constricted region called the centromere at which spindle fibres attach during cell division via a protein structure called the kinetochore. …