Biological Membranes - A. N. Ogurtsov 2012

Structure and Functions of Biomembranes
Membrane Structures of the Cell
Cell Nucleus

The Nucleus is the largest organelle of a Introduction/5.html">Eukaryotic Cell (measuring about 10 µm in diameter) that is visible under a Light Microscope. It is separated from the rest of The Cell by a nuclear envelope consisting of inner and outer nuclear membranes (Figure 13).

The region between the two nuclear membranes is known as the perinuclear (intermembrane) space. The outer nuclear membrane is studded with Ribosomes and is continuous with the rough Endoplasmic reticulum (RER). The inner nuclear membrane is lined with specialized Proteins, such as lamins, which serve to anchor nuclear structures (the nuclear lamina).

The nucleus contains virtually all of the cell's DNA. This DNA serves as the repository of Genetic information and is the primary site for its Replication and expression.

During interphase (the stages between cell divisions), the majority of nuclear DNA exists as heterochromatin, i.e., densely packed DNA associated with RNA and proteins.

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Figure 13 - Cell nucleus

Less densely packed DNA is referred to as euchromatin; this is the region where active METABOLISM/31.html">Transcription of DNA into RNA takes place.

The nucleus frequently contains a nucleolus, and occasionally multiple nucleolites. During Cell Division, the nuclear Structure breaks down.

Chromatin is organized into Chromosomes, which represent the highly condensed forms of DNA molecules visible under a light microscope.

The exchange of macromolecules, such as proteins and RNA, between the nucleus and the Cytoplasm occurs through nuclear pores (approximately 7 nm in diameter) formed by nucleoporin Protein Complexes. These pores regulate Transport Across the nuclear membranes. Peptides and small proteins, such as Histones, pass freely into the nucleus. Larger proteins (exceeding 40 kDa) can only cross the nuclear membrane if they carry a specific nuclear localization signal.