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The cell nucleus consists of nuclear envelope, nucleolus and nucleoplasm. Most chromosomes are located in the nucleoplasm, but portions of several chromosomes containing clusters of rRNA genes may get together in the nucleolus, forming the nucleolar organizing region. The major role of the nucleolus is to produce rRNA.
Figure 1-C-1. Schematic drawing of the nuclear envelope which contains two lipid bilayers. A mammalian nucleus has about 4000 nuclear pores, each is formed by over 100 different proteins.
Introduction to chromosomesChromosomes are the structures that hold DNA molecules. One chromosome contains a DNA molecule. Each chromosome has a p and q arm; p is the shorter arm and q is the longer arm. The arms are separated by a pinched region called the centromere. In order for chromosomes to be seen with a microscope, they need to be stained. Once stained, the chromosomes look like strings with light and dark "bands" and their picture can be taken. The picture, or chromosome map, is called a karyotype. The germ cell (sperm or egg) of a human being contains 23 chromosomes, labeled from 1 to 22 and either X or Y. The somatic cells (cells other than germ cells) of a normal person has 46 chromosomes. For other species, the chromosome number varies from 1 to 1260 (more info). A human somatic cell contains two chromosomes that determine the sex of a person. The two sex chromosomes are XY in male and XX in female. The gene (SRY) that is important for testis formation is located on chromosome Y. It is possible that a person with testis still exhibits female characteristics (more info).
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