The Cell Nucleus – An Explanation for Students

The Nucleus is the Central Organelle of the Cell

Sep 17, 2009

The cell nucleus may be the most important organelle in the cell for students to understand. It contains the DNA which controls nearly all cell activity inside the cell.

The cell nucleus is the centerpiece organelle of the cell. The actions of the nucleus control almost all other cell activity. Only eukaryotes have a cell nucleus.

What Does a Cell Nucleus Look Like

Under a microscope, the nucleus looks like a dark spot floating in the cytoplasm of the cell. Biology students can see that the outside of the nucleus is covered by a double plasma membrane which protects the nuclear contents from floating off into the cytoplasm of the cell and keeps other cell components out of the nucleus. The nucleus is fairly large, composing about 10% of the entire volume of the cell.

Ribosomes cover the surface of the nucleus and make it look bumpy. Outside the nucleus, the endoplasmic reticulum attaches to the outer layer of the nuclear membrane, in some cases creating a continuous sheet of ribosome-covered membrane that shifts seamlessly from nucleus to ER.

During cell division, the nucleus cannot be seen at all, since the membrane temporarily dissolves to allow the chromosomes to segregate and the cell to split into two. After cell division, the membrane is restored. A few types of cells, such as human blood cells, don't have nuclei at all.

What's Inside the Nucleus

The most important thing inside the nucleus is Nuclear DNA, which can be found in the form of chromosomes. During cell division, these chromosomes form into the familiar individual X shaped molecules seen in textbooks and science fiction movies around the world. When the cell is between divisions, however, these chromosomes relax and spread out, forming tangled strings of DNA called chromatin.

The nucleolus is another structure inside the nucleus. This structure builds ribosomes, which can then be transported out to the surface of the nucleus, to rough endoplasmic reticulum or out into the cytoplasm of the cell, where they build proteins.

Other, less defined structures also occur in the nucleus. These regions carry out functions such as providing a scaffolding for the nuclear structure, processing RNA inside the nucleus and regulating the transcription of DNA.

The Nuclear Membrane

The nucleus is surrounded by a double membrane called the nuclear membrane or nuclear envelope. It has an inner and an outer layer. This phospholipid membrane is porous, with many channels, gates and pumps crossing both layers for molecules to move in and out of the nucleus. This is important because the DNA inside the nucleus controls all of the functions of the cell, and it couldn't do this without being able to send molecules out into the cell cytoplasm. Molecules that regulate DNA also need to be able to move into the cell, and these membrane pores control that flow as well. Other things that cross the nuclear membrane include molecules that are the building blocks of RNA and DNA, molecules that provide energy to power the transcription of DNA and ribosomes that move out of the nucleus into the cytoplasm. The nuclear membrane can have up to 4,000 pores, allowing for sophisticated control over what moves in and out of the nucleus.

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