The nucleus is a large membrane–bound cell organelle which houses the chromosomes and which occupies roughly 10% of the volume of all eukaryotic cells. The nucleus is separated from the rest of the cell and the cytoplasm by a double membrane known as the nuclear envelope. The outer layer of the nuclear membrane is studded with small openings called nuclear pores, which allow for the controlled movement of selected molecules in and out of the nucleus. Most of a eukaryotic cell’s DNA is found in the chromosomes of the nucleus, while a very small amount of DNA is present in the mitochondria. All plant and animal cells with a nucleus and known as eukaryotic cells, (meaning true nucleus) while bacterial cells which lack a nucleus are known as prokaryotic cells.
The DNA–containing nucleus has been described as a balloon filled with thick solution with a fibrous mesh which holds the DNA in place and which moves molecules about.
The major components of the nucleus include the chromosomes, the nucleolus, the nucleoplasm, and the nuclear cortex. Chromosomes are made of DNA; the nucleolus manufactures ribosomal components; and the nucleoplasm is the fluid and filaments inside the nucleus. The nuclear cortex is a dense area on the
Cytoplasm —All the protoplasm in a living cell that is located outside of the nucleus, as distinguished from nucleoplasm, which is the protoplasm in the nucleus.
Genetic code —The blueprint for all structures and functions in a cell as encoded in DNA.
Organelle —Membrane–bound structures within the cell which have specific functions. Some organ–elles are mitochondria, chloroplasts, nuclei, and lysosomes.
Ribosomes —A protein composed of two subunits that functions in protein synthesis.
inner face of the nucleus, which tethers the chromosomes in place when the cell is not undergoing division.
Not all cells have a nucleus. Bacterial cells lack a nucleus and so do the red blood cells of mammals. Red blood cells (or corpuses) need to be flexible enough to get into tiny capillaries to deliver oxygen and nutrients to the cells. Some cells have more than one nucleus (multinucleated), for example, nutrient–providing cells in the garden pea plant and lilies both have some multi–nucleated, nutrient–providing cells. The long, tube–like skeletal muscle cells in vertebrates are also multinucleated.
The nucleus is the core of a eukaryotic cell. Its primary function is to separate events inside from the cytoplasm outside. This separation of space and time supports the careful choreography of the detailed molecular dance happening inside the nucleus.
A non-dividing nucleus (in a state) “resting,” is actually making the molecules which allow the rest of the cell to function. One of the most important events taking place in the nucleus is transcription, which is the transfer of the instructions on the DNA to the RNA. DNA is a stable store of genetic information, which must be transcribed (via RNA molecules) to construct the proteins coded in its blueprint.
Messenger RNA makes a “mirror image” copy of a stretch of the DNA molecule and then moves RNA out of the nucleus through the nuclear pores into the cytoplasm. There the RNA locates the ribosomes where it consumes the protein products with the help of transfer RNA molecules.
Prior to cell division, the nucleus replicates itself so that the two new cells will each contain genetic information. Several nuclear enzymes coordinate the replication of DNA. During cell division, the nuclear envelope breaks down, and equal copies of DNA and cytoplasm are partitioned into two daughter cells. After division, the nuclear envelopes reform in each daughter cell around its own copy of DNA. This fundamental sequence of events allows for the continuation of eukaryotic life during embryonic development and cellular regeneration throughout life.
See also Chromosome.
Agutter, P.S. Between Nucleus and Cytoplasm. New York: Chapman and Hall, 1991.
Alberts, B., et al. Molecular Biology of the Cell, 4th ed. New York: Garland Publishing, 2002.
Becker, W., and D. Deamer. The World of the Cell, 6th ed. New York: Benjamin/Cummings, 2005.
Louise H. Dickerson