The cell nucleus is separated from the hyaloplasm by a nuclear membrane. It is the cell's control center and contains the chromosomes, which carry the genetic information. The cell nucleus is part of each and every cell. The most important exceptions are the red blood cells, which only live for about 100 days because they lack a nucleus.
The functions of the cell nucleus are to ensure control of protein synthesis by the formation of transfer RNA, to take over control of species-specific cellular reproduction by the formation of special RNA molecules, and to store the chromosomes.
The cell nucleus comprises the following constituents (Fig 2-2):
- The nuclear membrane, which is an inner membrane, a lipoid layer through which exchange of materials between nuclear contents and hyaloplasm is possible. The outer membrane is part of the endoplasmic reticulum; it forms a narrow gap with the inner membrane, the perinuclear space, which is connected to the channel system of the reticulum.
- The nucleolus, part of an active chromosome; transfer RNA, with which protein synthesis is regulated, is formed in the nucleolus.
- Chromatin, which consists of largely unwound chromosomes from the resting or interphase nucleus. The chromosomes are long spiral threads of DNA molecules. They are always found in pairs. In humans, a diploid set of chromosomes comprises 23 pairs of chromosomes, ie, a total of 46 chromosomes. The DNA molecules carry the genetic information (genes); this is where the functions, structure, and timing for coordinating living processes are stored.